1940 Pontiac flat 6 overheating

[Gerty the Gremlin]

So, the other day I was driving my 40 Pontiac with the straight 6 in it, when I lost the belt and didn’t realize it until the temp gauge was pegged at 220.  I immediately stopped and check the motor.  I could hear the coolant boiling and there was an excess of blow by.  I put the belt back on and still let it sit for a while until the boiling stopped.  I started the car and all of the symptoms went away except…. The motor usually averaged a temperature of 180-185 but it was running at 200 since.  I decided to change the oil, and replace the thermostat cause I really didn’t know how hot it got.  
 

well, today I changed the oil and replaced the thermostat with a new 180 degree one.  Now, when I start the car it immediately begins to overheat, even at an idle.  Any thoughts?

[avgwarhawk]

I would say the boiling inside the water jackets may have knocked loose scale and clogged the radiator.  Did you confirm flow with the new thermostat?  Try to back flush the radiator.    

Edited March 29, 2023 by avgwarhawk (see edit history)

[Bloo]

Since it happened right after a boilover, I suspect you have a blown head gasket. There are kits to check for abnormal C02 in the radiator, indicating that combustion gases are leaking into the cooling system. If you are in the US you can get one of these on the "loaner" program at Oreilly or AutoZone. You "buy" the tool for some nominal amount, and when you bring it back in a couple of days they give you your money back. These use a blue fluid for CO2 detection and the bottle of blue fluid is the only part you must buy outright.

 

This test is not perfect, but it is real good. If it passes (fluid stays blue), combustion leakage is almost certainly not the problem, although the tool can miss once in a great while. If it fails (fluid turns yellow) that is 100% a failure and there are combustion gases in the radiator. Most of the time, probably 95% of the time, that means a blown head gasket. There are some other ways combustion gases can get in the radiator, like cracked blocks, porous heads, etc., but those are far less common than head gasket failure.

 

You should know the most common cause of overheating in a Pontiac Six is the water distribution tube. The same one fits from 1937-1954 sixes, with 1935-36 being similar but not exact. The originals were galvanized steel and rust out. When the tube has some leaks, water can circulate without ever getting to the back of the engine. Persistent boilover is the result. When the tube fails completely, the car becomes undriveable due to fast boiling. If you need one, California Pontiac Restoration has galvanized ones like the originals. Brass tubes exist and are desirable because they won't fail, but aren't made anymore. They do turn up on ebay occasionally.

 

On many models of Pontiac it is possible to remove the radiator, water pump, and part of the grille to get the old water tube out through the front. That was not possible on my 36 though. Read about my misadventures here, and there's pictures of a 1937-1954 aftermarket brass water tube, too:

 

[Bloo]

One more thing, your new thermostat should have a tiny bleed hole in it. A rattle valve will also work. If it has neither, you might have an air lock not allowing the thermostat to open.

[F&J]

2 minutes ago, Bloo said:

One more thing, your new thermostat should have a tiny bleed hole in it. A rattle valve will also work. If it has neither, you might have an air lock not allowing the thermostat to open.

i am glad somebody mentioned air trapped under t-stat....before chasing all sorts of things. 

 

start there first, which should cure the "instant" overheat, but then see if it still runs hotter than it did before the belt incident.  If it does still run hot but not boiling, i would not yet condemn the head gasket.  Do what was said in post2 about visual check for water flow with cap off, and perhaps back flush radiator, or first feel for cold vs hot spots on face of rad core.

[Rusty_OToole]

Flat HEAD six, flat HEAD six. A flat six is a horizontally opposed six cylinder engine as used by Porsche, Subaru, Corvair, Tucker and some airplanes. A flathead six, is a six cylinder inline engine with the valves in the block beside the cylinders.

Now that we have cleared that up on to your problem. Some good suggestions so far. I will elaborate on the test for the corroded water distribution tube. DO NOT TAKE YOUR ENGINE APART. You don't need to, in order to test it.

 

Use a hand held infrared thermometer for this test. With the engine running, check the temperature at the front of the engine and at the back. They should be the same. If the front is cold and the back is hot, that means the tube is shot and all the cooling water is streaming up the front of the engine, and none is reaching the back. Only if the tube is bad, do you need to fish out the old one and put in a new one.

 

Incidentally you can use the thermometer to test the rest of the cooling system, such as the radiator. If the rad has hot and cold spots it is a sign that it is plugged up and coolant is not passing thru certain areas.

[Gerty the Gremlin]

So, first I did the hard part first and flushed the entire cooling system and got some minor rust out of it.  After that I filled with water to confirm that the issue is still there or gone.  I used my thermal gun to check the motor all over and the temps are very close to one another at around 170-180.  With that being said JUST around the temp sensor it is warm around 200.  I’m probably gonna flush it one more time and check my wiring to make sure the gauge still has a good ground.

[EmTee]

Does this car have a pressurized cooling system?  If so, it would be worthwhile to do a pressure test.  See whether the system will hold 6 ~ 8 lbs. without bleeding down.  How do the spark plugs look?  Coolant leaking into a cylinder will usually cause the plug ceramic to look much cleaner than the others.

[Gerty the Gremlin]

Just now, EmTee said:

Does this car have a pressurized cooling system?  If so, it would be worthwhile to do a pressure test.  See whether the system will hold 6 ~ 8 lbs. without bleeding down.  How do the spark plugs look?  Coolant leaking into a cylinder will usually cause the plug ceramic to look much cleaner than the others.

I’ve determined that the cooling issue is within the gauge.  I’ve checked the entire motor with a digital thermometer and all is running cool and consistent.  I just have to figure out where the issue is with the electrical part of the temp gauge

[EmTee]

Sounds like there could be air trapped at the sending unit.  With the engine running, try loosening the sending unit until coolant comes out.  Then tighten it up again.  Process is similar to bleeding brakes.