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#1408074 Fri Apr 30 2021 05:34 PM
Joined: Mar 2019
Posts: 119
Based on what I can read and have observed the cooling system for a '49 3100 was not pressurized from the factory. However, in the operator's manual there is a note that the design of the radiator neck will allow the owner to install a pressure cap. I agree that the design of the radiator neck and the location of the vent tube will work with a pressure cap. As a side note I did see that the 5100 series used a 3 1/2 to 4 1/2 psi cap.

I am interested in your experiences and thoughts about this. Do you get any better cooling performance with a slight pressure in the system? If a pressure cap is used I expect it should be very low pressure (maybe 3 or 4 psi). My understanding is that the primary reason to pressurize a system is to increase the boiling point of the coolant.

Right now our '49 3100 tends to run a little hot (about 190 or so) with a 180 thermostat. It goes to this temperature when you idle right after driving for 10 or 15 minutes. When you start driving again the temperature drops back to about 180. Our ambient temperatures are only between 60 and 70 right now. I expect things will get worse as we get into 80 and 90 degree days this summer.

My thinking is that we have some gunk/crud in the block and head that hopefully we can flush out (this weekend's project). The engine was rebuilt at least 15 years ago so it sat exposed to the atmosphere for quite a while. We did have the radiator cleaned when we assembled the truck. I do not think that adding a pressure cap will address our concern.

Thanks for the ideas.

Joined: Sep 2001
Posts: 31,731
Bubba - Curmudgeon
try a 140-150 degree thermostat? []
Radiator cap "fully open at 170 degrees" 4.5 to 5.5 psi (1949)

Tim []
1954 3106 Carryall Suburban [] - part of the family for 49 years
1954 3104 5-window pickup w/Hydra-Matic [] - part of the family for 15 years
- If you have to stomp on your foot-pedal starter, either you, or your starter, or your engine, has a problem.
- The 216 and early 235 engines are not "splash oilers" - this is a splash oiler. []
Joined: Jun 2004
Posts: 8,619
If the system is in good condition you can run a 4 lb. cap. Pressure does increase the cooling capacity of the system.

They say money can't buy happiness. It can buy old Chevy trucks though. Same thing.

1972 Chevy c10 Cheyenne Super
Joined: Mar 2019
Posts: 119
We definitely do need a thermostat. When we ran it without one we barely could get to 140. The engine did not run very well.

A lower temperature one might be the solution.

Thank you.

Joined: Mar 2014
Posts: 3,205
I use a 160 degree thermostat and have the fan (electric) adjusted to keep the engine temp at 185~190. Seems to do very well in that range. The 235 has a nasty habit of building up sludge at the back of number 6 cylinder. There is a lot of slope to the engine and stuff naturally accumulates there. Hopefully this was cleaned out when your engine was rebuilt. This can make the rear of the block heat up and the temp sensor is back in that area, so it sort of skews things a bit. I also use a 3 row aluminum radiator and a 7 pound pressure cap. That and the electric fan made a big difference, but Davenport may be slightly cooler than DFW. We have summer about 300 days per year it seems.


1952 1/2 ton with 1959 235
T5 with 3.07 rear end
Joined: Mar 2010
Posts: 10,392
Check your timing with a timing light. Too far advanced will cause the kind of increase in temperature which you are experiencing. It will also cause poor fuel economy and reduced horse power.
Checking the timing is much easier than cleaning the water jacket while the engine is in the truck.

Joined: Mar 2019
Posts: 119
Great additional information.

I agree that the water jacket in these stovebolt blocks can get pretty cruded up. I cannot believe how much gunk I flushed out of the block in my ‘37 Master coupe. We can only assume they cleaned the truck block decently when it was rebuilt. Except I am leery of assumptions like that.

In fact the cooling system performance of that car is our baseline. That 216 has a stock radiator, 180 degree thermostat, and 13 degrees advance. Even on 90 degree days I never see over 190 when idling at a light. Most of the time it barely runs at 180. I expect the thermostat is a little off and is opening early. The temperature gauge in the car agrees with my infrared temperature gun.

We’ll see what we learn this weekend.

Joined: Apr 2020
Posts: 177
I would also check the accuracy of your gauge

Karl j Townsend
Joined: Apr 2009
Posts: 572
Originally Posted by TBUChevy
Great additional information.

I agree that the water jacket in these stovebolt blocks can get pretty cruded up. I cannot believe how much gunk I flushed out of the block in my ‘37 Master coupe. We can only assume they cleaned the truck block decently when it was rebuilt. Except I am leery of assumptions like that.

Once upon a time the chemicals used when "hot tanking" a block might have removed the junk in the water jackets. For a few decades, at least in California, all it really does is de-grease the blocks. I spent an inordinate amount of time poking around with a stiff wire at the back of my block clearing out the very hard black chunks of gunk. I think the shortcut of adding lots of radiator stop leak rather than fixing radiators contribute to this problem

Joined: Mar 2019
Posts: 119
Many thanks to all for the inputs and suggestions. We do have the situation under control. I like to try and remember to close my threads with a resolution post.

The primary change we made was to install a 160 degree thermostat. The gauge now indicates about 160 when driving and will go to just below 180 when you stop after driving for awhile. It drops immediately when you start driving again or speed up the engine to get a little more air and water going through the radiator. We are satisfied that this is acceptable operation for our use.

As we did our troubleshooting we did determine that the block and radiator are very clean and flow water very well. Using an IR heat gun I do think the gauge is reading maybe about 10+ degrees high.

The one variable left is the condition of the impeller on the water pump. We do not know what was done to the pump when the engine was rebuilt. My thinking is that if we can easily find rebuild kit we will do that just to eliminate another unknown.

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