So I Bought a 03 350, and lately its been having a random missfire. We already replaced all spark plugs gapped them then replaced the knock sensor and all plugs as well but keeps having the same problem car lags and doesn’t run at it full potential and occasionally it will die once I hit the red light. Have you guys experienced something similar and what did you guys end up doing. Thanks In advance

 

Did you pull codes to determine “random misfire” or just your choice of words for its behavior?

P0300 = random misfire but if lucky, you’ll catch a P030x identifying the cylinder(s).

When you did the plugs, did you check/replace any ignition coils? 80% of the time, it’s a bad coil (or more) OR a misplacement of the #4 and #6 coilpack.

 

Mic, I've been fighting this code for the past year. I've replaced all of my coils and sanded the plugs then tried replacing camshaft position sensors, MAF sensor and tested for vacuum leaks using starter fluid. I get no reaction from starter fluid. This misfire is killing me. Any thoughts?

 

Is yours a generic P0300 or P030x identifying the culprit cylinder?

I would inspect the entire ignition harness at this point. (Start with checking the #4 and #6 cylinder wiring to make sure they're not switched.) With all the seemingly obvious fail points replaced, can only surmise it comes down to bad wiring.

 

Generic P0300 has a laundry list of possible causes in the FSM. Have you ran through all of them? Have you done a vacuum test? Or compression and leak down test? Weak cylinder(s) can trigger P0300. I rebuilt my engine over it... 60% leakdown in like 3 cylinders.