2001 GMC Other

Engine Performance problem
2001 Other GMC Models V8 Four Wheel Drive Automatic 295000 miles

cylinder #4 and #6, missfire.
i changed spark plugs, wires, cleaned the injectures, change the gaskits on the intake manifolt, swich the coils between bank 1 and bank 2, changed the CMP sensore, change the fuel and air filter, and I changed the lifters, and I still have missfire on those two cylinder, , any idea, thx
September 3, 2010.

Engine running rough or misfiring can be cause by the following: 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. Idle air control valve.

Sep 3, 2010.