Overheating

TOM NELSON

MEMBER
2003 SUBARU IMPREZA
2.0L
4 CYL
4WD
AUTOMATIC
106,000 MILES

Engine is overheating. They replace the thermostat, it is fine for about a month, then it starts overheating again. It is also leaking small amounts of coolant when parked, but not enough to lower the reserve level. The temperature suddenly jumps up to the red limit, then quickly goes back down. Coolant can be heard boiling in the radiator.

The thermostat has been replaced four times now and I'm starting to think it's not the thermostat, but something they're doing incidentally. But what?

9 Replies

JACOBANDNICKOLAS

MECHANIC
108,217 POSTS

Hi,

If the thermostat works okay for a while and then it starts overheating, chances are it isn't the thermostat.

Does this happen regardless of driving speed? For example, does it only overheat when in stop-and-go traffic or does it overheat at highway speeds?

Next, you indicated a slight leak, and you hear boiling. Has anyone checked the radiator cap to confirm it is holding pressure? Also, was the timing belt and water pump replaced at 100K?

As far as the leak is concerned, we need to determine where it is coming from. What I suggest is to perform a pressure test on the cooling system to see if you can locate it. Here is a link that explains how that is done:

https://www.2carpros.com/articles/radiator-pressure-test

This may help you as well:

https://www.2carpros.com/articles/car-is-leaking-coolant

For the pressure test, you will need a pressure tester. Most people don't own one, but oftentimes a parts store will lend one to you.

Last is a possible head gasket issue. Take a look through this link and see if anything mirrors what you experience. Some of the checks are more involved, but there are a few very basic ones. See which you are comfortable doing.

https://www.2carpros.com/articles/head-gasket-blown-test

I would like you to try something for me. When the engine is cold, remove the radiator cap and start the engine. See if coolant shoots out of the radiator.

Let me know what you find or if you have questions.

Take care,

Joe

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Wednesday, May 25th, 2022 AT 9:19 PM

TOM NELSON

MEMBER
5 POSTS

Thanks, Joe. The radiator cap was the first thing they replaced, but no effect. They've pressure-tested the radiator several times and they swear there's no leak. I also had the water pump replaced (thinking it was a seal leak), which didn't help. There's no sign of gases or foaming and no coolant shoots out when the radiator cap is off. The oil looks clear and there's no steam visible, so it seems not to be a head gasket problem.

It had a blown head gasket about seven years ago and the timing belt was replaced as well. I checked all the hoses, and they looked fine, but I did notice coolant leaking out at the bottom of the reservoir. The reservoir is still full.

It only seems to overheat in stop-and-go driving so far. I'll have the reservoir and the tubing replaced tomorrow and see if that helps.

Tom

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Thursday, May 26th, 2022 AT 12:53 AM

JACOBANDNICKOLAS

MECHANIC
108,217 POSTS

Tom,

If it isn't overheating at highway speeds, that is usually the result of the cooling fans not running. I know that sounds basic, but have you noticed it coming on? Also, how many miles are on the timing belt that was replaced? If it's stretched, it could change timing slightly which too can cause the engine to run hotter.

Let me know if the fans are working and about the timing belt. Also, when the engine is at operating temperature, squeeze the upper radiator hose. It should be solid. If it is soft, we are losing pressure.

Joe

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Thursday, May 26th, 2022 AT 5:34 PM

TOM NELSON

MEMBER
5 POSTS

Joe,

The timing belt couldn't have more than 20,000 miles on it. The fans are working. They're controlled by a thermostat, so that's probably why they kept replacing the thermostat.

You may be on to something about the hose. It is very soft when the engine is cold and only partly solid when it's warm. The coolant in the reservoir moves up when I squeeze it, so I think it's holding pressure. But there's also a crunching sound when I squeeze it. They told me they flush the radiator whenever they replace the thermostat. Is it possible that small bits of rubber are slowly breaking off and creating a clog? That would explain why it stays fixed for a month after they work on it.

Tom

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Friday, May 27th, 2022 AT 1:51 AM

JACOBANDNICKOLAS

MECHANIC
108,217 POSTS

Tom,

Anything is possible. If you remove the radiator cap, is there any evidence that the coolant is contaminated? Are the hoses original? I agree with you that it seems something is moving around and restricting the flow of coolant. When the timing belt was replaced, did they also replace the water pump?

Also, you mentioned a small leak, but not enough to lower the reservoir. Are you certain the radiator is full?

Let me know.

Joe