Won't Idle!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

I have a 92 Silverado 305 engine TBI. I recently gave it a tune up like i have done in the past (plugs,wires,cap,rotor,airfilter,pcv valve,fuel filter and ignition module) but know when i bypass the EST to time it it runs fine but when i plug the EST back together and clear the codes the idle goes up to 1,000 rpms and back down to 100 rpms back up then back down and keeps doing that. PLEASE HELP i ran out of options of what to do or what could be wrong. I checked the Knock sensor by putting a 3.9K ohms resistor in place of it and the idle is very low does that mean the KS is bad if not what else could it be.:banghead:

Remove the old PCV valve from the trash, clean it out, and install it on the engine. Did that help?

Sorry that did not help cleaning the pcv.

The easiest way to find out would be to swap back in the old parts, as mentioned above. I can't imagine that the plugs or wires would cause surging like this, try igition module? Cap/rotor?

swapping old parts back did not do anything. Looks like i am stumping all who is trying to help on this matter. Has this problem not happened before sounds like this is going one for the books. Thank you Guys for trying to help me on this matter it is greatly appretiated.

How much did you adjust the timing?

When you 'clear the codes'...what is stored?

This may be a long shot 'cause I'm not really familiar with your TBI. I had the exact same symptoms on my '01 Infiniti I30 when its idle air control valve went bad. I don't know if you have that or not, I though I saw somewhere that '92s with TBI did have a IAC motor. Sounds like the same thing to me.

Good luck

Did you clean the throttle body and IAC passages during the process?

The real baffler is that the idle is apparently stable with the EST bypassed, but not with it connected. You may want to check the connections at teh HEI module in the distributor base and at the distributor shell. The HEI module itself may have a problem.

My Sisters's Tracker does that and these guys are right it is something to do with the IAC Valve. Try getting a new one. Ailing that maybe a bad sensor.

I would check if you disconnected the throttle position sensor when you were working on it. These will cause the idle speed to vary. If you replace it use a quality part. It could also be loose and moving. You could have loosened the air intake causing the engine to get improper air flow. Check all work and be sure you didn't unplug or left something disconnected.

Check simple things first. Maybe a vacuum leak?? Maybe you left something unplugged, or accidently unplugged something in the process?