2007 Ford F-150 5.4L misfire
We have a miss on 2 cylinders (#3 and #5) on this ford engine which we turned out to be coming from defective injector coils
which we just replaced. Road testing showed the engine still had a on and off miss on #3 cylinder. Swapped cylinder coils #3 and #4 but the miss stayed with #3. Check voltage drop between ecm driver terminal to #3 and it passed. Put a good known ecm but the
miss remained. Please help.
A quick way to resolve this is to use a vacuum gauge and get the engine to warmed up. Watch the vacuum needle for fluctuation. If the needle is dancing during the miss then you found your problem.
We remove the valve cover because the vacuum gage was dancing. Found #3 valve spring broken. RR the head and has been running a couple days now with no missing. We also found later that you can actually replace the valve spring without removing the cylinder
head but that will be for next time.
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