I have a 93 TBI Suburban. When I let off the gas it almost seems like it is on cruise control. It just keeps going and very slowly starts to de-accelerate. The butterflies operate freely and their action is smooth. If I shift the tranny into netural the motor still races. If I tap the gas pedal hard the idle will usually return to normal. Any ideas?

 

Sounds like your throttle is sticking caused due to bracketry, vacuum leaks. You will have to keep this thread TBI related to keep it alive. Since it is not 3rd gen related we should lock this. However, I do allow general TBI related questions to live because many 3rd genners use TBI systems out of other GM cars and since they use this site to diagnose their problems I usually let them stand.

 

Alot of times the pressure difference from vacuum as well as the force from the return springs can cause the tbi base to wear in an egg shape around the throttle shaft. Itll cause the throttle blades to bind up and hang open after you let off the gas. It could also be the IAC. If its sticking after it follows the throttle opening it can cause the motor to run on, but it racing sounds more like a sticky throttle

 

Try hooking up an external return spring and see if that helps. Don't go too heavy though, you might exelerate the wear that has already taken place. Spraying a little PB Blaster penetrating oil around the throttle shaft might help also.

 

It's probably idleing too high or the IAC is letting in too much air. Especially, since I noticed in your sig it says you have a GMPP 383. You need to modify the prom for that motor quite a bit. I bet your fueling and timing are way off and that's why its doing that.

 

I do have a GMPP HT 383 in the Suburban. I also bought a TBI from CFM Tech and matched injectors from Turbo City along with their chip for this motor. I did run a return spring on it today and I did not have the run on condition. I also ran my scan tool and was watching the IAC direction and position. On the way to work the IAC would be in the FWD position during accell and cruise , the position was around 130. On decel,the direction was REV and the position would drop to 80. On the ride home, position was 105 and decel was 52.

 

SOooo..., The return spring helped?

 

Yep, Sure did. I should get better gas mileage now to because it is so damn hard to mash the gas now. Next is to find the real problem.

 

The TBI unit came from CFM Tech and should have been rebuilt with new bushings. I think though that they get all of their bodies from Turbo City.

 

Try a lighter spring! leadfoot!

 

Build/get a scanner, and try seeing what the engines actually doing.

If the RPM are following the IAC counts, then it's normal. If the IAC counts are out of whac, then there's something else going on. Without any real data to look at you're just guessing or possibly throwing parts at it until you happen across the cure.

That being said, try cleaning the IAC and it's passageways.

 

Pulled out the IAC and there was a little piece of trash in there. Cleaned out the passage ways and also put on a new gasket, the old one was ripped. Removed the spring I installed and all seems to be ok right now. So not wanting to leave well enough alone, I installed a VFPR.