2004 3.9L Misfires P0300 P0304 P0305 P0306

[rides2fix]

Note: I started posting this problem under TPS at 10% when closed, but wanted a more appropriate title.

I have not been able to detect any vacuum leaks and have replaced the plugs and wires with not improvement. The coil pack ohms out OK and I have switched the cylinder pairs at the coil pack on the affected cylinders, but the misfires still occur on 4,5, & 6. My scan tool consistently shows misfires on these cylinders under the Mode $06 (diagnostic tests) data and shows that there are no misfires in cylinders 1, 2, & 3. Sometimes I don't have misfires reported in either cyl 5 or 6 so there is some randomness for whatever reason. I don't notice misfires too much at idle, although some do occur. While holding the throttle at 2500 RPM I can feel the car shake due to the misfires. The misfires can be heard when the engine is under a load and accelerating; even short quick accelerator jabs result in misfires you can hear. Since the misfires are in adjacent cylinder, I performed a compression check and do not see anything wrong there either. I have checked the fuel pressure and the mechanical gauge agrees with the scan tool reading with the engine off and does not suggest any fuel pressure problems at idle or 2500 RPMs. As a precaution, I did replace the fuel filter. I need to locate it, but I think there is an exhaust manifold leak, but not on the bank I am having the misfires on.

I think there must be some clue to what is going on since the misfires are in adjacent cylinders. Can I rule out evap emission issues - wouldn't that affect all cylinders?

Thanks for any suggestions or ideas.

John

[rides2fix]

I corrected a typo that incorrectly stated that I was now seeing misfires on cylinders 1, 2, & 3. I am seeing no misfires on these cylinders. Also updated the fuel pressure information.

Any suggestions on what to do next will be greatly appreciated.

Thanks, John

[rides2fix]

Turns out the left bank catalytic converter is partially restricted - honeycomb fell apart. I was not able to detect this with a vacuum guage and was not getting any DTC for converter effeciency. A professional pointed me in the direction of the exhaust and a back pressure test confirmed it.