Bad / Hunting idle 88 Duke when warm / hot (Page 2/8)
Donster MAR 19, 07:52 AM

quote
Originally posted by Donster:
I tried resetting the IAC valve using the procedures I found here on the forum and in the manual.
But after taking it out of diagnostic mode with the IAC unplugged, the engine will not start, so I cannot adjust the idle air screw. Is this an indication of a bad IAC?



I tried it again, but with the IAC disconnected, I cannot get the engine to run.
I'd hate to screw with the screw (no pun intended), as I have read so much negative about it here.
Since nobody here told me if that was an indication of a bad IAC, I have ordered a new one, which should be here in Germany by Friday.
Guess I'll soon know. :-)
theogre MAR 19, 09:43 AM
Static idle is "sealed" for good reason to stop ID10T from F'ing up this. Not just GM but Ford Chrysler and other to meet Fed and often State Rules.
But...
If the Seal is gone then you need to check because many think adjusting this "fixed" whatever problem and most only make more problems.

Even if the Seal is intact... Idle stop screw and point it hits and throttle pivot points can wear. Just how bad depends on many things and you may need to adjust idle for old cars. If really worn then may need a newer TB TBI or Carb.
GM cars are often better for this but still can happen. One of worse was first version of Chrysler Mini Vans w/ 4 cyl that had a very thin screw tip that could literally hammer a hole thru the steel stop and completely close the throttle.

All Fiero engines covered here from GM FSM, See my Cave, Idle Stop

Note that this problem Will Not cause Hunting Idle for most cars.
Crap power/grounds will and a few others.
Just 1 iffy ground in the cabin or engine bay can cause this.
Common grounds etc See my Cave, Wire Service
Don't just look at or even "test" grounds and power block under C500. Take apart all ends, clean and lube w/ silicon or brake grease at minimum. Either grease won't wash out easy and won't eat plastics.
Donster MAR 20, 12:48 AM
Thanks Ogre!
The articles in your cave will certainly help me hunt down any possible bad grounds or connections. Very informative....and your knowledge is amazing!

But perhaps you can answer this for me: If my hunting idle when warm is due to a bad or iffy ground, would this problem not also be when the engine is cold, not only when warm or hot?
Or is the circuitry in closed loop so dominant that it maintains a steady idle and only becomes evident once the ECM takes over?

I am still hoping that the IAC is the culprit, but will tear into my wires this weekend using your information.
Thanks again,

\D

[This message has been edited by Donster (edited 03-20-2019).]

fierofool MAR 20, 06:07 AM
Don, I don't know if the L4 IAC is the same or designed the same as the V6 IAC, but how are you adjusting it before installing the new one? The V6 has two style IAC's and their method of adjustment differs.

One has a smooth, round pintle shaft. That one can be screwed in to set the initial adjustment of 1 1/8 inch. The other has a slotted shaft and if it's twisted, it will destroy it. For the smooth shaft, retract the spring from the pintle head and turn the pintle head. For the slotted style, place your thumb on the end of the pintle, exert pressure and rock it side to side. The initial setting should be just a hair less than 1 1/8 inch from the tip of the pintle to the surface where the gasket rests on the body of the IAC. If the IAC is installed with the pintle extended more than 1 1/8 inch, it can be damaged.
Donster MAR 20, 06:56 AM
Charlie,

The V6 IAC is threaded to screw in.
The L4 IAC needs to be fastened with 2 screws. They are quite different.
The last one (out of the box) had the pintle screwed all the way in. I hope the new one is the same way.
Once it arrives and I install it I jumper pins A & B (diagnostic mode) with key on, engine off, wait 45 seconds or more, then I disconnect the IAC, take it out of diagnostic mode and that is when I would start the engine and try to adjust the idle air screw.
After that, I re-connect the IAC, and start up the engine. Optionally, I disconnect the ECM and re-connect it before driving at least 35 mph or more for about 20 or 30 miles, applying STOP & GO and also shutting off the engine a few times.
As they say: By the book!
fierofool MAR 20, 07:05 AM
It is quite different. anyway, look at the pintle shaft. I'm sure the same methods of retraction would be required before installing if the pintle is extended too far.
theogre MAR 20, 05:15 PM
Close loop happens after ECM sees a good signal from O2 sensor. As little as 30 sec after start.
O2 sensor Is Not a "one wire" but looks like it because O2 ground is bolted to the trans bell bolts.

ECM knows volts and iffy wires can lie to ECM for this and most anything else.

other iffy wires can cause a lot of headaches including Hunting idle
Donster MAR 28, 05:07 AM
Had the time to go through all my wires again, couldn't find any that looked iffy.
Took out the entire C500 and redid the wires.
Still waiting on new IAC and still idles like crap. :-(
Patrick MAR 28, 03:12 PM

quote
Originally posted by Donster:

...after taking it out of diagnostic mode with the IAC unplugged, the engine will not start... If I plug the IAC back in, engine starts again, but of course with the symptoms again once it runs warm.



I'm curious... what happens if you simply unplug the IAC while the engine is running? Does the idle continue to hunt?
Donster MAR 28, 03:16 PM
Stalls / dies.........instantly.

Installed a new IAC today, went through the procedure, symptoms remain.