Good morning,

I'm having a problem with the engine warm idle speed. Stone cold, the truck turns over ata steady 800 to 850 rpm's. As it warms up, it drops to 500 rpm's. This is warming up in the driveway.

First question. Isn't 500 rpm's low for warm idle?
I disconnected the MAP sensor and the engine started surging and I got a check engine light. I am thinking the MAP sensor is good.
Next I removed the rubber fitting to the vacuum line goingto the duty cycle purge solenoid. The engine briefly surged, then steadied back down to 500 rpm's. ThePCM must have adjusted the IAC stepping to account for the extra air flow.
I also removed the screws to the IAC, using astraight slot screwdriver as a spacer so the IAC plunger could not fully close, increased the rpm's to about 650. I read somewhere on the forum that you can manually set the step counts using the proper scanner . Would this be a viable fix?

Second question.When I put the truck in gear,the rpm's drop by 200. Is this normal?

Thanks,

Scott

 

does truck cut off?

if not.
your fine
you have a good running engine.

 

The truck runs pretty sweet during normal city and highway driving. It's when I stop (stop light or stop sign)that I'm having a problem.

Thelow idle speed is causing the oil light to comes on and the oil guage intermittantly goes to zero. The engine also starts running a little rough. I don't have this problem if I keep my foot on the gas a little.

Work I done on the engine and exhaustover time is I have replaced the cat and pre O2 sensor, timing chain, plenum pan, rotor, cap, plugs, spark plug wires, cam and lifters. A few weeks ago I had the battery disconnected for a couple of days while I replaced the stock steel plenum pan with a Hughes aluminum pan. I was hoping that would have affected the idle speed. It didn't. I hate to replace parts as part of shotgun trouble shooting. So, if anyone can help me get to the bottom of this, I will grateful.

Thanks,

Scott

 

for comparison, mine runs about 800-1000 rpm when cold, then drops to about 600 at warm idle. when shifting to drive or rev, it drops about 100 rpm but idles steady.

i had an old chevy truck that idled poor for a lot of reasons, sometimes for vac leak, and then later it had some kind of torque converter drag that would kill it at normal idle.

reset pcm first. disconnect neg battery and turn key to start for 30 sec.
you might replace IAC, but it sounds like its generally working right. not too expensive at $50.
check for vac leaks - even introduce some to see how it acts. disconnect/plug all vac to see if it improves.
good luck.