90 Tuned Port Idle Problem
Thread Starter
Instructor
Joined: Dec 2007
Posts: 111
Likes: 0
From: Muscatine Iowa
90 Tuned Port Idle Problem
I have a 1990 L98 6-speed that I am having problems getting it to idle correctly.
-Replaced TPS
-Replaced Throttle Body (reman)
-Replaced IAC
With the throttle cables unhooked, the car will idle fine, if you slowly rev the car up and let it down slowly it hangs up at 1300 RPM. If you blip the throttle and let it come back quickly, the car will idle back down.
The throttle body is brand new and works freely, the cables are unhooked, I tried it again without the TPS and IAC hooked up and it did the same thing.
It seems as though the vacuum will hold the throttle plate open (barely)unless you flip it quick and then it will idle back down.
Getting ready for a bon-fire after pulling all my hair out
Le Mans Master
Joined: May 2004
Posts: 5,866
Likes: 60
From: Hellinois
Finalist 2020 C4 of the Year - Modified
You've got to have something hanging up in the throttle body. Either the blades or the shaft somehow. Rev it up slowly, let off it slowly, then with it idling high just press the lever closed and observe the movement. It might take a few times to actually find it. Once you observe the movement then you can shut the engine off it will be a bit easier to spot with the engine off. One other thing, when the tb was rebuilt, did they put either bearings or bushings on the throttle shaft? If not then you may have a worn casting which is allowing the throttle blades to be sucked open a bit while the rpms are up. If this is the case then send it back to them with a jar of vaseline and instructions on what to do with it.
Burning Brakes
Joined: Apr 2004
Posts: 762
Likes: 2
From: Huntington ma
Double check the attachment/mounting of the TPS to the TB. I've seen replacement TPS's that have enough of a protruding screw, or rivet, holding the lever arm on that the protrusion gets jammed into the end of the Shaft sticking out of the TB when you tighten the TPS mounting screws down.
You would Think that since both the TPS and the TB shaft are rotating together it wouldn't matter, but it Will cause the stickiness you describe. If that's your problem then Careful grinding of the TPS screw, or the end of the TB shaft, to get clearance there will fix it. Or a washer under each of the TPS mounting tabs...
Also look carefully to make sure that the TPS arm/lever isn't rubbing on the TB housing and sticking - not following the shaft back down to idle. I've seen That happen as well. You said it does the same thing with the TPS disconnected, so likely not the case here, but check it anyhow..
You would Think that since both the TPS and the TB shaft are rotating together it wouldn't matter, but it Will cause the stickiness you describe. If that's your problem then Careful grinding of the TPS screw, or the end of the TB shaft, to get clearance there will fix it. Or a washer under each of the TPS mounting tabs...
Also look carefully to make sure that the TPS arm/lever isn't rubbing on the TB housing and sticking - not following the shaft back down to idle. I've seen That happen as well. You said it does the same thing with the TPS disconnected, so likely not the case here, but check it anyhow..
Thread Starter
Instructor
Joined: Dec 2007
Posts: 111
Likes: 0
From: Muscatine Iowa
Double check the attachment/mounting of the TPS to the TB. I've seen replacement TPS's that have enough of a protruding screw, or rivet, holding the lever arm on that the protrusion gets jammed into the end of the Shaft sticking out of the TB when you tighten the TPS mounting screws down.
I will check this out
You would Think that since both the TPS and the TB shaft are rotating together it wouldn't matter, but it Will cause the stickiness you describe. If that's your problem then Careful grinding of the TPS screw, or the end of the TB shaft, to get clearance there will fix it. Or a washer under each of the TPS mounting tabs...
Also look carefully to make sure that the TPS arm/lever isn't rubbing on the TB housing and sticking - not following the shaft back down to idle. I've seen That happen as well. You said it does the same thing with the TPS disconnected, so likely not the case here, but check it anyhow..
I will check this out
You would Think that since both the TPS and the TB shaft are rotating together it wouldn't matter, but it Will cause the stickiness you describe. If that's your problem then Careful grinding of the TPS screw, or the end of the TB shaft, to get clearance there will fix it. Or a washer under each of the TPS mounting tabs...
Also look carefully to make sure that the TPS arm/lever isn't rubbing on the TB housing and sticking - not following the shaft back down to idle. I've seen That happen as well. You said it does the same thing with the TPS disconnected, so likely not the case here, but check it anyhow..
Thread Starter
Instructor
Joined: Dec 2007
Posts: 111
Likes: 0
From: Muscatine Iowa
Thread Starter
Instructor
Joined: Dec 2007
Posts: 111
Likes: 0
From: Muscatine Iowa
I removed the TB and everything operates freely. When mounted on the car without the cables hooked up it operates freely. When you start the car, the vacuum seems to bind up the throttle plate. While it was off, I tried putting tension on the shaft every way possible and it still worked fine. The minute you start the car, the throttle will not return to idle on its own unless you blip it hard. I add another spring to help it close. It helped a little but the problem is still there.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Le Mans Master
Joined: Jul 2007
Posts: 5,043
Likes: 8
From: Tampa Florida
I removed the TB and everything operates freely. When mounted on the car without the cables hooked up it operates freely. When you start the car, the vacuum seems to bind up the throttle plate. While it was off, I tried putting tension on the shaft every way possible and it still worked fine. The minute you start the car, the throttle will not return to idle on its own unless you blip it hard. I add another spring to help it close. It helped a little but the problem is still there.
Thread Starter
Instructor
Joined: Dec 2007
Posts: 111
Likes: 0
From: Muscatine Iowa
I had the same problem a number of years ago here's my fix.....I've been running it for years with no problems. I installed an extra spring (acordion style spring....it is painted black and hard to see) to keep the TB shut at idle.....it adds a little extra postive feel to the pedal as well.
BTW---No codes.
Le Mans Master
Joined: Jul 2007
Posts: 5,043
Likes: 8
From: Tampa Florida
I put the helper spring on it already. It made a slight difference. I can't believe how freely this operates with no cables on it and the minute you start it up without the cables, the vacuum won't let it idle down unless you flip the throttle plate really quick.
BTW---No codes.
BTW---No codes.
Good Luck!
Thread Starter
Instructor
Joined: Dec 2007
Posts: 111
Likes: 0
From: Muscatine Iowa
Now he doesn't return e-mails.
