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Old 08-10-2005, 09:34 AM   #1
Jim85IROC
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Default part throttle stumble in 91 TPI Corvette

my 91 has developed a habit of having a nasty dead spot and/or stumble in the throttle at part throttle during acceleration. If I accelerate very easily it doesn't do it, and if I accelerate at WOT or close to it, all is fine. It's when I'm anywhere inbetween that it's having fits. It doesn't do it all the time, but it's doing it often enough to irratate the hell out of me. A while back I tried blocking the EGR to see if that helped, and it didn't. Just recently I replaced the 02 sensor, which also didn't help. When using my Auto X-ray, I couldn't find anything out of the ordinary, but I was concentrating on 02 voltage and knock sensor data more than anything else, so I may not have noticed a MAP problem if there is one.

I'm going to check the X-ray again, but has anybody else had this sort of problem? Right now by process of elimination, I've got to think that it's MAP related, but so far I've gotten no codes, my gas milage is unaffected, and drivability other than part throttle acceleration is totally normal.

Thanks for any help.
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Old 08-10-2005, 10:48 AM   #2
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I'm not sure if a dirty TB would do that or not??
You could try cleaning the bores and blades and then resetting the base idle.

Also, check the ignition for any loose wires.

Do you have any fuel pressure leak down issues?
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Old 08-10-2005, 10:55 AM   #3
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I'll bet it can just use a computer tune up.. Do you have any datalogs?
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Old 08-10-2005, 02:19 PM   #4
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I forgot to mention that I also swapped the TPS with the one on my IROC with no avail, and I swapped the throttle body with no improvement as well.

Why exactly would my car need a "computer tune-up"? I've made no changes to it that would require a PROM change.

I haven't checked any fuel delivery issues because typically fuel problems don't dissapear at WOT. Ignition could potentially be an issue, but again, I would expect that at cruise and WOT conditions, not just part throttle acceleration. The car has a new coil, cap, rotor, plugs, wires, and distributer ignition module. About the only part of the ignition system that I haven't replaced recently is the ESC.
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Old 08-10-2005, 02:29 PM   #5
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Has it always had this?

alot of the speeddensity cars come tuned from the factory with something left on the table.. alot of them have like a surge, deadspot, whatever.. Usually apparent to whoever owns the car and takes a sensitive driver to feel however it is still there.
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Old 08-10-2005, 02:30 PM   #6
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Have you checked manifold vacuum for a leak (though something like a leaking EGR valve usually gives idle problems too)?

You might check inside the dist cat to verify that the rotor is still securely attached and check all plug wires for firm "snap" into position.

I'm not familiar with SD setup so don't know what issues with the MAP might bring.
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Old 08-10-2005, 04:22 PM   #7
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I can try checking vacuum, although I'm not sure how I'd diagnose the vacuum when the condition occurs.

It did this before and after my new cap and rotor change.

This problem hasn't always been there, it's something that developed rather slowly over time.
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Old 08-10-2005, 04:56 PM   #8
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You can lay the vacuum gauge hose under a wiper blade with the gauge face taped onto the windshield.

I use this method with my FP gauge when I want to check for fuel delivery problems and it works well.
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Old 08-10-2005, 04:56 PM
 
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