P0304 - Cylinder 4 Misfire

1997 Ford Mustang 4.6L EFI engine with 67K miles. The engine is actually now in a Factory Five Roadster. It sat on a stand for 5 years but runs well now with the exception of the code. I keep getting a code P0304 (cylinder 4 misfire) and I’ve checked the plug - the code seems accurate; it is misfiring. Things I’ve tried: 1)swapped plugs, 2)this is a dual coil engine so I’ve swapped coils, 3)the #4 plug is the longest sparkplug wire so I’ve gotten high performance wires, 4)heavily grounded the AL head, 5)put dielectric grease on the plug, 6)pulled my hair out…still, ONLY cylinder 4 gives the misfire. Any ideas?

Try swapping the fuel injector with another to see if the miss moves with the injector.


Are you sure it is misfiring due to ignition? If you have an old timing light with an inductive hookup, put it on the #4 spark plug wire and run the engine. see if the timing light blinks as it should. If you don’t have a timing light, then take the old spark plug and lay it on the head. Remove the spark plug wire and attach it to the old spark plug. Crank the engine and check for spark.

If you are not getting a spark, then you need to inspect the wiring from the coil pack back to the computer, but I think you will find that you are getting spark. I think this engine still uses the lost spark system which means that it actually has four coils, two on each side of the engine, so if you lost spark to one cylinder, you would also lose spark to one other cylinder.

The high performance wires might be a mistake. There are different types of high performance wires but the correct wires should have about 4k ohms of resistance each. The resistance helps develop a good spark. OEM wires are usually the best.

Have you checked the injector for that cylinder? Have you pulled the valve cover and checked for a collapsed lifter. Because the hydraulic lifters do not ride on the cam shaft, a collapsed lifter may not make the typical clacking noise. The roller bearing in the middle of the rocker arm that the cam does ride on could also be bad but I think that would make a lot of noise.

Have you done a cylinder leak down test or a compression test yet? You could have a burned valve.

I swapped #1 and #4 injectors and the code stays with #4. I had done a compression check awhile ago and got 175-190 psi on all cylinders (#4 was 190). I have not done a leak down check and guess that’s next. If the lifter was bad wouldn’t the compression check fail?

The compression is fine. You might try listening to the injector with a mechanic’s stethoscope or long handled screwdriver and note if you hear it clicking while the engine is idling.
If it is not clicking, then you need to verify that power is provided to that injector and that a ground is being made and broken by the ECM.

Thanks. I’ll listen and check the injector electrical.

SOLVED! Thanks for the help. I did the old screwdriver stethoscope on the #4 injector and sure enough it was not clicking like the others. Wiggling around on the connector I found it to be a loose connection there. If I wiggled it just right the injector would start clicking and the engine idled better. I’ll replace the connector. THANKS!!!

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