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73-87 Chevy _ GMC Trucks => Fuel Systems and Drivability => Topic started by: Henry on January 25, 2022, 11:40:50 AM
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Hi Guys:
I have a driveability problem now that we are in winter: my truck will not come out of high idle when the outside air temperature is below approximately 50F.
My truck: 1976 C-20, mostly OEM. 350 V8, T400 trans, A/C, Qjet 4MV carb (divorced choke coil), standard thermostat. Only mod is a 4-row radiator instead of 3-row radiator.
When the temperature is above the 50s, the truck goes through cold start normally where it is in high idle and you can kick it down to regular idle within a few minutes or drive away and it goes down to regular idle after suburban driving for 5 minutes.
When the temperature is below the 50s, the truck starts in cold start but stays there no matter how long I let it stay at high idle. It will not kick down and if I drive away it will not kick down even after 10 minutes of suburban driving.
Inspection of my carb shows all linkages to be intact, no hose leaks, and choke coil to be free and in good shape. On the surface everything seems in order. I will go make some linkage adjustment checks and do some testing of the vacuum break diaphragm to make sure it is really ok.
So when I first started the truck with the hood up I noticed that the fan was blowing a gale force wind on cold start and appears to be running at engine rpm...and this made me suspect it may be the culprit of keeping the engine and carburetor too cold in cold weather. I have the OEM fan and a OEM style thermostatic fan clutch, standard duty. When the engine is off, regardless of cold or hot engine, the fan has resistance to spinning: no matter how hard I spin, it will not feel like turning more than 1/2 turn. On a cold day (about 45F) I cold started it and at high idle when the engine was cold I attempted to stop the fan with a rolled up newspaper: it took considerable force to stop it while the engine was running. When I released it the fan just started back up to what appears engine rpm driving a gale force wind across the engine again. So I thought the fan clutch was bad and went to a big chain parts supply store and bought a new standard duty fan clutch. This clutch is a Wagner brand and it cost about $35. I put it in the truck and gave it a week to settle in by driving it around town in hot and cold weather. So now I go through the same checks of the fan clutch and it behaves exactly like the old clutch. So I am feeling I dont really understand how this fan clutch works or maybe I have a bad new fan clutch.
So I still have to do the checks on my carb, but can anyone tell me how to really check if this fan clutch is working correctly?
Regards,
Henry
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The only thing to keep you at high idle is your choke/assembly.
What do mean by "divorced choke coil"?
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What do mean by "divorced choke coil"?
The thermostat mounts to the intake manifold versus a "married choke" where the coil mounts inside a housing on the side of the carburetor.
Henry, does the intake manifold become hot where the choke thermostat mounts? Exhaust flows from head to head through a crossover passage directly below the choke stove. Does the engine have a functioning heat riser valve/EFE valve? Is the choke t-stat cover in place to help trap the heat?
Viscous fan clutches typically are engaged when an engine starts. In really cold climates the fluid is so viscous that it can take some time to redistribute and release.
Edit: spelling
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Divorced means you'll NEVER get along ;D
They went to electric choke because those hot air chokes work for crap
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I have had mine since July 2015 and divorced choke still works great!
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Ah yes, the little coil spring wheel under a piece of tin, with a arm thingy pushing upward. Never owned one.
My current one draws hot air from using vacuum from a metal tube coil under the manifold.
Sound like it just needs adjustment since he is able to kick down under warmer conditions.
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Hi Guys:
So here are my answers to your questions:
-No, I am not sure my intake manifold gets hot where the thermostat mounts...I will check this out next time I run it.
-Yes, I have a functioning heat riser on my exhaust downpipe.
-Yes, the little sheet metal cover on my choke coil is in place, coil is clean and dry and moves freely.
-I live in north-east Texas (Dallas area) and we have mixed winters here so every week we get mild days (50s-60s) and cold days (20s-40s) so I have an opportunity to mess with the truck with different temps on a daily basis...I was hoping by driving on both kinds of days I could get the fan clutch fluid to settle to what would be normal levels in the reservoirs for appropriate fan speeds.
