Bob Shafto

Coolant Problem

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I'm losing coolant in my '36 Pontiac flathead six. There are no leaks on the garage floor. The thermostat isn't stuck. There is no water in the oil. The exhaust shows no sign of steam. The head gasket is intact. The engine runs beautifully with no misses, backfires, lugging, or other signs of mis-functioning.

 

If I fill the radiator half full, it will run at 160-180 on a very warm day for 30 minutes or so; then coolant will be forced out of the radiator overflow tube.  (It's pressurized to 3-4 lbs)

 

I took the head off. All the valves have a nice white powdery look, but one plug is black (the others white/light brown). That cylinder (#5), also shows some black deposit on the combustion chamber. 

 

These cars do have a water distribution tube, but it was replaced 25 yrs ago when the engine was rebuilt. With modern coolant rust inhibitors, it should still be intact.

 

Any thoughts on what the problem is would be much appreciated.

 

Bob Shafto

Member #800-555‬

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5 hours ago, Bob Shafto said:

If I fill the radiator half full, it will run at 160-180 on a very warm day for 30 minutes or so; then coolant will be forced out of the radiator overflow tube.  (It's pressurized to 3-4 lbs)

What happens when you fill the radiator to full.  Low coolant will cause the coolant to boil and percolate out just like you describe.

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21 minutes ago, Spinneyhill said:

Well, fill to just covering the top of the core anyway...

 

Thats not how it works on these. The radiator is crossflow. There is a petcock so you can see if it is full enough..

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Posted (edited)
7 hours ago, Bob Shafto said:

If I fill the radiator half full, it will run at 160-180 on a very warm day for 30 minutes or so; then coolant will be forced out of the radiator overflow tube.  (It's pressurized to 3-4 lbs)

 

Greetings from another 36 Pontiac owner.

 

There is a pressure cap?

 

Try loosening it to the first click and see if if it does better. If it does, there is probably combustion pressure getting into the cooling system. There are CO test kits (they have a plastic thing that goes in the radiator neck, and some blue liquid that turns yellow in the presence of CO) that could be used to confirm. In my area, Oreilly have these in their rental program.

 

I used one of those infra-red thermometer guns to check to see if my water tube was ok. There was no huge difference in temperature between the front and rear, so I guess it is.

 

 

Edited by Bloo (see edit history)

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I changed out the neck of the radiator years ago and fit a 3-4 lb. cap to it. It's always worked very well.  As you say, these radiators are not intended to be filled. I do believe combustion pressure is getting into the cooling system, but can't figure out how/why that is happening. Any possibilities come to mind?

Thanks.

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I guess I would be examining the head and block extremely close for porosity or cracks. I can't think of anything else (except the headgasket).

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5 hours ago, Bob Shafto said:

I do believe combustion pressure is getting into the cooling system

Compression test next?

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I agree that the testing kits that use a reactive fluid to test for the presence of CO2 in the radiator are invaluable for figuring out if your head gasket/head is intact. It involves a pressure bulb like a turkey baster, some blue fluid that reacts in the presence of CO2 (it's sensitive enough that if you breathe on it, it changes color), and a seal that fits on your radiator neck. Drain the coolant far enough to keep it from being sucked into the test cylinder, start the engine, secure the test apparatus to the radiator neck, and use the bulb to pull air from inside the radiator through the test fluid. It will change color if there's exhaust leaking into the cooling system. I have a setup that I got from a tool truck years ago and it has saved me from full tear-downs several times. Problems I thought I had, turns out I didn't. 


Before you tear things apart, let's make sure we know where the problem is. I'm betting you have a head gasket problem, but being able to test before disassembly might just save you a lot of time.

 

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Posted (edited)

If it turns yellow, thats conclusive. If it doesn't, there is probably no combustion leak, but it can miss occasionally.

 

You can also wave the probe of an exhaust analyzer over the radiator neck with the engine hot, if you have access to one, but not many people do. Generally you will see HC first, but HC can be there for other reasons. CO is conclusive.

 

EDIT: Oops, it is CO2 the blue fluid looks for.

Edited by Bloo (see edit history)
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Bloo- Is 100 percent correct. 

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You can test the fluid by exhaling over the opening of the tester while squeezing and releasing the rubber bulb several times. This way you will see the yellow color the fluid will turn. Then repeat squeezing the bulb several times in fresh air and the fluid will return to blue. 

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