1999 Ford F150, P0306 Misfire Cylinder #6

This 1999 Ford F150 Came in with a misfire on cylinder number 6. No big deal except that the spark plugs, spark plug wires and coil assembly have already been changed with quality parts. The other thing I noticed was that the misfire did not come in until the engine speed reached about 1500 RPMs. Diagnosing this wound up being easy after I thought about it for a little while.

To confirm the cause, note the RPM range when the misfire comes in. Disconnect the EGR valve vacuum hose and plug the hose end. Bring the RPMs back into the misfire range as previously noted. If the misfire is gone the EGR tubes are restricted.

To correct the condition, remove the upper intake and inspect the EGR orifices in the lower intake openings. I did not bother listing how to remove the upper intake, as that information is available in any repair manual for this vehicle.

The picture may be too dark but there are tubes with holes in them where I am pointing to.

A small piece of wire and a good shop vacuum and the debris (at least in my case) comes out easily. The same condition could have caused a misfire on any of the cylinders. Just coincidence that the #6 tube was open and the others were highly restricted. The one open tube was causing all of the exhaust air from the opened EGR valve to be dumped into cylinder #6. Thus causing a lean misfire on that cylinder.

Some new gaskets, put it back together and this one is done.

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