-Another thing I tried before my initial post was an attempt to take the fan and fan clutch completely out of the equation to see what the carb would do: I cut a piece of cardboard the exact size of the radiator and stuffed it between the grille and A/C condenser coil so as to block airflow through the radiator and hence fan to engine. I waited for a cold day and then cold started it...the fan and clutch were unfazed by my block...the fan still managed to create a good strong wind force against the engine as if there was no blockage...I was surprised. I do have the original radiator and fan cowling to duct the air. I drove off with it and the engine finally came off high idle after 10 minutes of driving and the water temp gauge never got above 1/4 even with the cardboard in front of the radiator.
I still have not done all my carb choke system checks and will do that late today or tomorrow. I also have the original air cleaner with the Thermac switch and flapper valve so I need to check this out as well and see if it is working and allowing warm air to the carb when it is cold. I will advise.
-Even if it turns out to be a carb adjustment that solves this very cold start issue, it bothers me that the fan clutch does not seem to work as I would expect.
Regards,
Henry
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Hi Guys:
So yesterday at late morning I did some more checks/adjustments on carb choke system:
-Ambient air temp was about 52F
-Cold started truck with a slight tap of acc pedal about 1" and let go...truck starts up immdiately in high idle
-After 5 minutes, high idle actually increases a bit by itself (I have touched nothing)
-At 6 minutes, I observe Thermac flapper valve opening in air horn
-At 7 minutes, I observe exh heat riser flapper valves starting to move and water temp is about 3/16 (half way between 1/8 and 1/4) gauge reading
-At 8 minutes, I give acc pedal a hard quick push to the floor and release and the truck comes off high idle to low idle
-At 10 minutes, the heat riser flapper and Thermac flapper valves are full open. The heat stove surface of the intake manifold is very hot
-At 15 minutes the radiator fan is blowing warm air now (was blowing cold air and air velocity at cold start and hot start seem same) and the water temp is almost to 1/4
-Another observation I made was that during the end of the high idle mode, the fan started making a cyclical moaning sound...this went away when the engine dropped down to low idle. I was not smart enough to put my hand in there to see if it felt like the air speed was changing.
So after all this I decided to just go ahead and make some choke system checks since the high idle seemed too high to me...it was really revving high. I followed my factory manual and since I have an early Q-jet (4MV) on a heavy duty emissions truck, these are the only adjustments I can make: fast idle, fast idle cam, choke vacuum break, choke coil rod length adjustment, and air valve dashpot adjustment.
-Fast idle: my fast idle was too high (2000rpm) so I adjusted to 1600rpm (low idle was observed as 650rpm which is spec)
-Fast idle cam: on spec, no adjustment
-Choke vacuum break: had to bend the vacuum break link arm a little bit
-Choke coil rod length: had to bend this a lot to get in spec
-Air valve dashpot: this is a touchy adjustment since there is so much slop in the link arm hole but I adjusted it anyway a little bit.
-I removed the cardboard air block I had in front of the radiator
-After all this I was a little surprised at how the choke coil rod length was so far off since I had adjusted it when I bought the carb a few years ago. Anyway, it was now time to see if these adjustments changed anything.
I let the truck cool off for 2 hours and the ambient temp slightly dropped to 50F. Truck started right up in high idle (slower than before though). I let it warm up for 3 minutes without touching anything and drove off in high idle. Within 3 minutes of city driving it came off high idle to low idle by itself. So far so good.
I let the truck cool off for 4 hours and the ambient temp was now 42-40F (late at night). Truck started right up in high idle. I let it warm up for 3 minutes and drove off without touching anything. After 7 minutes of driving around in high idle I came back home and it was still in high idle (seemed higher than when it started) and it would not kick down despite water temp being 1/4. Finally I did a real hard kick on the acc pedal to the floor and idle came down to low idle.
So I will do a couple more days of observations to see how it behaves and report back to this post. BTW, I was incorrect to say the fan clutch I bought was a Wagner brand...it is a Murray brand part number #2705.
Regards,
Henry
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Sounds like it's gunked up. Have you used some carb cleaner on the linkage?
Physically check to see that all components are moving easily.
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Hi Johnny:
When I did my carb choke system checks/adjustments I made sure all the linkages and external parts of the carb moved freely. The engine is actually very clean and the carb looks clean clean down the throats as well. I will keep driving it around on warm and cold days to try and quantify exactly how long it takes to warm up and how far down I need to pop the acc pedal to release it from high idle. Maybe taking 10+ minutes to warm up in 40F weather is normal...I wish someone could tell me if this is how this truck should behave with original equip. I still dont have a good understanding of the relationships of all the linkage and cams have with each other on the carb...I know how to adjust everything to specs but if I can understand exactly how they are working together maybe I can make a subtle adjustment somewhere that will allow the carb to kick down to lower idle sooner. These q-jets are a lot more sophisticated than aftermarket Holley carbs.
