1989 GMC C1500 Erratic idle, only at start up

GENERAL89

MEMBER
1989 GMC C1500
V8
2WD
MANUAL
272,000 MILES

This is an 89 GMC, with a 350 in it. I just bolted the throttle body back on and torqued it down to factory specs. I also disconnected the negative batt. Cable. I restarted the truck, and it ran fine. The next morning, I cranked the truck up. It cranked and started fine, but almost immediately the rpm dropped from about 1000rpm to about 400, and acted like it was going to die. It acted like a vacuum leak until I hit the gas pedal a couple of times. The rpm seemed to level out to high idle. Once it warmed up, it idled fine the rest of the day. This happens everyday, when the motor is cold.

I have tried warming the truck up, disconnecting the batt for at least 30 seconds, reconnecting the batt, turning the engine on for 5 seconds, turning the engine off for 10 seconds with the key on, and then cranking it back up. I drove at highway speeds for about 20 miles, and called it good. It had the same problem the next day.

1 Reply

CARADIODOC

MECHANIC
33,858 POSTS

Well, it's been a few hours and no smart people have replied, so I'll make a comment. This is a real common complaint with Chrysler vehicles and has an easy fix. When the battery is disconnected, the Engine Computer loses its mind. The engines run fine except for the low idle / stall concern. The computer must relearn "minimum throttle" before it will know when it must be in control of idle speed. The conditions required to trigger that to occur are very high manifold vacuum for at least seven seconds. All that's needed is to drive at highway speed with the engine warmed up, and coast for seven seconds without touching the brake or gas pedals.

I never heard of the need to do that with GM vehicles, but it's something to try.

Caradiodoc