I just purchased an auto 1998 Civic LX with a D16y7 a month ago. Just wanting a stock, reliable daily. The car is completely stock besides lowering springs.

When I bought it, I noticed in Park, once the engine warmed up, it would idle high and surge slightly around 1600rpms, but wouldn’t do it in Drive, and it drove 100% fine. After a week though, the CEL came on, so I had it checked at auto zone, and it came back as an IAC valve code (can’t remember the specific code atm).

So I ordered one off eBay since parts stores want like $200+, which is ridiculous. That made the engine run perfectly with no idle problems and the CEL went off.

Well after 3 weeks of daily driving, one morning it just started doing it again, but worse, and the CEL light came back on. So I thought it might be a vacuum leak since some of the air intake gaskets were tore, and the throttle body gasket was split. So I just ordered a cheap cold air intake off eBay instead of sourcing the stock intake gaskets, and replaced the throttle body gasket with a new one.

No improvement at all after installing those. So I scanned the CEL again, and got code P1509. Had to drive the car home, and it’s surging so badly that at any rpm below 2000, it just surges and bucks. Above that though, it’s 100% fine. Even pulls to redline at WOT with no problems.

I kept the original Denso IAC valve that I removed weeks ago, cleaned it like I’ve watched YouTube videos, and reinstalled it. Yet it still surges just as bad if not worse with that one. I’m really not sure what to do at this point. I removed the air intake to put my hand over the throttle body to see if it had any other vacuum leaks, and the engine died from my hand. Unplugged the IAC valve, and it still surged badly. Do you guys have any advice on what I should check for next? This is driving me crazy as I’ve barely been able to drive it since I bought it. Any advice would be greatly appreciated

Edit: the only issues I know of off the top of my head is that the car has an exhaust leak in the manifold right between the tubes, that someone already welded up once, but has developed a pinhole crack again. It’s always had it and still ran fine those 3 weeks with that leak. So I’m not sure if that’s a contributing factor or not

Would a pcm from a 1998 Civic auto work in my car that has a d16y8 instead of my y7? A friend of mine has a civic identical to mine and I probably could just borrow his pcm real quick to see if that’s my problem, but I’m not sure if that would work since we have different intake designs.

I just got a complete throttle body with a factory IAC valve, tps sensor, and MAP sensor off a Civic in a junkyard, and swapped it onto mine with no improvements at all. So now I’m thinking it’s either the wiring or the pcm

No I haven’t but i have been reading that that can cause my problem as well. Every time I’ve checked the coolant, it’s right to the top of the cap on the radiator. Could constantly removing and installing the IAC valves that leak coolant from those two small hoses every time I remove them possibly cause air to get in the lines?

Well I’ve jacked up the front end, removed the radiator cap, set the air to full heat on full blast, let it run for 30 minutes yesterday, and 30 minutes this morning. A lot of foam and bubbles have came out but it doesn’t seem like it ever stops. With no improvement in idle. Also unplugged the power steering pressure switch as I’ve read if they’re bad, it can cause high idle. No effect there. Unplugged the o2 sensor for the hell of it with no improvement either lol.

But one thing I’ve noticed is that if I unplug the MAP sensor, idle drops way down to around 1000rpms and sounds almost normal. I originally thought it would almost die every time I unplugged it, but it’s actually just dropping way down and idling normally

So here is what the coolant is doing. I’ve jacked the front end up and let it idle (surging the whole time) for 30-45 minutes on two separate occasions now with the heat on and it’s still bubbling/foaming up like this. A friend of mine thinks it could be a blown head gasket causing air to get into the coolant, thus causing my high idle and surging. But if that were the case, I stil don’t get the IACV circuit CEL I’m getting. The engine never overheats or loses coolant either

The overflow tank has been at the full mark since I bought it, so really the only difference is that I’ve had the fan on the whole time instead of off, and I didn’t hold the rpms that high. It stayed around 1700rpms in park, so the radiator fan did come on several times while it was running those two times, and the foamy coolant pours out of the open cap multiple times. But I will try it again with the fan off, and hold the rpms higher. Thank you for the clarification