I also talked to the guys at the car parts company that sold me the fan clutch and they gave me a phone number for the manufacturer of the clutch so I will talk to those guys on Monday and hopefully they can give me some useful tech info on the fan I bought.
Regards,
Henry
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Thanks for the feedback, very helpful!
Two things come to mind when you say high idle is intermittent:
The linkage needs more adjustment, as in elongation from the coil to the carb. Can you send a pic when hot and cold please?
The manifold, while getting 'hot' may not be as hot as it needs to be. Can you use a laser thermometer to measure? It may be the crossover exhaust ports under the manifold are blocked with crud.
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Hi Johnny:
So I did some driving yesterday with cold starts at 58F and 62F and recorded my findings. We are going to have 30F weather on Wednesday so I can do a cold start and record findings with these cold temps.
I talked to the manufacturer of the fan clutch this morning and he explained the way the Murray fan clutch is supposed to work and thought about what my problem may be. He said the clutch thermostat activates the internal valving of the clutch at 180F air temp which is about 190-195F radiator water temp. When the clutch is disengaged (below 180F) it still runs at about 45-50% the driven rpm. When engaged he said it runs at about 75-80% driven rpm. He said fully engaged, the clutch will only run at 800 rpm maximum regardless of driven rpm. He said that if my fan seems to always be running at the same speed that possibly my water thermostat is not functioning correctly in that it is not allowing the radiator to get up to hot temp and hence the fan clutch never getting up to the engagement temp. This does not explain why the fan is blowing so hard when cold. If my thermostat is malfunctioning it would be that it is stuck open...I guess this is a possibility and would explain why it takes so long to warm the water up and hence the long cold start problem. I guess I need to go buy a hand held rpm meter and see exactly what the fan speed is with respect to the driven speed when engine is cold and hot....maybe my perception of the fan running as fast as the engine is wrong.
You may be right in that my heat stove in the manifold may not be getting hot enough, quick enough for the choke coil to do its thing. I need to go buy a hand held thermometer to check this out and this will also allow me to check what that fan clutch and radiator temps get to. Next week the temps should get back to the 60Fs in the mid day so I can photograph the choke coil rod in hot and cold positions and recheck its length spec at the same time. Looks like I may need to look at the engine water thermostat as well.
Regards,
Henry
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Thanks for your persistence AND the education on how fan clutches work!
Personally I ditched mine in favor of an electric one. Instant horsepower increase BTW.
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I put in a fan clutch, is supposed to operate the way the guy says. I don't recall hurricane forces. I'll try to get out and see if mine sounds on its own. I won't be starting it for a while again.
But they're spot on. You're thermostat sounds stuck open. It should stay closed in cold weather and let the engine get warm. If it is only getting to 1/4, it should get warm even if there is something wrong with the clutch. I had an old car once that you could see the thermostat cycling open and closed half to 3/4, when driving in super cold weather, - 15F.
Sent from my SM-G965U using Tapatalk
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Hi Guys:
So it has been a little while since I have updated on my problem but I have bought digital hand held rpm meter and digital hand held temperature gauge and have been making test checks on the cold starting of truck and fan at various temperatures. Without burdening everyone with all the test data I can say that I think the engine coolant thermostat is working correctly, the heat riser valve is working correctly, the intake manifold heat stove is working correctly, and the thermostatic choke rod and coil are working correctly. The fan clutch may not be working correctly because I have recorded it running at the same rpm as the engine when cold in some instances and running at 75% engine rpm most of the time regardless of hot or cold engine. Rarely has the fan ever got as low as 55% engine rpm. After all these observations I think that even though the fan clutch may not be working correctly, it is not that big an influence on the cold starting behavior of the carburetor. In other words I dont think that the cold/hot air blowing across the thermostatic choke rod/coil has as much an affect as the coolant water temp or intake manifold temp. So regardless of what the fan is doing the problem is somewhere else...back to the carb.
