I'm prepping an 89 Civic Si for SCCA Street Touring class. Just recently after the car warms up the idle will fluctuate up to 1600 rpm then back to 700 rpm on a reqular basis every 2 or 3 seconds. Sometimes the problem will go away after a while. It sound just like when I used to run a full race high lift cam in Chevy small blocks. But honest, it stock.

Engine has stock ECU, 4-2-1 headers, supertrapp dumping three feet behind the cat, light wt UR race underdrive crank pulley, and a small 6 lb (actually its 7.3 lb but thats another thread) Braille battery. Engine is otherwise stock.

Any ideas on the idle problem. Thanks for any help you can offer.

 

Remove the idle valve, and check for debris or clogging.
Clean the idle valve, with a good brake or carb cleaner.
Re-install.

If this does not affect it at all, then you may have a faulty valve. And as a result should replace it.

 

Actually, that is a very common scenario for the ST Civics...don't know if its a combination of the mods allowed or what, but if you listen to the ST grid at Nationals, they pretty much all do that very thing. 1500-700-1500-700...
And most of them perform fine otherwise.

 

To make sure your idle problem is coming from the iacv and not from somewhere else remove the air intake tube to the manifold and cover the hole on the back side of the inside of the throttlebody if the idle smoothes out then your problem is iacv related if not the problem is elsewhere.
With the ignition on unplug the iacv and plug it in again,listen for a click as you do it if it clicks electrics are good.Remove and clean iacv as suggested above and reinstall.
If that still does not work the engine may need to learn how to idle again.

 

I didn't have this problem on my 91 Civic Si for ST. Sometimes it is the Idle Air Control Valve, sometimes a vacuum line.
I know on my 93 Civic Si I have the looping idle, my IACV was clogged a bit. Cleaned it and still have this problem. I think I narrowed it down to cracking vacuum line. Haven't had time to check.

 

Guarantee you its the alternator going and you are running off battery power. As the engine starts to die - the IACV pops open and it revs up.

 

Vacuum leak after the throttle body and your throttle body is trying to compensate use carb or brake spray with small hose to be percise and spray around the area where a possible air leak and when the idle calms down or changes all of the sudden then that is where you leak is change gasket or defected device. Start at the spring in your throttle body and go backwards check gaskets etc. and also check IACV. GL

 

check your idle screw it will back out and do this.

 

this can be caused by a wide range of things from Coolant needing bled, IACV bad or dirty, or bad vacuum lines. Really you could chase the problem all around and may never fix it. Luckily all it pretty much does is annoy you.

 

what everyone else says,

but my question is, i didn't know you could run a lighten flywheel in ST? thought it was just pulleys?

 

This.

Usually if the IACV can't set your idle correctly, it will seek from low to high. I've had the issue before in a fifth gen and this fixed it for me, I had backed the screw too far out and had to tighten it back in.

The easiest way to check that it is in fact the IACV causing the surging is to take the intake off of the throttle body and just put your finger over the IACV hole.

 

Did you ever figure out the problem?

 

Hi I am missing the 2 pin wire for the iacv on the back of the intake. I can't find it any where would sone have a picture of the wiring harness ? I have 3 valves on the throttle body and one on the back of the intake know as the IACV (M/T) any help would be grateful

 

I have the same issue with my ST Civic but I've got a 92 Si. I cleaned out my IACV and it helped a bit but it still jumps up and down about 300 rpm until it warms up. As soon as the motor warms up it sits at 900rpm and doesn't move at all. I even replaced all my vac lines. I'll try a couple more things but if it doesn't change I'm leaving it and letting the car do what it wants.

 

you need to reset your idle.

1) start the car
2) disconnect the IACV connector
3) CEL will come on, motor may turn off possibly
4) Adjust Idle screw until the motor starts to idle at 750
5) Turn motor off, reconnect IACV connector, and restart the motor
6) Target Idle should be close to 750-900, if not - adjust the idle screw to set idle.

Burping the coolant system works as well if there is air in the lines. For me at least, most of the time the idle screw needs adjustment.

 

i have this same issue with my 89 Civic LX its a 1.5 all stock no mods at all, but my idle is going from 800 to 200 and damn near choking it self out if i turn on the A/C it gets worse turn everything that takes power off and it idles normal for a bit then goes back to crap, i have been thru 2 Alternators, new plugs wires cap and rotor button, i have done a vacuum leak test, also a propane test (gave a little shot of propane to the intake car died immediately), took it to honda their master tech had it for 2.5 hours and couldn't figure it out, my diagnostic teacher cant figure it out, and my emissions at idle are ridiculously high but at 2500 rpms they are normal, and smell gas fumes like crazy at idle......im ready to blow this car up im at a loss here