My car started missing on cylinder three. I figured it was about to drop a valve seat, so I pulled the head and had valve seats replaced, machined, and new seals -- an overall head job. I put the head back on and started the engine. It started right up but still had a misfire in cylinder #3. I checked the compression on all cylinders 120 PSI +/- a pound. I checked the pulse to injector with tester and could see pulse on tester. Then I swapped injector two and injector three. There was no change in the misfire in three. I got new spark plugs, swapped two and three plug. There was no change in misfire. I checked the rocker arm swing, which seems average to others, and there were no engine codes and no engine light. I checked that the spark was getting to plug, and it had a very strong spark. No CEL or Error Codes.
Hello! We will be happy to assist. You have definitely been on the right path to finding the root of the misfire. However there are a few things left that I recommend checking. I suggest to check the spark at the spark plugs at each cylinder. There may be an issue with the coil pack causing the misfire. Also check the PCV valve because that is a known cause of a misfires on your particular vehicle. I suspect that the PCV valve and hose may be leaking and causing the misfire.
You may have the engine model which has the PCV valve located above the transmission next to the exhaust header. This engine model’s PCV valve is routinely looked over because most are unaware that there are two PCV valve systems and the first one is easily visible from the top. I also recommend checking the resistance and voltages at the coil packs with a multimeter. I have a friend whom worked as a technician at a Ford dealership once tell me of a problem with your generation of Focus in which the wiring to the coil packs were made too tight and the tension broke the wires overtime.
I would always recommend having a qualified mechanic, such as one from YourMechanic, assist you in diagnosing your engine misfire and assisting you in repairs.
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