Engine Performance problem
1998 GMC C2500 V8 Four Wheel Drive Automatic 194000 miles
Replaced plugs when noticing a drop in voltage from the alternator, plugs were way overdue. Truck ran great for a week then started running rough and getting progressively worse. Fuel pump failed, replaced pump and filter, did not correct engine running rough problem. Have since replaced cap, rotor, wires, tested fuel pressure good at 55psi. Everyday the truck runs worse then before, everytime I stop at a light it wants to stall it's running so rough, if I place the truck in neutral or park, it smooths out a little but still runs rough. I've tried injector cleaner and high octane fuel, no change. All of the above transpired over 3 weeks. Pulled all the plugs again, all appear to be firing though the trucksounds like it's misfiring. Any ideas? Running out of ideas here. Thanks
Check and test the following below: 1. Defective spark plug.
2. Inadequate spark/coil, defective spark plug wire.
3. Lack of compression
4. False air leakage.
5. Faulty fuel injectors.
6. Insufficient fuel pressure.
7. Contaminated fuel.
8. EGR valve that is leaking.
9. Oxygen sensors.
10. Throttle position sensor
11. Manifold absolute pressure sensor
April, 13, 2010 AT 8:24 PM
Today I ran a barrage of test still trying to solve this rough engine running problem. Pulled all plugs again, all are firing although both 1 and 2 are dirty, 3 thru 8 are all buring clean. Not sure if some indicator here. Additional, pulled oxy sensors and did back pressure test both sides, running 1-1.5psi, so I don't think a blockage problem. Did block plate test on EGR, sealing off ports, no change in engine operating so I don't feel the EGR is bad. Checked MAP Sensor checking DC voltage, input 5VDC, output 3.85VDC prior to applying vacuum, thought I finally solved the problem, replaced MAP, no change in rough engine operation. Checked Compression all cylinders, all at 140-145 (not cold), although not the best, truck has 194000 miles, so expected. No change in engine running, idles slightly rough, small surge about 100-150RPM, in drive engine runs very rough, sounds like it's missing off and on. Rechecked all fuel, pressure maintains 55 idle, slight increase under acceleration. Checked complete engine for possible vacuum leak, can't locate any possible leak and can't hear any whistling indicating a leak. Pulled Mass airflow to force service engine light to make sure it would come on, truck has had no fault codes prior to me tripping the light. I'm at a total loss here to solve further, any other help?