TBI troubles- "sticky" throttle

[Flmarine71]

Hi guys- just picked up a 88 silverado 1500 with manual transmission. Not used to TBI. The throttle/pedal has resistance when you first get into it and then lurches free. Happens when the truck is running and shifting gears (most noticeable in 1st and 2nd) after throttle goes back to full close. I've researched here and it seems the likely issues are:
1. Throttle cable catching- I replaced with new cable but same issue. When truck is not running, there is no initial resistance in pedal.
2. Dirty TBI- inside doesn't appear dirty at all but I went ahead and sprayed throttle body cleaner in and let sit with no real difference.

There is a 3-4 second delay when blipping the throttle by hand (cable disconnected) before idle goes back down. I am assuming this is what's causing the "lurching" once the resistance goes away.

Could a bad TPS sensor cause these symptoms? I need to get some probes for my multimeter and planned on checking per service manual. Any other items you recommend checking? Thanks.

[Boog]

Does it idle unusually low?
The idle air control slows rpm drop to keep the engine from dropping too fast and dying.

My '91 had a sticky gas pedal and when pressed harder to accelerate it would then lurch. Mine also idled real slow and often died when stopped with foot on brake and it in gear. The IAC controls idle rpm for the most part and I am told it can get lazy. If it allows the throttle blades to close too far they can stick in the bore as mine did. That's where you get the lurch.
There is a hole on the front of the throttle body on the driver side front you can see in the pics. Normally there would be a plug over it but yours appears to have been removed. In that hole is a torx head "minimum idle speed" screw. A T20 I think it is. It is factory set to the minimum idle speed necessary to keep the engine running in case the IAC doesn't. If that plug is missing on yours someone may have turned it out thereby lowering the minimum idle speed and creating your issue. I began by turning that screw in a half turn at a time to see how much difference it made. You are supposed to unplug the IAC while you do this and jumper pins A and B of the diagnostic plug under the dash. You can google the procedure but that's pretty much it. It is covered in some repair manuals.
Of course timing and such should be checked first and set to spec.

[Flmarine71]

Thanks Boog. The idle seems normal but the tach doesn't work so I need to double check.

[1976gmc20]

I thought this was just normal for these old pickups?

[Mr_Rich]

That looks like an Edelbrock TBI maniold which puts the throttle linkage in a slightly different position than stock. That's a difference right there that I'd chase to make sure there is no binding. That linkage does not look right at a closer look. Not right at all.

[Just call me Sean]

It wouldn't hurt to check for codes.

[Flmarine71]

Originally Posted by Boog

Does it idle unusually low?
The idle air control slows rpm drop to keep the engine from dropping too fast and dying.

My '91 had a sticky gas pedal and when pressed harder to accelerate it would then lurch. Mine also idled real slow and often died when stopped with foot on brake and it in gear. The IAC controls idle rpm for the most part and I am told it can get lazy. If it allows the throttle blades to close too far they can stick in the bore as mine did. That's where you get the lurch.
There is a hole on the front of the throttle body on the driver side front you can see in the pics. Normally there would be a plug over it but yours appears to have been removed. In that hole is a torx head "minimum idle speed" screw. A T20 I think it is. It is factory set to the minimum idle speed necessary to keep the engine running in case the IAC doesn't. If that plug is missing on yours someone may have turned it out thereby lowering the minimum idle speed and creating your issue. I began by turning that screw in a half turn at a time to see how much difference it made. You are supposed to unplug the IAC while you do this and jumper pins A and B of the diagnostic plug under the dash. You can google the procedure but that's pretty much it. It is covered in some repair manuals.
Of course timing and such should be checked first and set to spec.

I increased the base idle with 1/3 turn in of the screw and the throttle is much smoother now. It doesn't feel like the blades are catching in the bore anymore and drives like a proper truck. Thanks guys.

[Boog]

Good deal. Thanks for the update.