90 GMC Sierra 1500 350 NEW EVERYTHING idle problem

[koticimage]

Please help. I have brought my truck into 3 shops. Nobody has fixed the problem. I have a 1990 GMC Sierra 350 manual tranny. New engine, alternator, radiator, throttlebody, injectors, IAC, TPS, cap/rotor, coil, distributor, etc. The problem that I am having, is that after the engine warms up the idling is erratic. RPMs go from 650 to 900 then 550 and sounds like it is going to die, but never does. It then repeats the cycle.

[2000CAYukon]

I have owned my 90 GMC K1500 5.7 700R4 since new. I also had the idle problem you have.

I found that I had 2 vacuum leaks. One was the vacuum hose to the MAP. The other was the PCV gromet in the valve cover. An AC Delco PCV and new PCV gromet made the biggest difference.

It would also help to hook a scan tool up and see what the ECM is seeing from the sensors and what the fuel trims are at. In my case, before the PCV and gromet where replaced, I would see BLM reach 108 (128 is neutral); therefore the ECM was removing fuel and the idle would cycle up and down. I think the vacuum leak at the PCV would allow crankcase gases to build up and fool the ECM to thinking the engine was running rich.

//2000CAYukon

[maxwedge]

Another possibility is a poorly seating egr . Check coolant temps sensor also.

[koticimage]

Thanks for the replies. Those things check out. There are no vacuum leaks and EGR is new. I bought a new ECM, but haven't installed it yet. I was wondering if someone has ever had the PROM go bad. I had one of the shops swap another used ECM into my truck, but it made no difference. I bought the new ECM because I'm getting desperate.