I have a 92 civic hatch with a d16z6. I recently changed the head gasket. Just before the head gasket job, it started acting up and it hasn't gone away. On cold starts it idles around 2k for a minute or so and then drops and surges between 900 and 1400ish. It drives fine on throttle but as soon as you let off it starts roaming again. There's no vacuum leaks, i inspected all the lines and covered the throttle body and the engine died. I replaced the fuel pump about a year ago when I bought it. The coolant has been bled, I did the bleed procedure 3 times to be sure, it has a brand new tstat I replaced when doing the head gasket. Has a brand new fitv, I've ruled out the iacv, I've multimeter tested the tps and o2 sensor, I have no clue what it might be at this point. I don't want to keep throwing parts at it to no effect so I figured I'd post here.
1992 Civic Si, d16z6, obd1, stock other than an intake.

 

Describe how you bled cooling system and ruled out IACV.

 

I bled the coolant according the owners manual procedure. I ruled out the iacv through multiple ways suggested on this site: let the car warm up, unplug it all that does is lower the range of the revving but it still roams the same amount, I tried covering the hole in the tb, I took it off and cleaned it, I replaced it with one from a working civic, nothing changed

 

Are any CEL codes thrown?

Brief summary of what you actually did would help.

If you leave the IACV connected and cover the IACV hole inside the TB, does the surging cease and the idle speed drop to normal when the engine is warm?

 

No CEL


I don't remember exactly what happened from covering the iacv hole on the tb but it definitely didn't get rid of the surge, I think it just started to die, but that was a lot of troubleshooting ago lol

 

Does the CEL work?

Would be informative to do the IACV test I described and then post the outcome compared to the IACV hole uncovered.

 

Yes the CEL works, it was throwing code 43 when I bought it, that's when I replaced the fuel pump.
Assuming I ruled out the iacv correctly, what else could cause these symptoms?

 

Do the test. Post the results.

 

So you're telling me with the iac completely disconnected and even switched with a working one, would still cause the same symptoms? I already tested it multiple other ways, what else could it be?

 

Just humor me, assume it's not the iac, what then?

 

Vacuum, intake air leak, air in cooling system

 

"D16z6 idle surge, NOT vacuum leak..."
No air intake leak, no air in cooling system.
What else?

 

Just because you say this^, does not make it true, if we are being honest.

Do the test. Post the results.

NOTE: This^ test is diagnostic for more than the IACV.

 

I said I've done the test. The results were: the car started to die, no effect on the surge.

 

Then why even believe anything I put? Got damn lol

 

If correct^, then you have a vacuum or intake air leak.

Do you see how your results look contradictory from an objective standpoint?

 

I don't believe anything you say. Don't take it personally.

If you would like help solving your problem, then it's imperative that I question EVERYTHING you state. Make sense?

Hopefully, you'll eventually see that I'm on your side, not working against you.

 

I know they're contradictory that's why I haven't figured it out myself yet and am coming here, it doesn't make sense

 

Participate in the process and have an open mind. In the end, you will benefit.

 

I understand what you're getting at. My car currently parked far from my house and I can't easily get there just to perform one test unless there's a few other things I can try while I'm there, so if you could list or suggest some things other than vacuum leak, air in coolant, etc things I've already tried that'd be much more helpful

 

A two-wire IACV is easy to deal with. Warm up the engine then unplug the IACV. It should slow down to much slower than normal idle. The spec is 450 rpm, which for a four-cylinder engine is basically barely running. It should be obviously slow and shaking. Don't trust the dash tach to be accurate at idle speed. Expect the CEL to come on with a code 14 when the IACV is unplugged. It goes without saying that there must be no codes before you start to chase an idle issue. Don't expect it to run properly with codes set.

If it doesn't slow down, the bottom line is that the engine can't run fast, or at all, without air. You need to find out how the excess air is getting in. Call it an "unintended path" if you want to maintain that your car doesn't "leak." Start by checking the bypass paths through the side of the throttle body before the throttle plate. You can remove the air intake pipe and block them with your fingers.

If it does slow down, that means the ECU is commanding a high idle. A common reason for that is that the ECT sensor is out of spec and telling the ECU the engine is cold when it is not. Or the thermostat is stuck open and the engine really is cold.

 

Have you set the ignition timing with a timing gun or checked it?