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I'm up against something new to me. My '02 Civic has started losing coolant through the overflow reservoir. It will fill the overflow reservoir and you can hear bubbles gurgling in the overflow. The temp gauge is always normal and the cooling system otherwise seems to be working correctly. My first thought (as I expect many of you are thinking) was cylinder leak of some kind pushing exhaust gas into the cooling system, so I borrowed the combustion leak detector kit (this one: https://www.lislecorp.com/specialty-...-leak-detector ) from the parts store and ran the test a handful of times. The test fluid stayed blue throughout all tests, which is a negative indication of exhaust gases in the cooling system. This was my first time running this kind of test. Following the instructions in the test kit and a couple of youtube videos:

1) Allow engine to cool enough so I could remove the radiator cap. Remove radiator cap.
2) Idle engine until thermostat opens and coolant is flowing to radiator. Confirmed that upper rad hose was hot and coolant was flowing to radiator.
3) Suck out top couple of inches of coolant from radiator to avoid contamination of test.
4) Press tool into top of the radiator to get a good seal. Fill tool with blue fluid to line.
5) Use suction bulb to suck gases through the test fluid. There was obvious indications that gases were passing through the tool as expected.
6) Continue pumping gases through the tool for 2 minutes. I couldn't pump as fast as possible because the fluid foamed up, but I could clearly pump a substantial amount of gas through the tool over 2 minutes.
7) The fluid retained its blue color and showed no sign of turning green or yellow.
8) As a test, I refilled the tool and held it up against the exit of the tail pipe and pumped exhaust gas through the tool, and the fluid turned yellowish green.

Two main questions for those who are more experienced than I am:

What could cause a false negative on this test? It seems like such a foregone conclusion in so many of these kinds of posts, that I want to make sure I could not be missing something.

If we end up trusting the test results, what else would cause coolant to bubble out through the overflow reservoir with no indication of overheating?

Thanks in advance for your help.

 
Rep Power: 223

It's the headgasket, do the test with shop air in this thread
https://www.civicforums.com/forums/3...reference.html