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

Offline Henry

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Re: Engine fan clutch-is it working properly?
« Reply #30 on: February 24, 2022, 10:27:05 AM »
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

Offline JohnnyPopper

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Re: Engine fan clutch-is it working properly?
« Reply #31 on: February 24, 2022, 07:42:59 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.

Agreed but open or closed, his coil is running abnormally cool if you look at his readings at OT.
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: 1976 C20 Choke Thermostat and Cold Start Drivability
« Reply #32 on: February 24, 2022, 08:22:23 PM »
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.
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 JohnnyPopper

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Re: 1976 C20 Choke Thermostat and Cold Start Drivability
« Reply #33 on: February 24, 2022, 09:11:29 PM »
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... ;)
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: 1976 C20 Choke Thermostat and Cold Start Drivability
« Reply #34 on: February 25, 2022, 03:39:31 PM »
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

Offline JohnnyPopper

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Re: 1976 C20 Choke Thermostat and Cold Start Drivability
« Reply #35 on: February 25, 2022, 07:57:24 PM »
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... ;)
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: 1976 C20 Choke Thermostat and Cold Start Drivability
« Reply #36 on: March 11, 2022, 03:41:54 PM »
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

Offline Henry

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Re: 1976 C20 Choke Thermostat and Cold Start Drivability
« Reply #37 on: March 13, 2022, 12:40:50 PM »
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

Offline JohnnyPopper

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Re: 1976 C20 Choke Thermostat and Cold Start Drivability
« Reply #38 on: March 13, 2022, 01:53:30 PM »
Good stuff Henry!
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