The Stovebolt Site Forums The Parking Lot Big Bolts 1939 VB 1.5 ton truck 165 Deg. Thermostat...

After replacing the 180 deg. thermostat with a 165 unit, the truck runs exactly the same... Somewhere around 185 deg. right below the red mark on the gauge... I would have expected it to run about 170-175 degrees, so I am a little surprised... Is there anything I can do to make the truck run cooler? The radiator has just been cleaned out/overhauled... Thank you.

The impeller could be rusted badly in the water pump so that it is not effective at moving coolant. I think there is a coolant distribution tube inside the block that takes water to the rear of the engine. If so, it could be corroded or missing. Anyone know for sure if there is a tube in that engine?

Interesting point, I thought about the water pump impeler being corroded... Darn, they're so expensive! I don't know anything about the internal coolant tube, I have never of it before and cannot find anything in the 38 shop manual I have...

The 1938 Shop Manual refers to a full length water jacket (read the whole chapter):

There is no 1939 Shop Manual online. If you have one, check that Chapter.
A copy of the 1939 Shop Manual is readily available to buy.

The 1941 Shop Manual describes a cooling system very similar to 1938.

I guess I’m a bit thick headed but I don’t understand what is wrong with 180* engine temps? An engine when warmed up is going to operate at what ever temp it wants to run at regardless of what thermostat is installed. Take the radiator cap off and start the engine and let it warm up. If you see water moving in the radiator your pump is just fine. Non existent problem. IMHO

"An engine when warmed up is going to operate at what ever temp it wants to run at regardless of what thermostat is installed."?

What temperature will it run at with a :
160 degree thermostat
175 degree thermostat
90 degree thermostat

A more interesting question might be what temperature will it run at with no thermostat when:
- idling at 0 degree external temperature
- idling at 30 degree external temperature
- idling at 70 degree external temperature
- idling at 90 degree external temperature
or
- driving (less than 25 MPH or more than 55 MPH) at 0 degree external temperature
- driving (less than 25 MPH or more than 55 MPH) at 30 degree external temperature
- driving (less than 25 MPH or more than 55 MPH) at 70 degree external temperature
- driving (less than 25 MPH or more than 55 MPH) at 90 degree external temperature

Answer: "at what ever temp it wants to run at"?

Are the answers good reasons for using a thermostat, and running the engine at the GM recommended temperature?

When, say a 165* thermostat, reaches operating temp (165) it opens. However the engine water still continues to heat until the engine (not the thermostat) reaches its operating temperature. It is air/water flow that determines how warm the engine gets. Many folks in the warmer regions would take the thermostat completely out and the engine would operate just fine as long as there was air flow and water circulation. Yes the thermostat will have an effect on engine temps but unless it’s defective and won’t open it shouldn’t be the cause of a overheating situation. The main point to my post was the OP’s concern over 185 degree operating temp being too hot. Unless it continues to climb, 185* will cause no harm. Note; I’m not a scientist so I cannot provide the answers to Tim’s questions just real world experiences.

I am wondering what the heat gauge looks like in a 1939 truck. My 1951 Canadian GMC has nice large gauges and I can easily see where the temperature is sitting. Recently I rebuilt the motor and had the block bored and tanked. It now runs a little cooler. My 1938 Chev Master has a 216 and the temperature gauge is tiny. At the high end of the gauge the line has a small red segment. I would guess that it isn't even 1/8" in length. So it took some getting used to because when the needle moved into the red, I automatically thought that I was in trouble! I wonder if the 39 truck has a gauge like that.

My questions were not meant to be answered. They were hypothetical.
They were in response to the statement made in quotes.

"An engine when warmed up is going to operate at what ever temp it wants to run at regardless of what thermostat is installed."?

In general, for an unclogged/healthy block, that statement is incorrect.

Hello truckernix,

I have the same gauges as your 38 Chevy master... Yes, the needle is just a hair below the red and sometimes moves over ever so slightly at idle... Is your master temperature gauge around the red at idle after sitting for a few minutes or just below it while you drive? I will check the coolant temp with a meat thermometer after my next run...

Tclederman has a point, any engine will run as hot as it wants regardless of the thermostat. It is all bout water and air circulation but all engines are different. My (43 Jeep) flat head 4 cyl. with a 160 deg. thermostat never gets above 165-170 deg. of normal driving... After a hard run or in a very hot day it will be in the 175 deg. range but rarely... Another GM product I have (38 Century) with a 160 deg. thermostat runs about the same as my 216 (180-195 deg.) what I consider hot with a lower rating thermostat...So maybe these motors like the 185-195 running temperatures regardless...I really don't know the factory recommended running temps. for the 216 engines...

I forgot a detail about my Master heat gauge. According to the manual, the start of the tiny red line is 180 and the end of it is 212! So with my Master, it usually registers a little over the center mark which is supposed to be 160, and my thermostat is a 160. However a long period of idling will take it up to near the start of the red line. I have rebuilt the motor with the block being tanked, I have a new pressurized rad and I have installed a truck fan. The special fan and the new rad was to try and relieve overheating problems. I get very uncomfortable sitting in traffic watching the heat gauge going up! The sedans are known to run on the hot side. I would assume that the truck would have a much larger radiator. I know my 1951 GMC has a very substantial rad and it is a 1 Ton.

180 degrees is not a problem. With a pressurized radiator, 15 lb cap, and 50-50 antifreeze, it will not boil til over 250 degrees. That is not great for the engine, 250F, but it will not boil. If you have a lower pressure cap, the max temp for boiling will be lower.

Modern engines have better valves in them than the old stuff and the exhaust valve is the first thing to overheat in any engine. If you work it hard, the exhaust valve will be much hotter than anything else in the engine and 20 degrees of coolant temp rise will be something like 200 degrees of valve temp rise.

I don't know about phiilp's '38 VB, but my '35 VB is NOT pressurized at all. The 2000' climb from town to home (3000') over about 6 miles works the cooling system pretty well. Right now in the summer, the large and easily read gauge won't go over 165 during that climb. While 180 isn't dangerously hot, it is indicative of a problem. I totally disagree that an engine connected to a properly functioning cooling system will runs at what temp it wants. The only reason to have a thermostat is to have the engine run where you want it to.

A couple of questions; does your truck have a heater? If not, how is the heater supply/return plumbed? Some folks will simply put a piece of hose from supply to return. This allows some coolant to bypass the radiator, and that doesn't help. That circuit should be blocked off, not free flowing if you have no heater. Having a heater core in that circuit actually provides more cooling in operation, and doesn't subtract from cooling efficiency when it's not. That's not true of a straight piece of hose.

Are you running an antifreeze mixture or straight water? While antifreeze wasn't in common use until after WWII, is really is a good thing to use, as it does allow for better cooling and inhibit corrosion. If you run antifreeze, be mindful of overflow, it's sweet taste will have animals ingesting it, and it is poison to them. You can put an overflow catch bag on an unpressurized system, but you have to mount it on the firewall at the same height as the radiator coolant level if you do.

Running with no thermostat BTW, CAN make an engine run hotter. The coolant can spend so little time in the radiator that insufficient heat is transferred, resulting in a loss of efficiency.