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I have a 2008 Honda Civic with about 150,000 miles that recently began leaking coolant from the reservoir cap (overflowing) while the engine is hot. I have no visible symptoms of a blown head gasket such as milky oil/coolant or white smoke out of the tailpipe but I figured I’d check anyway. 


I had a family member, who’s a mechanic, use his combustion leak test kit to check for gas out of the radiator and he found nothing but I watched him do it. Rather than sit the tube straight into the radiator neck like usual, he instead put a no spill funnel in, filled it with water and began sucking the air from the top of it with the tube. He did this for about 5 minutes, didn’t see any change in the testing liquids color and sucked some gas from the tailpipe to make sure the liquid was working which it was. Afterward, he told me that my head gasket was ok and advised me to change the coolant filler neck and reservoir cap because they were both worn. 

I’ve done what he advised and nothing’s changed. I’m just wondering if the way that he used the combustion leak test would give me a false reading and if I should get my own kit and test it myself before I go about changing other parts trying to figure out what’s causing the coolant loss. I also wanted to ask if a bad thermostat could cause the issue since I haven’t changed it out yet.

1 Answer

A bad thermostat can definitely cause this.  Put in a new one.  A simple compression test could have told you if you had a head gasket problem.  Buy a Fail Safe thermostat, it fails in the open (not closed) position.