My 1996 F350 with a California Emissions 7.5, has had a miss in #5 cylinder for a bit. It does not smoke, uses a little oil(quart in 3000 miles) has 16 inches steady manifold vacuum, compression is lower than other 7 holes(115 compared to 145). Sets CEL with DTC P0305(misfire detected #5 cyl). When ambient air temps are real cold, under 45 degrees, when first started, you can hear a slight piston slap, which gradually decreases as it comes up to operating temps. Engine idle is very slightly rough, engine accelerates smoothly with no hesitation or apparent missing. The question I have is this: when I pull the spark plug on this cylinder, the plug looks as though it was just installed, no color, no oil, no unburned fuel! I played with it again this am, and felt all 8 injectors with engine running, all make a pulsing vibration, so it appears all are functioning, so why is there no color on plug?? Truck has 148,000, had new cat and muffler last year. Only problem I have had is I have had 2 rebuilt distributors fail, in the 3 years I have owned it, both failures were PIP sensors, now have a good used distributor with a new Motorcraft PIP installed. I think the distributor failure is what hurt the engine, as it sounded like a bunch of crossfires when it failed on Interstate 40. New engine on the way. Thanks for listening to my rambling! Dale

 

The spark plug from the #5 cylinder probably doesn't look bad because the cylinder isn't doing any work. To me a quart every 3000 miles is a lot of oil usage and it will probably ruin your new catalyst.

How do you know it is a piston slap that you are hearing?

 

Well, when the engine is cold, say 60 degrees or so, after the engine starts, you can hear a noise that sounds like some gravel being shaken in a can. And as the engine temp comes up, the noise goes away. Thanks! Dale