Intermittent Misfire

[ImDino]

I have a 2003 Navigator Ultimate with the 5.4L 32V engine (112000 miles). It has an intermittent misfire (violent 'bucking' at high speed when it happens). It seems to be worse on HOT days. The check engine light has never come on and there are no DTC's when I check with my OBDII code reader. All service is up-to-date (new plugs, fluids, filters, etc). The dealer could find no codes with their equipment. A mode 6 test was performed and they said everthing looked OK. They sold me a new battery (low CCA's) but could not offer a solution without a check engine light. Any thoughts? Bad coil, coil boot, injector, valve - how can you tell with no DTC's?

[pro-five-oh]

Its tough without DTC codes. I would first get your fuel pressure checked and make sure the fuel filter was changed. If that comes up with nothing, then you need to get all 8 coils tested (too much resistance, maybe).

[megaforcer]

I just had the same issue on my Town Car. Very light misfire at highway speeds. No check engine light. I hooked up a Snap On Modis to it and it showed nothing. I just changed all the plugs and coil boots and it took care of the issue. Coil boots are about 10 bucks at the dealer, its 80 bucks well spent. Try to change the plugs again, make sure you gap them right and throw some new boots on it, see if that works.

[Fordace]

TSB 06-5-13

ENGINE MISFIRE WITH DIAGNOSTIC TROUBLE CODE (DTC) P0301, P0302, P0303, P0304, P0305, P0306, P0307, P0308, AND P0316.Publication Date: March 7, 2006FORD:
2003-2004 Mustang
LINCOLN:
1999-2004 Navigator
2002-2003 Blackwood
2003-2005 AviatorMERCURY:
2003-2004 Marauder

ISSUE:

Some 1999-2004 Navigator, 2002-2003 Blackwood, 2003-2005 Aviator, 2003-2004 Marauder and 2003-2004 Mustang vehicles built before 12/19/2004 equipped with 4.6L 4V/5.4 4V engine may exhibit a runs rough, lacks power, and engine misfire, prompted by a MIL light and DTC codes P0301, P0302, P0303, P0304, P0305, P0306, P0307, P0308, and P0316.
ACTION:

Diagnose engine for engine misfire concern. If diagnostics determine an engine misfire which is characterized by leakage through exhaust valve(s), only replace the affected cylinder head(s).
SERVICE PROCEDURE



FOR VEHICLES WITH RUNS ROUGH/MISFIRE CONDITION:

Perform relative compression check on all cylinders. If abnormal, check cylinder leakage and actual compression for "low" cylinders refer to Workshop Manual (WSM), Section 303-00.
If leakage/compression check indicates a valve train concern, per valve train diagnostics in WSM, Section 303-00, replace cylinder head and cam assembly, pipe plugs and timing chain tensioner, per procedure outlined in WSM, Section 303-01.
If cylinder leakage and compression tests are inconclusive, perform misfire diagnostic, per TSB 05-22-8.
NOTE:DO NOT TRANSFER SPARK PLUGS. INSTALL NEW SPARK PLUGS IN REPLACEMENT CYLINDER HEAD AND CAM ASSEMBLY.

NOTE:THE ENGINE MISFIRE CONCERN IS TYPICALLY EXHIBITED ON THE LEFT HAND CYLINDER HEAD AND RARELY ON THE RIGHT HAND CYLINDER HEAD. UNLESS CONFIRMED, THE RIGHT HAND CYLINDER HEAD SHOULD NOT BE REPLACED AS AN "EXTRA MEASURE" SERVICE PRECAUTION.

NOTE:THE REPLACEMENT CYLINDER HEAD WILL NOT IMPROVE OR REDUCE ENGINE HORSEPOWER OR TORQUE ABOVE ADVERTISED LEVELS.

[ImDino]

Fuel filter is new. Plugs are new OEM with the correct gap. Must be the coil boots. I'll let you know. Thanks for your input!

[ImDino]

Coils were stress tested. Five bad ones (#2,3,4,7,8). Replaced all five and it runs great. Not sure why the check engine light never came on though???