1992 Corvette Runs Rough and Loss of Power over 3000 RPM

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All,
I have a 1992 Corvette Vin P 350 automatic and it has me stumped. Nothing has changed on the way it runs and I have replaced the following. New Opti Spark (TWICE) new GM Unit. New Coil, New Module,
New Fuel Pump, New Injectors, New Plugs (Double Platinum) Factory, New plug wires, removed and cleaned EGR works perfect, Checked compression all at 190/200 PSI. Bought Snap-On Scan Tool and checked system. Oh, there were no codes and the check engine light was not on. With Snap- On Scanner I get a (28 QDM fault) Fan relay circuit. Strange since the Fan(s) both work perfectly as does the airconditioner. Other than that nothing shows to be throwing a code. Ohm checked the coil and nothing out of range. Replaced knock sensors. This does not have an ESC box since it is a 1992. The ECM does ECS function. With the Snap- On code reader all readings are within normal and it says it is OK to drive. The timing, sensors, and such all read within normal range per the Factory Service Manual Indications Page. The timing moves advance and retard when the throttle is depressed. I am at a loss and want to correct the problem. The nice people at Vista Chevrolet refused to work on the car. I assume since it is a 1992 and ALDL they did not want to take the time to diagnose the problem since it is not OBDII where all you have to do is plug in and get a code. Oh, in park when it reved it does rev up but slowly and in gear it stumbles over 3000 RPM. It idles like it has a miss but was doing the same thing before I started.
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fuel pressure? original filter? fuel pump?
--
RS
2006 FLSTNI
2002 1200C
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---snip the parts I don't have an answer to---

had a GM dealer try and tell me they didn't work on an 84 vette any longer. a quick call to GM and then a return call from that same dealer wanting to make an appointment to work on my car. hmmm? naturally, I took it to a different GM dealer and told them that GM said dealers would still work on my 84.
my2
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Plugged cats perhaps?
Von wrote:

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never have liked "cats"... :-)
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> Von wrote:
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Speaking of Cats, I just had a Flowmaster installed on my truck, dealer even asked if i wanted wild or mild. I went with wild. I swear i cant even hear a difference! due to the 2 or 3 converters ahead of it.. FLUSH $$$
--
RS
2006 FLSTNI
2002 1200C
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I got flowmasters on my 84. converters ???? :-)
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Try watching the:
O2 Sensor readings when you bring it up to 3K RPM, and see if it goes lean or rich.
Block Learn and integrator are also an important diagnostic reading. 128 and 128 is optimum, but Chip updates can make these readings worthless.
Get back to me with the O2 readings as you raise the RPM slowly and the miss just starts.
RK

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Changed the fuel pump and filter and checked the pressure and it was within norms with and without vacuum.
Checked the converters and they are free and open.
The O2 readings were right on target.
128 is right on where it shows on the Snap-On scan tool.
No aftermarket chip and it is seated correctly. No FETs look to be defective on any of the QDM ports.
Vista Chevrolet sucks (delete expletive)... I thougth I would take it to Bill Heard Chevrolet next.
The 28 QDM fault is something that I have a call into Gordon Killebrew in Kentucky. He has been ill and out of pocket. He is the guy that worked the Corvette Action Center during the C4 build and now has his own company and does phone consulting.
I am at a loss here as all the usual suspects have been changed and or inspected.
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maybe lost the hi resolution signal on the Optispark?
http://www.aicautosite.com/garage/encyclop/ency04g.asp
Von wrote:

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http://www.google.com/search?source=ig&hl=en&q=bill+heard+complaints
Watch out for Bill Heard
I think its a clogged exhaust , cats or pipes. Hold your hand at the exhaust as some one revs the motor. Good luck I hope you let everyone know what you do find.. Kickstart
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I have flowmasters and the airflow seems fine with the stock converters (there are two)... The high resolution on the opti-spark checks out with the Snap-On scantool. Thanks for the insight on Bill Heard Chevrolet. Vista is owned by Bill Heard and they were not helpful. The exhaust sounds ok but I suppose I will take the converters down and check for blockage next.
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to agree with Bob and kickstart, sounds very much like a blocked exhaust from the converter or an internally failing muffler.
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Dad wrote:

For the L98, backpressure at either the A.I.R. manifold or at the O2 sensor should be under 1.25psi at idle and under 3.0psi at 2000rpm. LT-1 ought not be much different.
-- PJ
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I am going to pull the converters and physically inspect them. But, before I do I will perform the backpressure test.
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My guess is this was an assumption tied into the statement "exhaust sounds OK."
AJM '93 Ruby coupe, 6 sp (both tops)
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My son had a Fiero that a few years ago that suddenly would be hard to start law,lacked power and would not accelerate over 10-20 mph ..this happened overnight....we knew a used car dealer and he offered to have his expert mechanic check it out for us..
Later on a dirt bag looking tramp driving an old wrecker came to the house and said he was a mechanic and came to check the Fiero..( I thought no way ).he started the engine ( V6 ) and walked around to the back of the car held his had to the exhaust pipes and said it was clogged converter...car was towed away converter gutted and all was well ....cost zero....I guess when you work in a used car lot you get to see it all ...so all the expert computer type mechanics really don't always know the simple facts of what fails on older cars. The obvious is often over looked ..the insides of the converter had just collapsed and jammed the whole thing up..
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Call Chris over at Corvettes of Houston. He may be able to help you out.
Empty3
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