91 High Idle

Marshrat · September 3, 2006

I borrowed my friend's bread truck to move my furniture. In exchange I promised to see if I could fix the high idle and stalling problems. It is a 91 delivery truck with a brand new Chevy 350 TBI motor. I checked it out today and found the EGR valve corroded and sticking bad. Replaced the EGR and now no stalling, but was idling higher than before. Replaced the IAC motor and now it is idling better than before, but still sometimes idles too high. What else can cause this? I figure it has to be some sensor or actuator causing it. Motor has just been replaced a few months ago and all componets were swapped over from old motor.

Marshrat · September 3, 2006

Think I fixed it. My friend called and told me that the mechanic had bent the linkage on the side of the throttle body to keep it running. I bent the throttle stop lever back a little too completley clost the throttle plate at idle. Reset the computer, let it idle a minute and then shut off. Cranked it back up and drove around town for about 30 minutes and came back home. Idled perfectly, never stalled once, and was very quiet and smooth! Was pretty fun driving around town being the bread man!

I hope that's all that was wrong with it. Still looking for any info on anything else I shoould check/adjust on this new motor setup before giving it back to my friend.

Marshrat · September 9, 2006

Help! I still can't fix it. But this is what I have found out. Something in the ECM is telling the IAC to close a little less than it should sometimes at idle. I have cleaned the TBI, replaced the TBI gasket, PCV, IAC motor, EGR and gasket and checked the TPS and Engine coolant sensor values and are right on the money. No vacuum leaks anywhere. I can restart motor by turning ignition OFF then ON for a few seconds, then OFF and immediatly START. It will idle perfectly at 400-500 RPM quieter and smoother than any motor I've seen. If I disconnect the IAC and play with the throttle it idles back down quickly and never stalls. With IAC motor plugged in, it idles down slower and never goes below 1000 sometimes as high as 1800! Then I just turn off and restart and it idles good again. What else could it be? I'm going back out to check it in the meantime.

SirSmapty · September 17, 2006

Help! I still can't fix it. But this is what I have found out. Something in the ECM is telling the IAC to close a little less than it should sometimes at idle. I have cleaned the TBI, replaced the TBI gasket, PCV, IAC motor, EGR and gasket and checked the TPS and Engine coolant sensor values and are right on the money. No vacuum leaks anywhere. I can restart motor by turning ignition OFF then ON for a few seconds, then OFF and immediatly START. It will idle perfectly at 400-500 RPM quieter and smoother than any motor I've seen. If I disconnect the IAC and play with the throttle it idles back down quickly and never stalls. With IAC motor plugged in, it idles down slower and never goes below 1000 sometimes as high as 1800! Then I just turn off and restart and it idles good again. What else could it be? I'm going back out to check it in the meantime.

<{POST_SNAPBACK}>

Make sure non of the spark plug wires are touching an exhaust manifold. I had a TPI Trans Am that had symptoms like you describe and that was the cause.