Yesterdays events indicate the problem:
0 minutes: Cold start at ambient temp of 46F. Starts with engine 1550 rpm. Fan 1040 rpm...see photo of carb before start.
3 minutes: thermostat hsg 74F, heat stove 93F, radiator inlet 47F.
6 minutes: engine 2070 rpm, fan 1200 rpm, fan clutch 60F
8 minutes: engine 2300 rpm, fan 1400 rpm, thermostat hsg 127F, heat stove 120F, radiator inlet 51F
11 minutes: engine 2260 rpm, fan 1260 rpm, thermostat hsg 187F, heat stove 146F, radiator inlet 165F
15 minutes: full pop of acc pedal brings engine down to 920 rpm, fan 789 rpm
21 minutes: engine 850 rpm, fan 790 rpm, thermostat hsg 187F, heat stove 150F, radiator inlet 165F
At this time I turn off the engine and observe the thermostatic choke coil rod has moved up .40 inch from cold starting position...see photo. After 15 more minutes I restart engine and it is still warm and in low idle so I drive off and all is well.
I arrive at a destination after 10 minutes of driving and turn it off and let it sit for 5 hours and the ambient temp drops to 38F.
0 minutes: Cold start at ambient temp of 38F. Starts with 1825 rpm. Fan 1120 rpm...carb linkages look like when 46F hot.
3 minutes: Engine 2000 rpm, fan 2000 rpm
6 minutes: Engine 2300 rpm, fan 2300 rpm, heat stove 107F, thermostat 131F, radiator inlet 81F, choke rod has pulled way down from initial engine start position(?)
15 minutes: Engine 2300 rpm, fan 2300 rpm, heat stove 138F, thermostat 187F, radiator inlet 160F, choke rod is still way down and hasnt moved...it appears to want to move up but something in the rest of the carb linkage is forcing it down.
16 minutes:shut engine off and inspect carb...see photo of carb. I use my finger to flip the cam arm upwards from being in a forward position and the choke rod now pops up and the choke air inlet plate swings open. The cam arm was stuck in a position. I put the air cleaner back on and the engine starts up in low idle and I drive off with it behaving like it is warmed up now with not problems.
It appears something is going on with the carb where the cam arm linkage gets into some position where the choke rod cannot move up when it is real cold....anyone have any more ideas after looking at my photos?
Regards,
Henry
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I'm a little confused as to the wild goose chase that seems to be going on. Are you suspecting the fan clutch of causing your choke issue? If so, don't go down that rabbit hole. Sounds like your fan clutch is working perfectly and you should replace the choke coil. If you move the choke rod by hand does it pop off of high idle?
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I think your manifold is clogged up where the exhaust gas is supposed to pass.
Evidenced by how low the temp is on your coil area.
The stove will run cooler than your manifold, but not by much.
Check the temp of the manifold near your stove against the temp of your exhaust manifold.
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Fwiw I remembered today to check my fan clutch. Ermine cold in the garage, about 45F, I didn't drive it this week, it's easy to turn, but not so easy for it to free wheel.
Good luck, it sounds like you're on track!
Sent from my SM-G965U using Tapatalk
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Hi Guys:
Vile, I agree with you that the fan and fan clutch are not the problem with the cold starting issue...even if the clutch is not working as I would expect...I have moved that off the table to another day.
Johnny: I would agree with you that the intake manifold heat stove surface does not seem to get very hot when the ambient temps are lower than 50F. Looking at my current data:
Ambient temp=63F, warmed up heat stove=170F
Ambient temp=46F, warmed up heat stove=160F
Ambient temp=38F, warmed up heat stove=138F
And all the above is recorded when the intake manifold thermostat housing is 187F...I dont have any recorded data of my exhaust manifolds when warmed up but I recall taking a measurement of them and getting 400-550F depending on where I measured on them....I need to take some more temp measurements of the heat stove with respect to other engine surfaces over time and see what the final temps come out to be when everything is finally hot when I get another day in the 30s and 40s.
I will also do what Vile suggests and that is to grab that thermostatic choke rod and push it up when I am stuck on high idle and see what happens.
You guys may be on to the fundamental problem here.
