You seem to have the best handle of anyone on this, so here goes....

I have a 1987 Iroc with 5.7L... it will not idle once it comes down from starting cold... it surges and hunts between 400 to 700 RPM... it even would die before I replaced the IAC valve.. now it just idles roughly... it has 25,000 miles, and is cherry. Internally stock.
I have added an adj. fuel pressure regulator set at 46psi idle with vacuum line off... I also have an elec. gauge to monitor fuel pressure... the regulator IS making a knocking sound... I can hear the diaphragm in it... but I don't think that is causing the problem.
Here's what I've done... I've checked all vacuum lines for physical integrity, and done a fairly good job of listening to them with stethoscope... don't hear any leaks. I've replaced the plugs with AC delco R43ts gapped at .035 in anticipation of future nitrous, replaced the sparkplug wires with the best I could buy, new rotor and distributor cap, new O2 sensor, new IAC valve, new CTS sensor, checked the MAF wires for integrity, used a propane bottle and hose to check for intake leaks... checked for exhaust leaks...

Tomorrow I should receive a replacement EGR valve if I want to do it... if I have the plenum off, what else do you suggest I replace or check? Can I check the egr valve without removing it first to see if it's stuck open?

Will having my vehicle scanned help me in this case, or will it only show the ECM making the commands, and not tell me if the physical items that could be causing this are WORKING?

Man, I am at the end of my rope on this.... PLEASE help, bud.

Thanks in advance.
Jethro

 

Jethro,

You've got mail...

 

Today, I had ordered a replacement EGR valve, but I will cancel it... it's not OEM and I had real concerns there... now I am certain. Instead I will find someone with the AC Delco part once I pull it off.

I blew out the passages of the IAC and throttle body while they were off... using parts cleaner and then compressed air to remove it totally.

I forgot to mention an important variable... the car is running rich.
The plugs show it, and the muffler is showing signs of richness.

Thanks, and I'll post a follow up