Engine stumbles after letting off throttle

[87ChevyR10]

Ok, truck has had this issue for a while now. It doesn't affect performance. Throttle response is fine. However, any time after I get off the throttle, the idle takes a dive and recovers. I have a feeling it's a vacuum issue, but could be something else. Truck has a 350 TBI w/ smog crap removed.

Any ideas where to begin looking? Thanks!

 

[chengny]

I am not familiar with GM TBI systems, but that is a textbook symptom of an IAC valve failure to open.

The valve itself could be sticking/sluggish and might just need cleaning. Also could be trouble with the wiring that connects the ECU to the valve actuator - or a failure of the ECU to command it open.

The ECU probably uses inputs from the TPS or MAP/MAF sensor (or both) to know when to command the IAC open. Maybe check them too.

 

[89Suburban]

Mine does the same exact thing by only at a certain temp/humidity. It will do that surging thing up and down on certain cold starts for a minute or two then goes away. But in cold weather I have that cold start issue with too much fuel being injected and having to tap the pedal down a tad to have it fire up on the first crank. It's still on my to do list to investigate this. Need to get my hands on an ALDL cable.

 

[87ChevyR10]

Ok, cool. So, how easy is this IAC valve to fix/replace and where is it located? Thanks!

EDIT: Also, is this IAC valve responsible for keeping the idle even at low RPMS? For example, I barely have my foot on the gas cruising thru a parking lot. The engine RPM's constantly rise and fall by a few RPM's. Would fixing the IAC valve correct this or is that a separate thing? Thanks!

EDIT 2: Is this the IAC thing you describe? http://www.autozone.com/autozone/parts/Valucraft-Idle-Air-Control-Valve/1987-Chevrolet-R10-1-2-ton-P-U-2WD/_/N-iigfvZ9gsof?itemIdentifier=966181_0_0_

 

[89Suburban]

Yes that is it, there is a certain procedure to install it.

 

[87ChevyR10]

Also, are there any noises associated with a bad iac valve? Such as buzzing? I hear buzzing coming from left side of tbi when facing truck.

 

[chengny]

Yeah but isn't that just the pintle?


Since he bounces back to normal idle RPM fairly quickly after a decel, that would seem to indicate that the pintle is receiving a signal and trying to stroke open. Often the case is that valve body is just gummed up. You should really try cleaning it before buying a new part.

Pop the valve open (the part shown above will come out of the valve body) you can even leave it wired up if there is slack.

Clean both the operating part of the valve (the pintle) and the body using off the shelf solvents. I use PBlaster first. Squirt a bunch in the body and on the pintle. Let the body soak for a hour or so. While you're waiting, keep the upper part wet and work the valve up and down.

After awhile blow (or wipe) the PBlaster out of the body and switch to carb cleaner. Spray both halves of the valves until clean. Reassemble and test.

Also, are there any noises associated with a bad iac valve? Such as buzzing? I hear buzzing coming from left side of tbi when facing truck.

Actually it's the opposite. A buzzing sound from a solenoid operated valve generally indicates that it is functioning. But that doesn't mean it can stroke - just means that the coil is energized and is trying to shift the movable part.

 

[87ChevyR10]

Great! Good info! Something else to add to the "TO DO" list when the temps start dropping. The truck is quite drivable as is. I only notice the idle dive when the engine is still cold. Once it warms up, it no longer does it.

Thanks for the help!