I also need to check out my heat riser(manifold heat control valve aka exhaust flapper) because although it rotates freely against the spring pressure when cold, it stiffens up when hot; it does slowly rotate to open position when the engine warms up and finally goes to full open but I noticed that when it is hot and open, I can push the counterweight with a stick and it will move to any other position and stay where I put it...sometimes when I come back from a drive it is always in a different position.
The other thing that may or may not be related to this cold start problem that bothers me a bit is the way the engine races to about 2200rpm from 1600rpm on cold start in cold weather. When the temps are above 55F the engine goes into cold start mode and runs at high idle of 1600rpm and then warms up. When the temps are lower than 55F, the cold start begins at 1600rpm and within 2 minutes the high idle increases to about 2000rpm...when ambient temps are lower than say, 45F the high idle increases to 2200rpm. Does everyone agree with me that this is not right and there is a problem here?
Regards,
Henry
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Rising from 1600 to 2200 is normal.
It's due to the gas atomizing as the engine heats up.
Measure the cast iron that the stove is bolted to. Should be much hotter than the stove.
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Use AC Delco 10-4020 Penetrating Lubricant (https://www.amazon.com/ACDelco-10-4020-Rust-Penetrating-Lubricant/dp/B008I2H55U) on the heat riser valve shaft.
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Is the Vacuum break diaphram functioning?
Check link: https://www.thirdgen.org/forums/carburetors/634914-quadrajet-manual-pdf.html
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Hi Guys:
Increasing idle: So Johnny are you saying that the increasing idle at very cold start is due to all that extra fuel from a very rich mixture being finally vaporized somewhere in the intake manifold as it warms up?...I could buy that except that when the engine finally does get hot idling for 10 minutes after the cold start you would expect all the puddled fuel to be vaporized and used up and then the rpm to go back to 1600rpm, but in my case it does not...it stays stuck at 2200rpm. Maybe because the intake manifold is still too cold even after 10 minutes with water the overly rich mixture continues to puddle in the manifold. So maybe the symptom of increasing idle is also related to the manifold not getting hot enough just like the heat stove...? I will do some temp measurments of the manifold runners next time I do testing. BTW my heat stove measurements are on the intake manifold just below where the heat stove attaches.
Vac break diaphragm: Geezer, I think it is working properly, but not 100% sure....you can pull a vac on it from an external source and it will hold...but I guess I do not know if it is working properly when actually hooked up to the carb...I have seen it moving to different positions during my warm up tests. Thanks for the great carb service book!...it has a great section on the choke system that i need to read...and see about the vac break diaphragm.
Heat riser: BD, thanks for the link to the solvent for the heat riser...I will buy some. I noticed that it has testimonials from people who say it works better than PB Blaster and Kroil...I got plenty of old car rust problems I can test it on...especially those studs and nuts on the exh manifolds to downpipe connections.
Gimme a few days to wait for some cold weather to do my heat riser and choke rod/coil checks and I will report back.
Regards,
Henry
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Try tapping the accelerator as the engine warms and speeds up. This should let the choke release and get off the fast idle cam, returning the engine to it's regular idle speed. Might not happen all by it's self while you are in the warm house drinking the morning coffee. That fast idle cam holds the primary throtle plates open slightly while the choke coil enriches the fuel mixture by closing the choke plate.
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At first start, cold vaporized gas is being drawn down to the flat surface of your intake manifold, then into the tunnels.
As that surface heats, the cold gas is vaporized to a finer degree, less liquid, more atomized, more combustible. Higher RPM.
Think about it: the fast idle cam is actuated by the coil. When the coil is heated, it releases tension to the fast idle cam through its linkage.
Either your coil is defective, or, it's not getting hot enough to release it's tension.
The temps you stated as being at the stove, but now on the manifold, are low for being under direct influence of exhaust gas passing under the manifold. Some readings are not even as hot as the engine operating temp.
I have a temp gauge and will measure mine at operating temp.
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Hi Guys:
So here is a one day cold start test with a lot of temp measurements:
-3:41pm dead cold start with ambient temp=51F, heat riser is closed and I lube it with heat riser lube.
-0 minute, since truck has not been run for a few days I floor the acc and then hold half way and crank...after 10 revs it fires up.
