Ashley P Posted May 24, 2022 Share Posted May 24, 2022 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? Quote Link to comment Share on other sites More sharing options...
JohnC Posted May 24, 2022 Share Posted May 24, 2022 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. Quote Link to comment Share on other sites More sharing options...
Ashley P Posted June 6, 2022 Author Share Posted June 6, 2022 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. Quote Link to comment Share on other sites More sharing options...
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