Author Topic: 1976 C20 Choke Thermostat and Cold Start Drivability  (Read 1736 times)

Offline Henry

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1976 C20 Choke Thermostat and Cold Start Drivability
« on: January 25, 2022, 11:40:50 AM »
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

« Last Edit: February 24, 2022, 04:33:53 PM by VileZambonie »

Offline JohnnyPopper

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Re: Engine fan clutch-is it working properly?
« Reply #1 on: January 25, 2022, 01:11:35 PM »
The only thing to keep you at high idle is your choke/assembly.

What do mean by "divorced choke coil"?
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline bd

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Re: Engine fan clutch-is it working properly?
« Reply #2 on: January 25, 2022, 02:08:43 PM »
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
« Last Edit: January 26, 2022, 10:37:07 AM by bd »
Rich
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In other words... if people learn by making mistakes, by now I should know just about everything!!!
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Offline VileZambonie

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Re: Engine fan clutch-is it working properly?
« Reply #3 on: January 25, 2022, 06:24:05 PM »
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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74 GMC, 75 K5, 84 GMC, 85 K20, 86 k20, 79 K10

Offline Rapid Roy

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Re: Engine fan clutch-is it working properly?
« Reply #4 on: January 25, 2022, 08:10:32 PM »
I have had mine since July 2015 and divorced choke still works great!
1974 Cheyenne 10 LWB STOCK 350 W HEI /TH350/AC/4 BBL Quadrajet
Mopar by Birth
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Offline JohnnyPopper

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Re: Engine fan clutch-is it working properly?
« Reply #5 on: January 25, 2022, 11:17:35 PM »
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. 

1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline Henry

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Re: Engine fan clutch-is it working properly?
« Reply #6 on: January 26, 2022, 10:19:36 AM »
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

Offline Henry

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Re: Engine fan clutch-is it working properly?
« Reply #7 on: January 28, 2022, 12:48:57 PM »
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

Offline JohnnyPopper

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Re: Engine fan clutch-is it working properly?
« Reply #8 on: January 29, 2022, 02:10:41 PM »
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.
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline Henry

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Re: Engine fan clutch-is it working properly?
« Reply #9 on: January 30, 2022, 11:01:05 AM »
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

Offline JohnnyPopper

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Re: Engine fan clutch-is it working properly?
« Reply #10 on: January 30, 2022, 02:17:57 PM »
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. 
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline Henry

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Re: Engine fan clutch-is it working properly?
« Reply #11 on: January 31, 2022, 12:49:39 PM »
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


Offline JohnnyPopper

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Re: Engine fan clutch-is it working properly?
« Reply #12 on: January 31, 2022, 02:17:53 PM »
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.
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline 1980K20

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Re: Engine fan clutch-is it working properly?
« Reply #13 on: January 31, 2022, 11:39:08 PM »
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.

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Offline Henry

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Re: Engine fan clutch-is it working properly?
« Reply #14 on: February 13, 2022, 01:13:08 PM »
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