-1 minute, heat stove (manifold below it)=71F, thermostat=78F, intake runner #1=56F, heat riser=209F, engine rpm=1800
-5 minute, heat stove (manifold below it)=103F, thermostat=141F, intake runner #1=101F, heat riser=360F, engine rpm=2250
-heat riser not noticably opening yet
-9 minute, heat stove (manifold below it)=129F, thermostat=188F, intake runner #1=138F, heat riser=445F, engine rpm=2400
-heat riser opening about 50%, choke coil rod up about 1/16 inch
-14 minute, heat stove (manifold below it)=155F, thermostat=195F, intake runner #1=166F, heat riser=530F, engine rpm=2200
-heat riser opening about 75%, choke coil rod up about 3/8 inch
-19 minute, kicks down with barely tapping acc to 880 rpm, heat riser is full open and choke coil rod is up about 1/2 inch.
-20 minute, turn off engine.
-1 hour later I observe the heat riser is fully closed and not sticking, and I spray more lube on it. Choke coil rod is still up about 1/2 inch.
-0 minute, engine starts immediately without touching acc to low idle, water temp gauge at full cold.
-1 minute, drive off, water temp gauge at 1/16.
-8 minute, arrive at destination with engine at low idle, choke rod still up 1/2 inch, heat riser is closed and a little bit sticky(it will open full up but release back about 1/2), heat stove(manifold below it)=173F, intake runner #1=176F.
-shut engine off at 5:13pm
-5-1/2 hours later ambient air is 40F, heat stove=75F, choke rod up 1/4 inch, heat riser closed.
-0 minute, I start engine with a light tap of acc and it starts right up at high idle. I notice that the tap of acc has drawn the choke coil rod way down.
-1 minute, engine rpm=1850
-4 minute, heat stove=102F, heat riser=225F, choke coil rod has not moved at all.
-6 minute, heat stove=107F, heat riser=336F, intake runner #1=127F
-9 minute, heat stove=135F, but the choke coil rod is still way down. I am able to kick it down with 1/4 acc tap to medium idle of 1500rpm...the choke coil rod has now moved up by 1/8 inch.
-11 minute, heat stove=160F, heat riser=473F, but the heat riser has barely started to open.
-14 minute, heat stove=157F, choke coil rod is still at 1/8 inch. 1/4 tap of acc kicks it down to 1300rpm.
-16 minute, heat riser is finally full open
-17 minute, will not kick down to idle and choke coil rod is still at 1/8 inch. I take some real long needle nose pliers and attempt to pull the choke coil rod up but it wont budge as if the linkage it is connected to it is holding it down for some reason.
-19 minute, I just drive off at 1300 rpm high idle.
-21 minute, while driving it comes off high idle by itself.
-29 minute, I arrive at destination and it is at low idle now (by itself)and I notice the choke coil rod is at about 3/8 inch up.
-So I am not sure what to make of all this, but for a start, it does seem to me that the heat stove may not be getting hot enough and not quick enough as well...seems like it should warm up within 10 minutes even at ambient temps lower than 50F...also I am wondering why I was not able to pull the choke coil rod up manually. Even though I read that Delco carb book I am still a little confused as to how all the choke linkages and cams work together since it is hard to see everything with the carb on the engine. So I bought a scrap Qjet 4MV carb on Ebay just so I can study how the linkages and cams work together...still waiting for it.
Regards,
Henry
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Henry,
Thanks, that was very helpful!
The same exhaust gas that is heating your heat riser to triple digits is designed to pass under your intake manifold.
It obviously is Not. Pull your intake manifold and clean the gunk out of the passage between the heads.
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Hi Guys:
Some more observations/questions:
Yesterday I ran some errands with truck: 47F ambient start temp, did some suburban driving for about an hour and half. Engine started at high idle and after letting it warm up for a couple of minutes I drove off in high idle...within 5 minutes of stop and go driving it came off high idle to low idle. Regardless of whether it is in warm up mode or fully warmed up it drives very well with no stumbling, hesitations, or stalling. 3 errand stops it starts right up in low idle. When I finally get back home the ambient temp is about 45F. With engine idling, heat stove checks 170F, thermostat checks 193F, intake manifold at runner #1 checks 180F, radiator inlet checks 180F. Heat riser counterweight is up which I believe indicates it is closed. It is free to move open with a stick but sticks a bit on the return to closed position. So this data probably doesnt give any indication on the heat stove during warm up time since the heat stove at this point could be fully hot simply from the overall heat of the engine running for 1-1/2 hours.
