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Anyone have a sugesstion?1994 running rough

1406 Views 4 Replies 3 Participants Last post by  Gary Carstetter
Hi everyone,
1st thank you for looking, and hopefully someone will have an answer.
My 94 standard coupe started running rough after warming up (closed loop). These are the replacement parts and tests my mechanic (certified GM) has completed. The car is still not running properly. Any suggestions that will help solve this problem would be qreatly appreciated.

Replaced: complete distributor(cap and rotor included), plug wires, spark plugs, fuel filter, air filter, ignition coil, all eight fuel injectors(Accel stock replacement), used PCM with used internal knock module(original PCM and module now reinstalled in car).

Swapped with 96 LT4: Manifold Absolute Pressure sensor(MAP sensor), ignition coil module, Mass Air Flow sensor(MAF sensor) and duct work.

Checked: fuel pressure is 35psi at idle and 46psi under load during condition(in spec), high and low resolution signals from distributor steady, graphed throttle input(OK), fuel trim readings well within normal ranges, performed engine top engine clean to remove carbon, checked exhaust backpressure(well below spec), coolant reading always steady around 200 degrees, checked EGR valve operation(operating propery in service bay-no way to really test during condition), load tested all power and ground feeds to PCM, injectors and ignition coil, disconnected tachometer wiring from coil-same condition, recorded multiple data "snapshots" to graph and send to GM technical assistance, road tested with a vacuum gauge during condition(steady reading).

Thanks again for checking this out. Gary
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Your certified GM mechanic can't help?

Check for any trouble codes?
Sorry forgot to mention that the car shows no codes.
Running Real Rough When Hot

I would start by doing a HOT compression check to see if all 8 cylinders are up to snuff. If you have a weak cylinder no amount of "tuning" will ever help it.

It's possible you have a leaking valve or a lifter that is rapidly bleeding down. I have also seen many valve guides wear out so bad the rocker arm was sitting real ****eyed on the tip of the valve. So I suggest pulling the valve covers and check things out.
Funny how you mentioned the valves. The mechanic scoped a couple of valves today and informed me # 1 exhaust valve looked pitted. He was going to do a test Monday to verify a valve
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