TBI sensitive off idle

[kenh]

1990 C1500, 5.7, 220K on the clock. Runs like a top but....

Come to a stop light it is difficult. The throttle is very sensitive, in it is hard to leave the stop without a surge of power. Sometimes to the point of almost spinning the tires. It will spin in wet or over a painted line. Reminds me of the "hit" of a two stroke MX bike.

Is there a way to soften this "hit"? Maybe open the throttle plates a tiny bit more and let the IAC close some to maintain idle speed????? Thinking out loud on this.

Other than this the engine has smooth power delivery.

Thanks!

Ken

 

[351FUN]

Something's gotta be up, touchy throttle is not a TBI trademark. Even spinning tires is a task usually. I'd check out the cable and linkages, see if something's up with them.

 

[PlayingWithTBI]

The throttle is very sensitive, in it is hard to leave the stop without a surge of power.

I don't remember, do you have a modified TB? Something like the "Ultimate Mods"? That may cause harder than normal acceleration. The ECM can be tuned to accelerate easier by flattening out your SA and VE at lower MAP vs RPM tables.

 

[kenh]

Engine and TB is bone stock. The TB could use a rebuild as it's dirty and I don't know the history of it. Only owned the truck for about six months now. I'll check out the linkage for anything that will hinder it's smooth operation.

Thanks for the tips!

Ken

 

[Pinger]

Engine and TB is bone stock. The TB could use a rebuild as it's dirty and I don't know the history of it. Only owned the truck for about six months now.
I'll check out the linkage for anything that will hinder it's smooth operation.
Thanks for the tips!

Ken

If this is an issue with the throttle being difficult to begin with then thumping open when the resistance is overcome - there's a current thread about this.
Thus far, it seems to relate to the throttle plate sticking in the body under vacuum (ie, only when the engine is running). I got my Vortec to do the same when I backed the throttle stop screw out. Immediately resolved by winding the screw back in so that it stops the butterfly - not the butterfly bearing on the throttle body.
If this is your issue - look for the thread. Can't remember its title but it's recent.

 

[Pinger]

This one >> https://www.gmt400.com/threads/88-1500-5-0-throttle-sticking-closed.50383/

 

[kenh]

If this is an issue with the throttle being difficult to begin with then thumping open when the resistance is overcome - there's a current thread about this.
Thus far, it seems to relate to the throttle plate sticking in the body under vacuum (ie, only when the engine is running). I got my Vortec to do the same when I backed the throttle stop screw out. Immediately resolved by winding the screw back in so that it stops the butterfly - not the butterfly bearing on the throttle body.
If this is your issue - look for the thread. Can't remember its title but it's recent.

Yes it feels like the throttle plates are stuck then break loose. When I had the TB off I could see the plates were not completely closed. IIRC the engine should idle in that 200 to 300 RPM range with the IAC closed???? The plate opening "looked" about right for that.

Hence my question if the plates could be opened a bit more and let the IAC close some to maintain the correct idle speed. The idle speed screw is still has the factory seal so I know it has never been messed with.

I need to take a close look at the throttle shaft and the TB to see if some wear is present. That might contribute to the issue.

I'll take a look at the link provided!

Thanks!

Ken

 

[Schurkey]

Throttle cable frayed?

Connect a scan tool, look at TPS data.

 

[L31MaxExpress]

Something's gotta be up, touchy throttle is not a TBI trademark. Even spinning tires is a task usually. I'd check out the cable and linkages, see if something's up with them.

My TBI swapped G20 van was always very touchy on the throttle compared to the stock Q-Jet even with a 305 it was easy to spin both of the tires off the line and it was on 275/60R15s with a 3.08 gear and a locking differential. That is until I installed the later model TBI and throttle cable that had the throttle cable bracket bolted onto the throttle body and used a progressive linkage via an eccentric cam. That made 1/2 pedal about 1/4 throttle and tamed it down tremendously.

GM used a throttle cam and lip/tb housing designed to shroud off half the throttle up unitl about 1/4 throttle on the Vortecs to tame them. They also used a progressive throttle cam on the later TPIs and LT1s to make them softer than the early TPIs off-idle. When I TPI swapped the G20 van with the early style throttle body it would spin the tires with a light touch of throttle.

 

[L31MaxExpress]

You can see how responsive the early non progressive linkage stuff is in this video clip. Just a light touch makes the engine rip with acceleration if everything is in spec and the engine is running well. My brother moved the old G20 after I had backed it off the trailer and laid 20 ft black marks in the parking lot of my shop. I said what was that for. Said he barely touched the pedal and it just lit the tires up. He was obviously acustom to the DBW of his 1.8L Sentra at the time not the 500 ft/lbs of a TPI 383 on a hair trigger.

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