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98 Chevy 5.7 Misfire
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Question
gmax1
My 98 K3500 with 161k was throwing a lot of codes. Replaced all 4 O2 sensors. Then Mass air flow sensor, new AC Delco platinum plugs, new Auto Zone plug wires, New NAPA distributor, tried switching NAPA cap and rotor with new GM brand. Mechanic (Jeff- former Honda, Toy, Subaru dealer mechanic) found cracks in intake manifold gasket and replaced with new upgraded Felpro brand. Still running rough. Jeff unable to find cause.
Brought truck to Dick- former GM dealership mechanic. Dick went through all of the diagnostic proceedures Jeff went through. Truck runs okay on start up but within 1-2 minutes starts running rough at idle and gets worse when put into gear. This has been problem from beginning. Number 4 and 8 plugs skipping the worst. All plugs on that side badly sooted up. Other bank of plugs (odd #) looks good. Sometimes misfire carries over to #1 plug. Truck is blowing black smoke and burning excess fuel. Compression in all cylinders is 175 to 185 with one cylinder showing 160.
Dick pulled O2 sensor before catalytic on passanger side and engine quieted right down. Says truck has plugged catalytic so new cat and muffler assembly for that side ordered from GM and installed. Ouch! Slight improvement but still running very rough, no power, misfires on 4 and 8, and spark as before advanced by 22 degrees.
Pulled NAPA distributer with the new GM cap and rotor button and switched it with a used GM distributor and again used the new GM rotor and cap. No significant change. Replaced central port fuel injector unit with one from a junk yard that supposedly came off a running engine. No significant change. Fuel pressure is 54 or 55 psi.
Headed back to junk yard tomorrow to switch used CPFI with another used one they have that is also supposedly off a running engine. If this doesn't work my next step will be to bring truck to GM dealership and have them diagnose it for possibility of installing new GM CPFI as a cure but that's a $500 - $600 fix and currently I have almost $2000 into labor and parts with Dick. Dick thinks the CPFI may be the problem. If it isn't he does't know what to do next as he has tried everything in his bag of tricks and has consulted with other mechanics he knows. We're both frustrated. Nothing anyone has done has resulted in significant improvement.
This thing is beating my wallet hard. Any suggestions would be appreciated.
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