Long convoluted story here...

1998 Civic LX AT with 215k on it. I have run a bunch of searches on several forums for this already, so I have some of the basics.

Before this all started, I had none of these problems. It ran fine.

Started with a noise that was coming from the pulley side (later found it was the AC pulley bearing that grenaded, but did not know that right away). After a day or so of that, the engine wouldn't start. Checked fuel pressure - within specs. Checked spark, and thought perhaps it was weak, but was not sure. Since Distributor is #1 cause of non-start, I replaced it and the rotor and cap. Rotor and cap were pretty worn in any case, but this did not solve it. Great - I am officially a Parts Swapping Monkey (tm) now.

Finally checked the timing belt marks and it had jumped a tooth. Bingo. I figure the AC pulley bearings may have bound up for an instant and caused it to skip. That bearing was not just noisy, it had disintegrated. No biggie now, so I put a new belt on and got the timing marks all lined up right (tensioner and water pump were fine so I left them alone - no need to change things on a car this old without good cause). Replaced the AC pulley/coil/clutch too. Got it all back together and went to set the timing advance due to replacing the Dist. Started and ran great. When I warmed it up and let it drop to idle, it started surging. Would drop to 1000 or so then kick up to near 2000 then drop back down and repeat about every 1-2 seconds. Took it for a drive, and no change after it is parked and let idle again (drives just fine).

Looked for a potential vacuum leak I may have caused during the timing belt, but cannot see anything obvious until I noticed what looked like a chunk of valve cover gasket sitting next to it. Take that apart and indeed the gasket had lost a chunk (it's old and been apart a few times, so no big surprise). I replace that, and all the seals in the kit figuring that has to be the source, but once again, the same surging idle.

Unplug the IAC and it starts and runs at 1500 or more steady (and throws a CEL, of course).

While I was replacing the timing belt, I also checked some general things and added some coolant as it was low in the rad. I mention this as people talk about bleeding air that can cause the problem, but I only topped it off, not a total drain, though it did take a bit to fill it. Temp gage never spiked like it would if it saw air during a drain and refill, so I am not suspecting an air bubble in the coolant, but I figured I should throw it out there.

Obviously I need to check more thoroughly for vacuum leaks using propane or carb cleaner. Any ideas on what to do next? This is driving me nutz as it has been down over a month now due to limited time to work on it.

Thanks for any and all help!

-Dave

 

More diagnosis tonight and no real progress. I went after finding vacuum leaks. Started by using a propane torch (unlit, of course) and could find no locations that changed idle at all. Switched to carb cleaner and still found nothing. I also do not hear any type of whistle that would indicate a vacuum leak, so I am starting to wonder if that is the issue at all...

Also, took the IAC off to look it over and clean it. It was in very nice condition but I gave it a blast of carb cleaner anyways. While I was down there, I pulled the PCV valve which was fairly gunked up, so I replaced that. Still no improvement.

Talked with a mechanic at NAPA while getting the PCV Valve, and he suggested the TPS, but my reading online indicated the TPS is rarely a problem. Per his suggestion, I unplugged it while running. The car continued to rev up and down for a couple cycles and then the idle rose to 2200 or so and stayed there. This was pretty much the same behavior that I got when I unplugged the IAC the other day. I'm guessing both of these behaviors indicate that the IAC and TPS are OK, but am not sure. I did ohm-out the IAC and both 1-2 and 2-3 read in the 22-25 ohm range (manual says 16-28, IIRC), so no obvious short or open.

At this point, short of just changing parts like the IAC (which is awfully pricey to just swap without good reason), I am truly out of ideas. If there is a vacuum leak, I cannot find it (and I am doubting it exists) and whatever else it may be is nothing I have been able to find.

HELP!

 

Cooing system must be full, or you get coolant splasing on the ECT which makes ECU think temperature is varying.

Throttle cable must go slack and throttle plate fully closed when gas pedal not pressed.

Plastic FITV on thermostat housing easy to break and causes massive vacuum leak. FITV should be closed when engine above 60 F.

Take the air cleaner off and feel for air going in the bypass holes, one leads to the IACV it should be fully closed when idle is that high warmed up. Bottom line is that too much air is getting in somewhere.

 

Will check. Pretty sure fitv started with '99 models, so not on mine. Have had the air cleaner off a lot, but will check the bypass ports more closely,

thanks!

 

Had time to check into this more tonight, finally. I warmed it up until the surging started and then plugged the opening to the IAC that is above the throttle plate with my finger. Immediately the idle settled down to a normal low idle and stayed there until I removed my finger. I have been reluctant to just replace the IAC as it is pretty pricey, but this would seem to point to a problem with it, or with its gasket.

mk378 or anyone else - do you agree with this? I don't have much else to go on here, so that seems to be the only likely cause now. And thanks a lot for that tip!

-Dave

 

I'm not sure how to test a 3 wire IACV. The two wire ones are simple, they snap shut when unplugged.

If one of the coils on yours is open it's definitely bad. They also can be intermittently open, so check when hot as well.

You should also ohm test the ECT sensor, if the ECU thinks the engine is cold due to bad ECT readings it will increase the idle.

 

Thanks. I had previously ohmed out the IAC and it tested fine. Both pins 1-2 and 2-3 should be 16-28 ohms, and they both were so no short or open in the IAC motor.

I got the computer connected to it today and cleared the codes I generated when I previously unplugged stuff while diagnosing it. I looked for a readout or control on the IAC but that was not there. It might be because the car is so old, or because I don't have the Honda extended package on the software. Either way I learned something as it did show the engine coolant temp. It was rock stable at about 188 the entire time (while the idle was surging). That tells me there is no air in the system. The idle stabilized perfectly when I plugged the IAC intake with my finger again too.

At this point I figure my odds are good that the IAC is bad, so I ordered up a replacement. We'll see in a few days...

I appreciate the tip on plugging the IAC port. I wouldn't have thought to do that, and it looks like it points to the problem. Thanks again!

 

We have a winner!!! The IAC part finally came in today and I replaced it tonight. That solved the problem. It was obvious from the moment I started it after replacing it, but I proved it out by warming it up fully and all is good. Then I was finally able to set the timing correctly too. Also good.

Thanks MK for that tip. That saved me from giving up on it and bringing it in. Still a pricey part at $180, but a lot less than it would have been if I brought it in.

Thanks!

Dave