Misfire 3.8 Chrysler Town & Country, 185K mile, 2003
 

Symptom:
Codes PO303, PO306, PO300; very rough idle and misses at all rpms.
Sooty wet exhaust. Mostly highway miles, but some in-town miles also.
I have notice headlight lights slightly dim at deceleration at 20-30 mph.

Diagnosis completed so far:[*]Compression tested all cylinders (cold engine and dry cyl). Pretty consistent ~145psi all cyl.[*]Performed leak down test. Homemade tester @60 psi .034” orifice and got 10% leakage.[*]Pressured each cylinder to 70 psi @ TDC and no bubbles in radiator.[*]Fuel pressure at fuel rail 45 psi. I have not checked the flow, but van will easily run 85mph.[*]Performed smoke test & sprayed starting fluid. No obvious vacuum leaks.[*]Looked at valve springs on cylinders 2,4,6 no broken springs. Haven’t check cylinders 1,3,5.[*]Clean & bench tested injectors. Moved #3 & #6 injectors to difference cylinders, but miss stayed at 3 & 6.[*]Ohm tested Coil – prim & second at spec and all cylinders have spark. I did not load test the coil.[*]All plugs (platinum NGK) look tan and no excessive wetness or carbon. New Mopar wires 3 yrs ago.[*]Checked sensors IAT, IAC, TPS, MAP, PCV. All appear in spec per shop manual (no EGR valve on engine).[*]Checked continuity on power and signal wires to coil – all are good[*]Checked for chafed/burnt wires by exhaust cross-over pipe. All were good as was the connector.[*]Checked connectors to PCM. All connection look like new.

Had local shop hook to more sophisticated test equipment and he found nothing. I am not sure what he did, but he said the PCM is good. He said he thinks the issue is carbon around the valve seats and he could fix for $1000. I checked with some dealers and they agree it could be carbon on valve seats, but I am not convinced this would make such a rough idle and miss at all rpms. Anyone with good diagnostic knowledge, please chime in. This one has got me stumped! Thanks.