So at this time, I loop a wire around the carb throttle so I can rev it and observe the heat riser. When I rev the engine up, the heat riser does not move from the closed position(see photo). I would expect the counterweight arm to swing open when the engine is revved up or throttle popped...but it does not. Can anyone tell me if it should?
Johnny, I agree I need to take the intake manifold off and see if the passage for exhaust gas is obstructed...I am going to slap myself real hard if it turns out I put in manifold gaskets that do not have an opening for this passage between the head and intake manifold. I will keep you posted on how this turns out...may be a few months before I do this.
Regards,
Henry
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Henry,
Thanks for the pic of the heat riser! Mine is moved closed by a vacuum motor that then opens when the engine reaches operating temp.
The riser is designed to restrict exhaust gas, and to force it through the intake manifold until everything gets toasty.
Have you observed it moving at all? It may be the problem causing all of this.
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You just reminded me of that can of heat riser lubricant we used to douse those things with. ;D
If it's stuck closed, your choke coil would heat up faster not slower. You do know you can convert to electric choke right? I mean if you are trying to preserve 100% authenticity, I can see messing around for weeks with this otherwise, time for an upgrade.
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Hi Guys:
Johnny: yes, when the engine is going through the cold start mode I see that counterweight on my heat riser swing to the rear over time which indicates it is going from closed to open...when the engine is fully warmed up it is fully open. Then when I drive off and come back home it is sometimes closed again or fully open or partially open...after warm up the position it is in does not seem to affect engine performance.
Vile: yes, I have been spraying that AC Delco heat riser lube on it recently to make sure it is freed up. It seems to operate correctly on cold start but after engine is hot it seems to have a mind of its own...does not respond to engine speed as far as I can tell. Yes, I understand I can upgrade to a newer improved design but I am keeping this truck original. This heat riser is still readily available and does not cost much so I will probably just buy a new one and install it...just a PITA to install considering the studs that always get very rusty.
Vile, considering this whole thread has really been about cold starting problem (and not fan clutch really) do you want to change the subject line of it and possibly move it to a different category of the forum?
Regards,
Henry
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You just reminded me of that can of heat riser lubricant we used to douse those things with. ;D
If it's stuck closed, your choke coil would heat up faster not slower. You do know you can convert to electric choke right? I mean if you are trying to preserve 100% authenticity, I can see messing around for weeks with this otherwise, time for an upgrade.
Agreed but open or closed, his coil is running abnormally cool if you look at his readings at OT.
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More than likely the crossover passages in the cylinder heads are coked. Another possibility is that the butterfly is broken loose and twisting around its shaft or missing completely from the heat riser valve. That was not uncommon in those days.
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Agreed, changing out the heat riser is a PITA, buy a stud replacement kit for both sides. Less profanity... ::)
Use anti-seize on everything.
If that doesn't do it, time to lift the intake and clear the heads and manifold of 'coke' per bd. Keep a shop vac handy... ;)
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Hi Guys:
Yeah, I am going to replace my heat riser valve and take off my intake manifold to see if it is coked up. Both activities will probably not occur until this spring or summer because they are big jobs (for me) and are messy with a lot of drama (rusty studs). I am in a part of the country where temps get above 60F in March/April so the cold start problem goes away fairly soon.
I feel confident the heat riser is not working properly because I think once it is heated up, its thermostatic coil spring is supposed to have greater spring force than its return spring and hence hold it in the open position. This only happens at warm up and after driving around the thermo coil spring seems to weaken under greater heat soaking and the return spring has more force which shuts the valve or at least balances the coil spring and so the valve just opens to a position of whatever engine vibration dictates. I am confident the butterfly valve is intact and in one piece because when I installed a new exhaust system about 1 year ago it was still there and in good shape. So although the heat riser needs to be replaced, I dont think it is a factor in my cold start problem.
Intake manifold: Having the exh passage ways coked up or closed by the wrong intake manifold gaskets is the most likely cause to my cold start problem.
Carb choke linkages: I am still waiting to get a scrap 4MV carb so I can study the linkage action with cams...so once I understand exactly how the cams and high idle steps are supposed to work I can determine if maybe I have a subtle adjustment or something wrong on mine that also may be necessary to address for cold starting.
When I get my intake manifold off I will also note if I see any oil sludge formation which may indicate if my engine runs too cold.
