Driving with Radiator Cap Loose

[R01k]

Do you see something strange in this picture?



I bet you do. Yeap, I have been spilling coolant like a sprinkler for several miles during the last week. See the radiator cap partially loose. The thing is that while changing the oil 8 days ago I accidentally unscrewed the cap one step. Today I open the hood to check the oil and found the view you can enjoy above. However, after letting the car cool down completely the coolant level shows about 3/4" higher than the Full line, where it has previously been. So, IMHO I could present 2 hypotheses to explain the raise of the liquid: 1) Air went inside the system or 2) the thermostat has sent too much coolant to the raidator, what do you think?

[hank]

I would opt for the air, due to the massive water carnage. Bleeding on such a small setup should be rather quick though.

[M-88]

Unrelated - but that's one of the things I like about the mirage. I HATE cooling systems with a pressurized overflow tank! Ones with actual radiator caps are the best.

I don't think you lost that much coolant - pull the cap and see how much is the the radiator and how much is in the overflow tank.

The radiator cap allows the system to pressurize, let extra coolant out into the expansion tank (when the engine gets up to temperature enough and the coolant pressure gets high due to temperature) and then allows the coolant to be sucked back in.

With that said, if the radiator cap is lose, the cooling system won't build pressure and MOST of the coolant is just going to go into the overflow tank when the car gets up to temperature. I'm willing to bet your overflow tank is a bit more full than usual.

Solution: Tighten cap down, take car for a drive and get the coolant up to temperature, then let it cool back down. I'll almost bet that it'll suck (vacuum) all of the fluid back into the radiator. Then go based on the level that is in the overflow bottle. If you need to add more, add more. Not likely you will, though.

It's a pretty common trick on an older vehicle if your radiator pops a leak to loosen the cap to avoid it spraying out of the radiator.

[R01k]

I'm willing to bet your overflow tank is a bit more full than usual.

You're right on the money here. The coolant is about 3/4" beyond the Full line.

[hank]

Wow, that was really insightful. Thanks a lot for imparting that information I never understood the mechanics of the overflow bottle, I just thought it was there to cycle coolant and act as a chamber for maintaining pressure.