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So I've had a mysterious coolant leak for the last few months that I haven't bothered much to try & track down. Just top off the reservoir & move along. Well a couple weeks ago I decided to pressure test the system & see if I could track down the leak. Car off, pump the tester up to 15psi & let it hold for 10 minutes. Didn't budge. So I relieved the pressure, then hooked the gauge back up. Started the car up. Within 15-20 minutes she'd spiked up to 29psi before I finally had had enough & wasn't trying to push it any further.

It looks/feels like a standard head gasket blown. But I'm just not trying to dive into that without doing a little more research. It's got 188k on it, I just did the timing belt/water pump two years ago & have never had any issues with over heating.

I'll do a compression test before I start the tear down just to check everything else. I've already got my head gasket kit & new head bolts sitting here waiting for me to start. But I figured I'd check in & see if there's any other (far flung) idea that might be the cause.

Thanks.
 

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I would think head gasket also. Not on my IS, but on my older Civic, I had a mysterious coolant loss like this. Wasn't burning, it would just keep overfilling the overflow and drain the radiator. The explanation I read was that the air pressure from the motor running is leaking air into the coolant system around the headgasket and pressurizing it, essentially pushing the coolant out of the system. I replaced my headgasket and the problem went away immediately.

Test the headgasket and see what your pressure is in each cylinder.
 
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