GM TBI Engine Idles High

If you’ve been to this site before, you might be familiar with my 1991 Chevrolet C1500 project truck. Long story short, I needed to swap the V6 in the truck for another engine (it had a rod knock). I got another 4.3L TBI V6 from a salvage yard. I recently got to the point of starting the new engine up, and it would idle at 2,000 RPM for 30 seconds or so and then drop to 1200 RPM. Both of these speeds are too high for idle.

I reviewed my copy of the Haynes manual for this truck (you should buy one here if you haven’t, they’re extremely helpful: 1988 - 1998 Chevrolet and GMC Pick-up Repair Manual). Here’s a list of troubleshooting items for a high idle:

  • Sticking throttle linkage
  • Idle speed incorrectly adjusted
  • Valve clearances incorrectly adjusted

I’m surprised they didn’t include “vacuum leak” as a possible cause. That’s essentially what mine ended up being, except it wasn’t located at a vacuum tube or port. I found my solution here: [SOLVED] extream high idle problem on 1990 chevy 1500, 4.3 v6 TBI, with this post:

Hi NID FTL

Two things come to mind very quickly.

1. Leaky throttle body base gasket.Very comon on this engine.I don’t know if you serviced this when you pulled the intake.

2.Misadjusted or bent throttle linkage.You don’t know what was done before you got to it.

If these ck ok there are other more complicated issues you would need a scan tool to ck.

IAC count, EGR percent,TPS etc.They all have an effect on idle speed and may not set codes in an OBD 1 system.Don’t forget a leaking pcv valve (Too obvious).Also a bad coolant temp sensor or connecter.Try wiggle test on 2 wire connecter near thermostat housing with it running.You may get lucky.Harness has been known to go bad at sensor connecter.

Good luck,Cardoc.

I popped my throttle body off and the gasket was in terrible condition with multiple sections missing, as you can see in the picture above. I still had the gasket from the previous engine and throttle body, so I threw it on there to see what would happen. Boom, perfect idle! So if your GM TBI engine has a very high idle, check your throttle body gasket. It might be letting in too much air and be causing your issue.