Surge issue

Tim T · October 19, 2015

Ok guys here goes. I'm lost, my mechanic is lost and I need some help. I have an 89 GMC Jimmy 4wd 5.7 tbi. The truck starts ok and runs fine cold. Once it warms up it surges. The rpms drop and then surge back up. It happens in or out of gear and only at idle. After replacing the 02 sensor, MAP sensor, EGR valve, ECM, ECM prom, having the TBI rebuilt, replacing the fuel injectors, plugs, wires, cap and rotor it still surges and runs ritch. The truck is in correct time and the TPS is within voltage. Now it will idle ok with surges at 3-6 second intervals. Turn on the A/C or heat and the rpms drop and the surging starts. Turn the ac or heat off and the surging continues. Any one have any pointers?
Forgot to mention truck goes from lean to rich. You can smell it and see it from the exhaust. I have checked for vacuum leaks as well as had the engine smoked. No leaks anywhere

Doug_Scott · October 19, 2015

Back in the old days an engine that was "hunting" (the term I was taught to describe what you are describing) almost certainly had a major vacuum leak. It should have been the first thing checked. Easy enough to check, first take brake booster vacuum line off of brake booster and pinch the line off close to end. If no change, take line off engine end, and plug the port on engine. If still there, take a good look at the pcv hose(no idea if your truck still has a traditional pcv valve) between base of throttle body and pcv valve. Squeeze that hose, it should not be soft, and most definitely should not be collapsed. Remove hose from engine side, and plug port that hose connected to.

If it is still hunting, you will need something you can spray along the intake manifold where it meets the cylinder heads looking for vaccum leak. Any change you get in idle quality should be duplicated after you have completely checked both banks of cylinders. If possible, disconnect EGR vacuum line(again, if equipped).

To me, this should be the first thing checked whenever you have an idle quality issue, and for sure it should be checked before any money is spent on parts. A vacuum leak can make a number of sensors "appear" to be defective.

Tim T · October 19, 2015

Doug. I did all that with the exception of the brake booster. I also replaced the PVC valve. The mechanic also "smoked" the engine and found no leaks either. The vacuum leak was my first guess as well.

grampadirt · October 19, 2015

Check the IAC motor.

Tim T · October 19, 2015

I replaced the iac. Watched it go to zero once the exam took over.

grampadirt · October 19, 2015

Go to alldatadiy.com and join up,I did years ago.Tons of info there,it's helped me many times.For $26.95 for the first year and $16.95 for each sudsequent year it's well worth it.It should help you.

If the IAC valve goes to 0 counts(@idle)it cuts off all air to the intake and probably why your engine runs rich.The Engine Coolant Temperature sensor can have an effect on this.Sounds like your engine runs fine in Open Loop then when it warms up and goes to Closed Loop it starts having a problem.