I will keep you posted.
Thanks,
Henry
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Good info!
In the meantime, (if you don't have 'observation fatigue') you can measure the temp of the exhaust pipe beyond the heat riser vs the driver side until you see it open. Should be noticeable if the riser is working properly.
I wouldn't give too much energy to the linkage et al, seems like it's a heat issue, as in, 'not getting enough' heat.
Having said that, you can't go wrong deepening your understanding... ;)
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Hi all:
I have not taken my intake manifold off yet, but in the mean time I decided to buy a new thermostat coil so I could determine if this may be a contributor to the problem. I made a test plate to mount my old thermostat coil next to the new coil and see how they compare when being heated. At room temperature the new coil end is just a hair above the old coil end...so I did not think this was significant. When both were chilled to 40F in the fridge, the coil ends were at the same position. Then, from this cold soak I immediately started heating my test plate to the 190F temp range. The rate the coils expanded were just about the same and at the final temp of about 190F, the coil ends were very close to the same position and had the same amount of resistance to finger pressure. The coil ends were about 1-1/16" above the mounting surface of the thermostat coil support. So I am going to say that my old coil is good. I have an even older coil that I replaced many years ago thinking it needed replacement (probably during a carb rebuild), and so I tested this coil with the new coil and it tested the same as well...so it is also OK. See photos.
I also decided to buy a new exhaust heat riser (about $62) and another used OEM intake manifold from eBay($60) so I can restore it and just replace my old one to reduce the vehicle down time. I have my scrap Qjet carb now (eBay $40) which actually looks rebuildable so I will clean it up and study the linkages and have it as a back-up carb. This is turning into an expensive endeavor but I would probably just spend the money on beer anyway.
I will measure exh temps on left and right side before I change out the heat riser. I am slowly eliminating all the possible problems and it appears a coked up intake is still the most likely culprit. Will keep you posted.
Regards,
Henry
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Hi Guys:
So yesterday I measured the exhaust pipe temperatures just aft of the heat riser on passenger (right) side and the exh manifold on the driver (left) side. I have OEM style single exhaust pipe so the downpipes from the exh manifolds meet together to a single pipe. The purpose of this exercise was to see if the thermostatic heat riser position had any effect on the exhaust temps...and also to just observe how my heat riser was behaving.
-Cold start at 56F ambient air temp, right exh=52F, left exh=49F
-1 minute after start right exh=420F, left exh=469F, high idle 1600rpm, riser closed
-2 minute after start right exh=482F, left exh=545F, high idle 1600rpm increasing, riser closed
-4 minute after start right exh=519F, left exh=595F, high idle 1600rpm increasing, riser closed
-6 minute after start right exh=539F, left exh=655F, high idle 2287rpm, riser closed
-8 minute after start right exh=580F, left exh=668F, high idle 2378rpm, riser closed, riser temp 455F
-10 minute after start right exh=635F, left exh=750F, high idle 2350rpm, riser opening, riser temp 485F
-13 minute after start right exh=673F, left exh=750F, high idle 2290rpm, riser 1/2 open, riser temp 530F
-15 minute after start right exh=680F, left exh=741F, high idle 2290rpm, riser 3/4 open, riser temp 567F
-18 minute after start right exh=606F, left exh=702F, kick down to low idle 860rpm, riser full open, riser temp 543F
-21 minute after start turn off engine
-16 minutes after turn off, riser has moved to 3/4 open and I start truck and drive off.
-12 minutes of suburban driving I stop and heat riser is closed. I turn off truck.
-10 minutes later I start truck and drive off.
-10 more minutes of suburban driving I stop and heat riser is 2/3 open. I turn off truck. Heat riser can be moved with a stick to any position and stays where positioned.
So after this test I would conclude that my heat riser is malfunctioning. I works during cold start but when the engine is hot I would expect it to stay full open with thermostatic spring pressure and spring back to open when moved manually. My records show I bought this riser in 2004 from that well known Chevy truck parts company in Kansas for about $45. I have only put 26K miles on it in all these years. It is most likely not Delco brand and is aftermarket manufacture and probably this is all of the life I can expect out of it.
Next exercises is to study carb linkage and replace intake manifold...will keep you posted.
Regards,
Henry
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Good stuff Henry!