2007 Ford F-150 5.4L misfire

Truck problem:

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.

ATS Adviser:

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.

FINAL FIX:

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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