Hello, I am new to the forum and need some assistance. I have a 1992 Z28 305, 176K, TPI. I have recently had the engine rebuilt, 20 over, everything new. New cat, and Magnaflow. New MAP, IAC, TPS, and TBI. Symptom: Idle surges when warm, TPS voltage set at .90, 02 voltage does not move much, between 640-712. Cannot time it because idle is too erratic. When lead is unplugged to set timing it does not change idle, nothing?? I have checked for vacuum leaks, replaced fuel pump, filter, nothing works. Does anyone have any ideas? I am pulling my hair out on this one. What do you think about changing the 02 and the EGR? Is it possible I have a bad ECU?

When i replaced my engine, i had a similar problem, turned out to be a coolant temp sensor, just unplug yours when the car is on, if it stops surging than you need a new one. My mechanic mentioned something about the car not know the temps at startup and such and because of that it was putting in too much or too little fuel. Also, i recently developed another small surge, turned out the second time around, it was my o2 sensor causing it.

Those 2 sensors were the only sensors i reused from my old engine, i have come to the conclusion that old sensors, and new engines dont mix well. Both parts cost me about 30$ total from pep boys.

Always start with the little things before going and replacing something expensive like the ECU

Is it TPI or TBI...?

Thanks I'll try those two sensors....and update. I have it at the mechanic now since I couldn't figure it out.

Any idea why the timing lead doesn't make a difference in idle when unplugged?

I think he means he put a new throttle body on his TPI engine, As far as i know there is a TB on the TPI engines?

Yeah, it's tuned port injection with a throttle body on it....thanks

Okay, here is a quick update. Chevy mechanic says that the EGR maybe sticking....due to the fact that now the engine idle only surges/hunts for idle after it is warm. So he says that it rules out the coolant temperature sensor? Do you believe that they are correct? I personally do not....please let me now your opinions.

Also, they still don't know why the timing lead makes no difference with the idle when it is unplugged...any ideas?

I had that same symtom on my 89 IROC. It only surges at idle after warm up right? After replacing a couple of sensors, found out I had a EGR leak. It ended up to the EGR gasket only NOT THE EGR itself. I almost bought one when a mechanic had tested the vacuum and verified it was only the gasket which only costs $5.00 bucks instead of $200 for the EGR itself. Check that out first.

my new egr only cost 65$, but i have a TBI, not sure if there is a difference, if its surging while warm it sounds like your mechanic could be right, check to make sure its not a gasket though, i believe you can push open and close an EGR with your finger to test if its sticking or not, but dont hold me to that. Also a coolant sensor is only 9 bucks, but mine would surge when cold and be fine while hot, so the complete opposite of your problem.

Z28 is back home and fixed. The carbon canister vacuum lines were backwards and teed off to the MAP Sensor back to the intake for the option of cruise control. My Z does not have cruise control. After the lines were ran correctly the fuel trims normalized and the idle surge stopped and the timing could be adjusted. Which was 6 degrees off. It runs like a dream and very smooth. Hopefully if anyone else has this same situation they will check their vacuum line routing for accuracy. If anyone has any questions feel free to let me know I'll help where I can.