Idle Issues 4.3 TBI
 

I am helping a friend of mine figure out a problem on his vehicle. He has an '88 GMC with a 4.3 TBI V6 that seems to have a too slow idle speed. In gear, it will idle slow, then it drops off to almost stalling. Of course, the slow idle speed causes it to chug badly. When I started it cold, the idle speed didn't seem to increase as much as a normal TBI engine does.

It runs fine under all other conditions, without any missing or chugging. It also has a new cap, rotor, fuel and air filters, which his mechanic changed to fix the problem, but of course didn't.

My vehicles are all carbed, so I am no expert on fuelies. However, I am thinking this is an IAC problem. Possibly carboned up, or not functioning properly? This vehicle is used for a lot of short trips, and probably almost never fully warms.

Any suggestions or advice appreciated.

Sure sounds like a IAC problem.

One quick test you can do, If it has A/C that works, you can turn on the A/C and see if the engine speeds up just before the compressor engages. The ECM should see the A/C demand and increase the idle speed before the A/C clutch engages.

Thanks for the response. I had actually performed the check that you mentioned, but I neglected to mention it in my post. When I turn the A/C on, I do not notice any difference in the idle speed before the compressor engages. So I guess this too would suggest the IAC?

Any other advice appreciated.