Q: Replaced bad lifter but still getting intermittent misfire code on #6 cylinder - wires, coil, plugs all replaced so now what?

asked by on October 12, 2017

Bought 1999Dodge Ram Van 3500 last October. Need to get smogged and reg. OBD2 showed misfire(P0306) on #6 cylinder. Replaced plugs, wires, coil,cap and rotor. Still blew code for intermittent misfire on #6. Found bad lifter. Took apart and replaced lifter. Checked injectors, wires, ran Sea Foam thru and Marvel Mystery oil . Cleared code...stays clear for a couple-few days then pops code P0306 again. Plugs look fine. Compression is same on all cylinders. Runs good, but every time I try to smog it it blows code on 6 before going in. Already paid $130 for 2 tests, ECU not ready because of code popping up. My friend mechanic thinks it might be the camshaft. OBD2 Showed upstream O2 sensor code and p0306 again. Waiting to get both 02 sensors now, plus new dipstick. Can't shake this P0306 - code keeps ruining my day, my life at this point. Must get smogged and reg NOW (was due in April!) Anything else besides canshaft that could cause this intermittent misfire on only one cylinder?

My car has 140500 miles.
My car has an automatic transmission.

You might have a vacuum leak around the intake to that one cylinder. If so, that can cause a random misfire. If the lobes on the camshaft are worn, that can cause a misfire and other inefficiencies as well. I would first check for a vacuum leak. If no leak, secondary firing patterns on the misfiring cylinder should be examined on an oscilloscope. The waveform will help narrow the potential causes to fuel versus ignition. Another possibility is the random misfire is due to intermittent faults in the injector and/or ignition components so although these components appear to be in spec during static testing, in fact they are not reliable and are later, in service, operating intermittently. Finally, the ignition and fuel driver circuits in the PCM for the misfiring cylinder could be faulty; that is the absolute last thing to test though unless you happen to have handy a known good PCM that you can substitute. If none of these possibilities pan out, you can request a misfiring diagnostic and the responding certified mechanic will get this resolved. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.

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