Injuneer

If I read that correctly, the problem starts when you reset the IAC. Before that point, the IAC was drifting +/- a few counts - nothing unusual - and it was holding the idle extremely steady.

Why did you reset it at that point?

Are you ruling out the AC as a possible cause of a sudden drop in RPM?

Zee man

Yes the a/c has no effect. On or off its the same. And No, I'm sorry , I am scanning because the IAC, or more correctly, because of the symptoms.

If I stop at a random light. The rpms climb up to 1500, from a nice normal 575. It'll vary a bit, a couple hundred each side of 1500 rpms. Fluctuate. Sometimes it willbounce from normal to as high as 1800, all without any outside input. Warm, cold, in gear or park. Doesnt matter. When it is at its worst, I have caught it and been able to scan it with scan 94 95 .So, when it has acted up at home. AndI have had access to my laptop I can plug in and watch the IAC counts. When I use the Iac reset button I get the 3188, target rpm . Like it was shut down for a split second.
The IAC numbers are the only thing acting out of the ordinary. Like I said at idle they can scale up as high as 80, 100 isn't that unusual when it is doing it either. But I haven't seen any of the other numbers change. On top of that, its intermittent, but the longer I run an IAC the worse it gets.If I replace it, it'll be ok for a couple days then start acting up.
Sorry for the confusion

Zee man

Just to add. When I reset the IAC , with either method. It will come down, but its a craoshoot if it will stay there. Sometimes it will, othertimes it climbs right back up.

Zee man

So, if I am to understand, the IAC number I see is a representation of where the computer wants the IAC to be. So the computer is ordering that position for some reason.

Injuneer

Yes.

Have you tried cleaning the IAC passages?

I've been helping someone with a similar problem, but more extreme... IAC counts go to 160 and RPM to 1,800. Odd thing with that particular scan is that the MAP drops to an irrationally low level. With his cam, MAP is normally at 50kPa. With the increase in IAC counts/RPM the MAP can drop as low as 25kPa - a value normally only seen on closed throttle decel. I see something similar in your data during the 1200 RPM excursion.

timo_pajala

this is just my guess but mine did the same thing and the reason was in the throttle body not the iac. Mine leaked and the hard part was that it didn´t do it regularly. My friend suggested changin it after we had changed almost everything else.

Zee man

The IAC passages are spotless.the whole tb is clean. I cleaned it everytime I changed the IAC motor.

I wouldn't have a problem changing the tb except . The only thing the tb could do is leak more air into the system. So why would the computer order the IAC to open and allow even more air if its already lean?

97WS6SCharged

If the leak is intermittent then the computer will close the IAC to try to get the idle down to target when the leak is open. When it closes the idle will drop below target so the computer quickly opens the IAC to compensate. An unmetered leak is possible since the ST fuel trims are showing slightly lean over all. You might also check the PCV valve and rubber fittings around the metal line since it feeds directly into the idle air passages in the intake manifold.

Zee man

I sprayed carb cleaner all around while it idled, everything seems good, no apparent leaks. Since I changed the IAC on Thursday its been just fine. I had one bout of high idle right after I changed it but its been rock solid ever since.
So I guess we will wait and see.

Injuneer

Are you sure you have all the screws for the TB bottom plate in the correct positions? If not the plate can leak. Have you bypassed the coolant?

Zee man

Yes, the coolant portion is bypassed. and yes the screw are correct.

Zee man

Been a week now with no further problems. Must have been bad parts.

Zee man

Or not. Here we go again! IAC is jumping around again intermittently, confirmed the numbers with GaryDougs scan tool. Swapped in another IAC, a used factory one a buddy had laying around. Its calm now, but, for how long?
I swapped out the MAF, no change. I also swapped out the air temp sensor, no change. Also swapped back to a factory coil and a different ecm. no change. I need to try the MAP the next time it does this, just need to find a good one to borrow for a bit.

So to refresh. When it is having its problem, it does the following. the RPM's climb up to 1200 - 1500 at idle, in gear or park. the tach twitches when it is doing it too. I also get spark knock on acceleration. I have noticed today a lurch at cruising speed, almost like I hit a bump in the road, randomly here and there. No codes or dtc's are stored .

Ya'lls thoughts would be most appreciated.

Chimera96

high idle can be vacuum leak....maybe a hose is cracked and leaks "intermittently" due to heat and or vibrations of driving causing your random issue

Injuneer

His problem is the PCM raises the IAC counts, and the engine surges to 1,500 RPM. If there was a vacuum leak, the PCM would close the IAC down until it bottomed out. The question is, why is the PCM raising the IAC counts, when the idle is at the programmed value?