Up & down idle 88 Duke when warm

[Donster]

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?

\D

[GTRS Fiero]

Just a guess.  When at this step, the IAC's pintle is fully extended, and doing nothing.  Nothing to do with the IAC.  It sounds as if the idle screw is at fault, as you suspect.  Note the position, then turn the idle screw a half turn.  Lefty-loosey.  Try starting.  You may have to open the idle screw a bit more.

Sorry, my IAC article is not ready, yet.  I only have a few notes, and the article is at least 2 months out.

Get me a picture of that idle screw, please.

[Donster]

Today I replaced the fuel pressure regulator, tore apart the C500 block next to the battery, traced all my ground wires, replaced the one going from the block to the deck lid, checked all fusible links and still have a hunting idle when warm/hot.
Still waiting on my new IAC so I can install and try to adjust the idle air control screw.

The German Karmann Ghia Convertible Club would love to have me as a new member! 

[TopNotch]

Have you considered a throttle body rebuild? Replace all gaskets, O-rings, and anything not metal in the throttle body. You can get rebuild kits at auto parts stores over on this side of the pond.

[Donster]

Have you considered a throttle body rebuild? Replace all gaskets, O-rings, and anything not metal in the throttle body. You can get rebuild kits at auto parts stores over on this side of the pond.

$38.00 with express shipping.

OK, TBI rebuilt, new IAC (I think it is now the 3rd).
I have gone through all my cables, looking for a bad ground, and have replaced about everything that can be replaced..........see here:
EGR
MAP
MAT
O2 sensor
IAC
TPS
CTS
PCV
Injector
ALL TB gaskets & O-rings (cleaned entire TB housing)
Fuel pressure regulator
ECM
Ground cables

Still hunting when warm/hot. Sometimes also stalls.
What I have not yet replaced are the 2 ignition coils...........could they be at fault?
Right now I'm at a point where I am willing to replace just about anything, as I am getting a bit POed, to say the least.

[Fierofool]

If it's any consolation, I had the periodic problem with the 88 Duke Coupe.  Sometimes it would idle without wavering, though at about 1500, and other times it would be up and down with a periodic stall.  If my memory is working, I seem to remember that Pat's 88 has a very high idle, too. 

[TopNotch]

A duke will have a very high idle at startup, but if should slow down after about a minute or two. Also, if you turn on the AC, the idle will go start cycle again -- very high for a while, and then slow down.

[Donster]

If it's any consolation, I had the periodic problem with the 88 Duke Coupe.  Sometimes it would idle without wavering, though at about 1500, and other times it would be up and down with a periodic stall.  If my memory is working, I seem to remember that Pat's 88 has a very high idle, too. 

And, did you get it fixed?

[Donster]

What I have not yet replaced are the 2 ignition coils...........could they be at fault?

[Fierofool]

Never got it fixed, Don.

[TopNotch]

What I have not yet replaced are the 2 ignition coils...........could they be at fault?

Usually if a coil is bad, the engine will miss or die at the most inappropriate time. It will be worse when accelerating. Been there, done that.

[GTRS Fiero]

Maybe a video of the issue?

What happens if you unplug the IAC, and operate the throttle manually to keep the engine tunning?  You should be able to do so, and keep a fairly steady RPM.  Or, try to keep a steady RPM with your foot.

[Donster]

What happens if you unplug the IAC, and operate the throttle manually to keep the engine tunning?  You should be able to do so, and keep a fairly steady RPM.  Or, try to keep a steady RPM with your foot.

If I disconnect the IAC, it stalls most of the time. When it doesn't stall, the idle still hunts.

[Donster]

The Ogre has told me (multiple times now) that I should look for a bad ground or "iffy" wiring and that my coils are not likely to be the cause, which coincides with what Pat said.
I've taken apart the C500 next to the battery and redone all of the wires.

Perhaps someone here can point me in the direction of possible bad wires/ground? Or are there too many?
Wich are the ground wires that could have such an effect on the idle?

I ordered a scan and diagnostic kit for the 87-88 Duke yesterday, what exactly, when I scan, should I look for?

\D

[GTRS Fiero]

Well, you could look for any value on a sensor that fluctuates in sync with the hunting idle.

Separately, you could use a meter to check the resistance from the negative battery terminal to the engine block, to the transmission, to the O2 sensor, etc.  Also, check the ground to the ECU.