The engine runs smoothly when idle at a stop, but exhibits slight roughness during motion-to-stop transitions. A P0305 diagnostic trouble code (DTC) is detected using an OBD2 scanner. Despite recent maintenance efforts, the issue persists. What are the most likely causes and recommended troubleshooting steps for this specific fault in a 1998 Ford Mercury Villager?
I'm planning to replace the spark plugs and ignition wires today. I'll also use a fuel injector cleaner as part of the treatment. I'll update this thread with results once completed. Thanks for the suggestion!
I've recently replaced the spark plugs, ignition wires, distributor cap, and rotor. I added a fuel injector cleaner to the gas tank and also used a 50/50 mixture of gasoline and injector cleaner at the intake manifold. After clearing the code, I ran the engine for approximately 15 minutes and re-scanned it. The P0305 code has returned as pending. The I/M monitor shows 'OK' for misfire detection, but the oxygen sensor monitor displays an 'inc' (indicative) status. The EGR system is functioning normally. Should I consider replacing or repairing the oxygen sensor? Any insights from Jeff?
An 'inc' status on the O2 sensor does not necessarily mean a faulty sensor—it could indicate that a full drive cycle hasn't been completed or that misfires are too frequent for the O2 sensor to properly monitor. You likely don’t need to replace the EGR or oxygen sensor. The issue may stem from engine problems specific to cylinder 5—such as a fuel injector malfunction, vacuum leak, or poor combustion. I recommend focusing on these areas. Good luck and please keep me updated.
Thanks for the input. This afternoon I'll perform a pressure/vacuum test to check for leaks. Afterward, I plan to remove fuel injector #5 for cleaning and inspection. I'll post updates as progress is made.
I haven't yet tested or replaced the injector #5. I tried using original Motorcraft spark plugs, which improved idle quality—but the P0305 code remains pending on the scanner. What do you think, Jeff? Is this pointing to a deeper issue with cylinder 5?
Next steps should include conducting a compression test and measuring injector resistance to rule out mechanical or electrical faults in cylinder 5.
Has any recent tune-up or engine maintenance been performed? A check of spark plugs, wires, and ignition components may be necessary.