After doing several searches on both of these topics I have found these forums are packed with knowledge. I am sure with no doubt over time I will find my answer to my problem.

Here goes – I have owned a 1987 with a L98 for 15 years and has run fine. Basically it never gets driven more than 1,000 miles a year.
I think the problem started four years ago. After changing the intake manifold because of a coolant leak I also thought it would be a good time to change the distributor cap and rotor while it was out. Engine started fine but died after 3 minutes. While doing so I forgot a wire or ground strap on the ignition coil and it burned out the ignition module. I change the module and its has ran ever since. But with a HIGH IDLE of 1,100 to 1,200 and 800 in gear. I had a shop check for a vacuum with a CO2 smoke machine and they found nothing. Said it could be an internal leak so I changed the intake again with the same results.
Since I rarely drive it I have not had time to look at it for three years.
Recently I checked the timing. Base setting is fine 6/8 degrees. Idle lowers to around 900. When you hook the EST wire back up, the idle returns to 1,100. I checked the overall timing and it’s like it is locked in around 37 degrees. It will not move even as the engine RPMS increase over 3,500. I also replaced the ECM three years ago with no results. No codes are posted either. I thought about the IAC valve but have been focused on timing.
Sorry my first post is so long, just trying to supply what I have done so far.

JIM

 

!) the IAC valve is probably dirty or needs replacement, 2) the TPS (throttle position sensor) might be bad or in need of adjustment and 3) the throttle body might need to be cleaned.

 

I wish I had some advice my friend, but I'm just looking at your problems hoping to solve mine....

 

with post #2, my bets on the IAC.

 

I am still wondering why the timing would be locked in at at 37 degrees?

I will Certainly look at the IAC valve.

 

You should follow the FSM on setting Min Idle with the TPS and IAC procedures. You can also find the procedures online on many sites or threads.
You might also look a the Throttle Body to see if it needs cleaning. It is a simple job to really rebuild it by taking it apart and cleaning the body (I let it soak in Carb Cleaner for a day) and with a gasket kit, re-assembled it. Then Setting the Idle/TPS/IAC, my idle problems were resolved.

Check the MAF and its connectins. Use some MAF cleaner on the inside of the MAF.

Good luck, just a few items to check or do.

 

Idle is controlled by a stepper motor which moves a pintle in and out which allows air into the throttle body. This movement is called counts and the greater number of counts, the greater the air flow. Idle is determined by load (a/c on or off) and the temperature of the coolant (the lower the temp, the higher the idle, the greater the number of counts). A scan would help you see what those counts are, what the coolant temp sensor is sending to the ECM and what the ECM is trying to make it idle at (called Targeted Idle) but in view of the fact that you've ruled out a vacuum leak, your IAC may just be stuck. I'd do this, especially since you've disconnected it's electrical connection to remove the intake - which sometimes confuses the IAC and ECM. Press the accelerator slightly and start it. Let it run for 10 seconds and then turn it off for 5 seconds and then restart it. If that gets it right, you've fixed it and you can go do something else. If it doesn't, ground the diagnostic link and with the key on, you should be able to hear the IAC motor running as it extends the Pintle all the way into the throttle body cutting off any air through it's passageway. Then disconnect it and start it. It may not idle (depends on if the engine was warmed up first), but if it does, it should hit around 450 rpms. In any event if you got the Pintle to move doing this, plug it back in and repeat the procedure for starting it with the accelerator slightly depressed and running it for a couple of seconds, then off for a few and then restarting it. If that doesn't get it working, I recommend you scan it so that you can see what the ECM is doing or trying to do. If everything is normal - air is coming in from someone else or the throttle is hanging open (though I believe you've allready ruled out the linkage).