Author Topic: 1976 C20 Choke Thermostat and Cold Start Drivability  (Read 4311 times)

Offline VileZambonie

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Re: Engine fan clutch-is it working properly?
« Reply #15 on: February 13, 2022, 01:41:56 PM »
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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74 GMC, 75 K5, 84 GMC, 85 K20, 86 k20, 79 K10

Offline JohnnyPopper

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Re: Engine fan clutch-is it working properly?
« Reply #16 on: February 13, 2022, 02:16:22 PM »
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.
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 #17 on: February 13, 2022, 11:30:04 PM »
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!

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1952 International L112 - waiting for EV conversion
1967 Camaro - 250 six cyl, 200-4R
1980 K20 4x4 Fleetside - 350 SBC roller Cam, TH350

Offline Henry

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Re: Engine fan clutch-is it working properly?
« Reply #18 on: February 14, 2022, 12:49:00 PM »
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

Offline JohnnyPopper

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Re: Engine fan clutch-is it working properly?
« Reply #19 on: February 14, 2022, 01:12:24 PM »
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.
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 #20 on: February 14, 2022, 01:59:42 PM »
Use AC Delco 10-4020 Penetrating Lubricant on the heat riser valve shaft.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Geezer77GMC

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Re: Engine fan clutch-is it working properly?
« Reply #21 on: February 14, 2022, 02:12:57 PM »
Is the Vacuum break diaphram functioning?
Check link: https://www.thirdgen.org/forums/carburetors/634914-quadrajet-manual-pdf.html
« Last Edit: February 14, 2022, 02:18:35 PM by Geezer77GMC »

Offline Henry

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Re: Engine fan clutch-is it working properly?
« Reply #22 on: February 15, 2022, 11:14:41 AM »
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

Offline Geezer77GMC

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Re: Engine fan clutch-is it working properly?
« Reply #23 on: February 15, 2022, 06:55:03 PM »
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.

Offline JohnnyPopper

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Re: Engine fan clutch-is it working properly?
« Reply #24 on: February 17, 2022, 11:10:47 AM »
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.
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 #25 on: February 20, 2022, 04:03:49 PM »
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

Offline JohnnyPopper

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Re: Engine fan clutch-is it working properly?
« Reply #26 on: February 21, 2022, 02:23:38 PM »
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.
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 #27 on: February 23, 2022, 01:43:25 PM »
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

Offline JohnnyPopper

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Re: Engine fan clutch-is it working properly?
« Reply #28 on: February 23, 2022, 03:33:21 PM »
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.
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 VileZambonie

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Re: Engine fan clutch-is it working properly?
« Reply #29 on: February 23, 2022, 05:42:11 PM »
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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              ⌠ŻŻŻŻŻ'   [☼===☼]
              `()_);-;()_)--o--)_)

74 GMC, 75 K5, 84 GMC, 85 K20, 86 k20, 79 K10