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1998 F150, 4.6 misfire


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I've been waiting MONTHS to ask the pros here...I've worked on said 4.6 because it had random misfires.  I replaced the OLD spark plugs and a coil or two, the truck missed on the customer on the drive home.  They returned and #3 (IIRC) showed some misses, so it got a coil.   They called a couple weeks later and said it was missing again.

If I notice (feel) the engine missing, both the misfire monitor or cylinder contribution screens are likely to show NO misses.   At least for a while.  Then they might show about a dozen misses on a particular cylinder.   After several more seconds of missing, a DIFFERENT cylinder will show misses.   If run long enough, most every cylinder shows misses.

Fuel has been run very low and twice I've added a few gallons, so I don't suspect contaminated fuel.  The miss is sudden/violent enough to make me think it's lack of spark.  (And fuel trims work OK when starting fluid is added.)

What have I missed? 

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It might not be a spark issue.

Check fuel pressure, fuel pump and check for vacuum leaks.

A gas cap vacuum leak or vacuum leak at the gas fill tube to tank, o-ring on top of tank where the pump is -- can cause 4.6's to jerk/miss like a mofo. 

My GT would miss/jerk because vacuum leak at tank.

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  • 2 weeks later...

I found a small vacuum leak at the pcv elbow, but the misfire remains.   Fantastic.   Fuel trims looked good before and after the vacuum leak repair, so I wasn't hopeful.

Dang the Ford scan data that won't show every misfire that I can feel.

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