Hi there. Just joined today1/3/22
I have a 99 civic EJ (D16Y8)with basic minor mods. The past few months every once in a while it started overheating but very intermittently. I got no symptoms of blown a head gasket and I’m not Leaking anything.
When It does overheat I notice that the overflow bottle has gained fluid and is over the maxline by a decent amount. Anyone got any ideas for a d18y8 what could be making this happen. Thermostat is good.
Also Anyone Know what the Air Assist Control Solenoid ( on the Thermostat Housing) does and could it be a causing this issue
i need as many ideas as I can.

 

Head gasket. Gasses are pushing the fluid back into the overflow reservoir. I bet when the engine cools down the overflow bottle empties out again? Might be minor, but dollars to donuts, I bet it's a HG.

 

Sure sounds like HG to me. No shade on OP, but I wonder what these "minor mods" were, and if any of them made the car more inclined to overheat.

Also, I will tell you that just because it didn't overheat recently doesn't mean it's not an HG. My EX d16y8 had a very small HG leak when I bought it—so small I wouldn't have known if a mechanic hadn't told me; it ran and drove fine (though I should have wondered why the inside of the coolant overflow tank was getting so black and greasy—live and learn!) Then one day, the little leak became a big leak, with all the symptoms you describe.

A lot of people will tell you a blown HG will give you white smoke from the exhaust and/or milky-looking oil. I did not have either of these symptoms. An HG can blow in a many-splendored variety of delightful ways, with different symptoms depending on where the leak is. Mine was just letting exhaust into the cooling system, the oil was never involved AFAICT.

FWIW, replacing the HG on these cars is not that hard as HGs go. I did it—twice!—and I'm kind of an idiot. (Enough of an idiot that the second time the HG turned out to be fine; it was actually the intake manifold gasket that had failed due to my own lousy job of cleaning the mating surfaces during the first HG fix. Oops!)

If you try it, I recommend the YouTube videos from Gasket Masters and Big Dog 50001. Make sure you have a way to get to work while you're doing this job, though, because it probably will stretch over a few days when you have to stop because you don't have the right tool, etc. Also, you'll probably want to take the head to your local machine shop to be checked and milled flat if necessary. Good luck!

(PS: That last intake manifold nut is a bitch. I hear you can get to it fairly easily with a 1/4" flex head ratchet, though—if I ever do this job again I'll definitely get one of those first.)