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Coolant boiling but not overheating?

My son has a 94 Escort LX and this weekend he noticed that when he’d stop the car he could hear the coolant boiling. The car temp gauge never registered too hot, and it never lost coolant. We checked the oil and it is not milky.

About 2 years ago it got a new water pump. Last summer a new thermostat, and then earlier this year a new radiator.

Last weekend he was stuck in a huge traffic jam. (4.5 hours for a normally 45 min. drive). I’m wondering if that could have weakened the radiator cap. He’s picking up a new one today, but wonder if there is something else we should check?


How does he know it was the coolant boiling? The reason I ask is that sometimes air conditioners will gurgle if they’re low on refrigerant, and fool you into thinking it’s the coolant.

Did you pop the hood and see if coolant was bubbling in the overflow tank? By ‘stop the car’ do you mean ‘turn off the car’, or ‘stop at a stoplight’?

Thanks. Shadow, would it do it if he never uses the AC? It may be low, but he never turns it on to find out.

Texas, stop the car as in turning off. Not sure if he saw it bubbling in the tank. I’ll ask when he gets back.

But this weekend was the first time he’s heard it.

If he doesn’t use the AC (or the defroster), then he could have an air bubble in the heater core, especially if he heard the bubbling from inside the car. He might need to bleed the cooling system.

Thanks! How would that happen? I could understand if it was just a short time after the new radiator, but it’s been 3-4 months and several thousand miles since.

I vote that a new radiator cap will solve the problem.

Maybe he didn’t notice it before.

Make sure all radiator coolant fans are operational. If you have 2 fans when you turn on ac both should be running. Also check coolant level when cool in the radiator and make sure it is full. Also are you sure you have a proper coolant to water mix?

BTW, a defective radiator cap will let air into the system over time, so if the cap is bad, it may have recently started to fail.

Thanks. We’ll check the fan with the AC tomorrow. They did put new coolant in with the radiator. I’m assuming it’s the right kind, but it’s been 3-4 months since, wouldn’t we have noticed issues earlier.

My son is going to get the cap tomorrow, hopefully that’s it.

Classic head gasket pinhole leak of combustion gasses into the coolant system.

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I have had this happen on my 84 Corvette after driving in hot heavy traffic. The temp gauge got to about 240 deg F and then spiked higher when the engine was shut off and the fan quit running. It bubbled in the overflow jug for a little while and then cooled off. Went back to normal and didn’t return until the next time I got stuck in hot stop and go traffic.

Seems like you are getting to the pressure relief point on the radiator cap and as the coolant expands and escapes into the overflow jug, now it boils in ambient pressure, not the 19 psi, or whatever your cap is.

I had that same symptom w/my truck one time, no AC on the truck. I could definitely hear a boiling sound coming from the radiator. Only happened when in 4WD, going up long, steep hills. It turned out to be the radiator cap wasn’t holding pressure. A new cap fixed it.

@insightful I have to agree; these symptoms point to a small exhaust gas leak causing the boiling but not raising the total engine operating temperature.

If there is a small exhaust gas leak into the cooling system, then shouldn’t a mechanic be able to put an emissions probe into the top of the radiator opening and sniff it?

It would also show small bubbles with the cap off and the hot engine running.

Thanks. The boiling only happens after he shuts off the vehicle. How could that be exhaust?

The boiling only happens after he shuts off the vehicle. How could that be exhaust?

Because the coolant is just below its boiling point while being circulated, but at shutdown, the “heat soak” raises the temperature to boiling (most likely near the exhaust manifold). The temp sensor is in a pocket of gasses and doesn’t reflect the true temp. Of course, it could be something else, but after the slow progression of replacing everything else in the cooling system, the road usually leads to the head gasket.

Thanks! He’s out with his girlfriend now. I guess we’'ll see what goes on when he gets home.