1991 Camaro Z28 5.7 TPI
Car will not idle, runs rich, boggs or cuts off when throttle opened, basically spitts and sputters really bad. Timing will advance sometimes, and sometimes it wont. I have replaced the ecm (3 times), knock sensor, map sensor, coolant temp. sensor, Ign. module, plugs & wires, cap & rotor, etc. I had a very respectable local mechanic to hook it up and check it out on his machine, and he told me that all the sensors were working properly, and he didn't know what it could be, but was probably electrical. I checked tps, and fuel press. reg. as well. I also cleaned the iac. This is a nice car that i do drag race on weekends. It has hooker headers, flowmaster exhaust, BBK/Eddelbrock twin 58mm throttlebody, K&N filters, hypertech prom, accel coil and wires, trans. shift kit, etc.. When running properly, it goes 8.99 in the 1/8. Not great, but good for a stock engine. I need some help...any I get will be greatly appreciated. Please, somebody tell me whats wrong with my Baby...

I had that problem and changed my coil to solve it. It was stock though. Get someone to test the coils output through various RPM and under load.

I tried swapping out with the stock coil off of my 88 IROC, didnt help at all. I guess it could have been bad as well...the 88 has been sitting (not ran in a few years).

EGR valve under the plenum? Sometimes they get rusty and the frame breaks, or some soot gets onto the seat and becomes a vacuum leak or diaphram breaks and the valve falls open.

The egr looks ok. Its only a little over a year old...but i will check it a little closer.

As I read your post, I suspect the problem arose overnight so to speak.
Since the engine was OE'd with an O2 sensor and you have headers on it now, you have transfered the O2 sensor and have been running it ever since.

A couple things you might do, not necessarily in this order:
1. Pull a spark plug to see if it is fuel fouled, wet. If wet, too much fuel or not enough spark.
2. Pull the two-wire coolant sensor plug off and sub in a 0-1K or 0-2K pot and slowly reduce rsistance to see/listen if the rpm's smooth out. Is there resistance across both wires in the coolant plug? This resistance reads back to the ECM if good. Should be a high and low reading by reversing the leads.
3. Eliminate ECM ignition timing by disconnecting the single tan wire, This is used to set initial (base) timing. However, it will set an ECM trouble code.
4. TPS, does it have a smooth transition from idle to WOT? No open holes.
5. At ALDL connector, short pins 1 & 2 together, turn on ign, no start, for 15 seconds, turn off ign, unplug IAC, and remove jumper from pins 1 & 2. Start engine, This is base idle, 500-600 rpm, neutral, no accessories on. Vacuum should be around 18-20 in./Hg. and steady. Turn ign off, reconnect IAC, idle should be ECM controlled regardless of load applied.
6. Charging voltage steady? No great jumps of more than 0.3 volts.
7. Charging voltage at distributor power wire?

You may need to disconnect battery to erase DTC's and take car for a cruise up tp 55 mph for it to relearn.

Havnt had time to do any work on it lately, but will get back to it in the next few days, but yes, the headers have been on for a few years and the O2 sensors are in them, were replaced a couple years ago. Car ran great, just all of a sudden, started doing this, then stopped and ran good for about an hour, then bad again. All started with a few loud backfires when i would take my foot off the gas while driving (running rich I assume), then it got like it is now, wont hardly run at all...all within the same day I believe.
1. Plugs were fouled...black and wet with fuel-Replaced with R45TS Delcos
2. Havnt pluget in a pot, but have replaced the coolant temp. sensor. (replaced it twice a few months ago before we found out ecm had gone bad, then replaced it again when this problem started)
3. left that wire hooked up when i checked the timing so i could see if it was advancing properly....it did, but I checked it again later with wire still plugged up, and it didnt. I plan on checking more into this in the next few days as I have time.
4.TPS checks fine, voltage moves with no open spots that i could see.
5.Have not did this yet, does this basically just reset the idle?
6. Seems to be good now, I did find that the battery and alternator had both started to go bad after this problem started, i replaced them both, thought that would be it actually...but not that lucky evidently
7.Havn't thought to check voltage at dist. power wire, but i will now
Thank you for all your suggestions, i will try these things as soon as i can find the time and will post my results. If you can think of anything else, anybody, please let me know. I am ready to get my Baby back on the road....and track!!!

Number 5: I may be mistaken, with jumper in place, key on, engine off, unplug IAC. Try to unplug with key off after jumpering first to see if IAC stays extended. Yes, this is base idle setting.

ECM is in diagnostic mode and opens EGR solenoid, extends IAC into its seat, and opens charcoal cannister purge solenoid.

You might unplug the O2 sensor as ECM would go to default setting, i.e., 0.450 volts, perfect 14.7:1 ratio.
You're certain there are no vacuum leaks?

ok, i finally got it fixed just before i went completely crazy..... there were 2 problems. The first was the adapter that hooks the hypertech chip to the computer, basically allows use of a maf chip with speed density computer. The second was the # 1 and # 5 injectors had shorted out almost completely (less than 1 ohm on one of them, and just a touch over 1 ohm on the other) causing that whole bank to go lean while the computer was telling the other engine bank to run rich, and take timing out instead of adding like it should. Thanks everyone for your help. I am greatly appreciative.