Bad / Hunting idle 88 Duke when warm / hot (Page 1/5)
Donster MAR 09, 09:47 AM
OK, before somebody tells me to look for a vacuum leak...been there, done that, all lines replaced, except brake booster line, but it is also OK.
Also the vacuum lines to the fuel vapor canister, also switched them around, just in case the PO had messed with change.
TV cable is also adjusted right.
Let me list what I have replaced after checking for vacuum leaks.

O2 sensor
TB gasket
Oli & filter
Ground cables

Still hunting when warm/hot. Sometimes also stalls.
Runs great when cold.

Here my question; can a faulty oil pressure switch cause idle problems? I know the fuel pump relies on information from the OPS.....and I also know that I accidentally shorted the OPS a while back, trying to connect it correctly again (someone from the Georgia Fiero Club told me I probably blew the fuel pump fuse, which was dead-on!), because the PO hacked around on the cables (only 1 cable connected to the OPS). I'm suspecting that something has been wrong with the OPS from the beginning. The PO also installed an 87 OPS instead of an 88 OPS, which I have, plus the 88 connector.
So while watching my instruments, I notice the fuel pressure takes a dip, then right after that, my idle takes a dip. It almost sounds like not enough fuel when the idle dips.
Does what I am explaining here make sense? Could it be related? If the moon is in the seventh house and the wind is blowing just right?

Frustrated in Germany...........Don

[This message has been edited by Donster (edited 03-13-2019).]

Blacktree MAR 09, 12:31 PM
There's a common misconception that the OPS acts as a fuel pump cut-off switch. It does not. It's actually a back-up for the fuel pump relay. If the relay is working properly, then the OPS will have no affect on fuel pump operation. The only exception would be if the OPS switch sticks in the ON position, causing the fuel pump to run continuously.

You said that you checked the brake booster line. But did you pinch it closed, to make sure the booster itself isn't leaking?

Also, I don't see any mention of the O2 sensor. When the engine warms up, the O2 sensor comes online. That's a possible culprit.

Best of luck.
Donster MAR 09, 03:12 PM
Thanks Blacktree,

I actually did forget to list the O2 sensor. It has been replaced.
And break booster was tested in both directions.
viperine MAR 10, 12:32 AM
Had this issue in a 2.8 V6. Timing was just not done right. 10° off, it runs great cold. Idle wants to hunt when warm. In my case, I had the distributor O-ring replaced and we painstakingly marked the distributor. The ringer was that the oil pump driveshaft came out with the distributor. In hindsight, we lined our marks up, but probably missed a tooth on the distributor/cam gear. Not certain of 4cyl timing procedures, but it's worth ruling out.
Donster MAR 11, 07:17 AM
I think I have narrowed it down to the IAC. Seems that I got a bad one out of the box.
Will test with IAC procedures from here on the forum and update tomorrow.
theogre MAR 11, 11:05 AM
Iffy power/grounds will cause hunting idle.
Alt brackets are ground for the alt.
Crap Wires to alt or battery or "block" under C500
and others can cause this.

Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

Donster MAR 13, 05:20 AM
Thanks Ogre,

But I've checked ALL cables, positive AND negative/ground and they seem OK. Ones that were questionable I replaced, but symptoms persist.
Could a bad exhaust also be the cause? Not enough back-pressure?
Donster MAR 13, 05:25 AM
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?
If I plug the IAC back in, engine starts again, but of course with the symptoms again once it runs warm???
JMTUT MAR 13, 10:14 AM
If you reset the IAC, did you drive it over 35 MPH to let the computer reset?
Donster MAR 14, 10:49 AM
Yes, I disconnected the battery, reconnected it, drove for about 60 KM at 35 mph and more, with stop & go and full stops with engine off in between.