Cooling System Pressure Problems


Asked by Aug 02, 2015 at 08:32 PM about the 1985 Jeep CJ7

Question type: Maintenance & Repair

All pertains to a 1985 Jeep CJ7 2.5L 4clylinder:

About a month ago 2 leaks broke out in my cooling system. One around the water
pump the other was a freezer plug. I replaced the water pump and replaced the leaking
freezer plug(which literally had a small whole blown through it) along with 2 other
freezer plugs near by. I then put the system back together and have been having
problems ever since.

I bled the cooling system to remove the air and now new leaks just keep springing up.
First the thermostat housing, which I replaced along with the thermostat. After this
leak I ended up getting a radiator cap pressure tester which I put on the system. I
pump it up to 16psi, the radiator cap is a 16 pound one, and no leaks. I then run the
car for 10 minutes and the cap pops up under the pressure allowing fluid to volcano
out, instead of releasing water through the overflow tube. I believe this is due to way
too much pressure. I then put the radiator cap pressure tester on the car and let the
car run. The system built up to 20psi in about 5 mins and I shut down the car and
released the pressure. Didn't want any hoses popping or anything else in the system.

I though maybe the new thermostat wasn't opening causing the pressure to build up,
so I pulled it out and tested it. The thermostat worked fine. I ran water through the
radiator, thinking it might be clogged, water runs through fine. So all the parts of the
system seem to be working.

I then suspected a problem with the head gasket so I preformed a compression test on
each of the cylinders. Each cylinder built up to 140psi after the engine turned over 4
times. No liquid was evident in any of the spark plugs. It doesn't seem like any liquid
is in the oil or oil is in the coolant fluid.

So I'm running out of ideas any other ways the pressure could be getting this high?

3 Answers


When the coolant heats up it expands and the pressure in the cooling system increases. With a 16 PSI cap and a 50/50 mixture of antifreeze and water the cap shouldn't "pop" until about 260°F. Because you've had 2 freeze plugs rust through and the thermostat housing spring a leak I'd suspect that you have a lot of corrosion and "junk" in the cooling system. I also suspect this due to the age of the vehicle. Is the engine running hot or flat out overheating? The fact that the needle on the pressure tester didn't bounce when the engine was started rules out a head gasket failure as does your compression test results. My thought is you have a localized overheating problem that's not being picked up by the temp gauge or light sending unit because it's in another part of the engine's cooling system. In this case I'd recommend a good chemical flush BUT be prepared for something else to spring a leak as a result. I'm thinking that some of the smaller passages in the cooling system are blocked by rust and corrosion. A chemical cleaner such as Prestone Super Flush should clear it out. I've used that product with success in the past. But as I said, be prepared. Something else is bound to start leaking. HTH. -Jim

1 of 1 people found this helpful.

Unfortunately the temperature gauge has gone out on the vehicle and I have been unable to fix it. That's another issue though. I ordered a cheap IR temp gauge gun the other day that will be here tomorrow, should help me measure what's going on. I've Flushed the system with chemical flush 3 or 4 times already. The water coming out of the flush is still dirty though, so I may need to flush some more. Thanks for the advice

1 of 1 people found this helpful.

You're welcome. Glad to help. -Jim

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