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2004 Ford F150 5.4L Misfire Diagnosis: P0305, P0304, P0300, P0316, P0171 After Spark Plug Replacement

Model: 04 f150 5.4l Fault Code: P0300 Posted: 2016-12-20 19:54

After replacing all spark plugs in my 2004 Ford F150 5.4L engine, the truck began running significantly worse and triggered multiple OBD-II diagnostic codes: P0305 (cylinder 5 misfire), P0304 (cylinder 4 misfire), P0300 (multiple cylinder misfire), P0316 (misfire during first 1000 RPM), and P0171 (system too lean on bank 1). I have already inspected all coil-over-plug (COP) assemblies, which are functioning properly. Despite this, the issue persists. I suspect a fuel or ignition system problem beyond just spark plugs. The truck is now showing signs of poor performance and increased emissions. I am seeking advice on next steps to resolve these codes, especially considering possible issues with fuel injectors, fuel pressure, or internal engine mechanics. Any insights from experienced F150 owners or technicians would be greatly appreciated.

Related fault codes
P0300P0304P0305P0316
Comments (6)
Anonymous 2016-12-20 21:46

I experienced the same issue on my 2006 Ford F150. It started with P0305, and after replacing spark plugs, multiple misfire codes appeared. The mechanic recommended changing the fuel filter, along with all ignition modules and the injector in cylinder #5—this helped somewhat but I still see P0305 and P0305PD. I tried Berriman's B-12 additive, which improved performance slightly. Now I'm planning to replace injectors in cylinders 5 and 6 to rule out a faulty injector. I've already replaced all components recommended by OBD-II code diagnostic sites but the problem remains unresolved.

Anonymous 2016-12-21 08:00

Misfires can originate from ignition, fuel delivery, or mechanical issues. First, confirm spark presence at each cylinder using a spark tester. Always use OEM or properly matched replacement parts. Check fuel pressure and KOEO (Key Off Engine On) pressure drop—look for drops of X psi within 5 minutes. Perform compression and vacuum tests to rule out internal engine problems. A diagnostic scanner provides live data that can help identify patterns, but these are fundamental checks every technician should perform.

Anonymous 2016-12-22 11:19

Replying to woakley144: I had the same issue on my 2006 F150. After replacing spark plugs and components, P0305 and P0305PD persisted. I used Berriman's B-12, which helped slightly. Now I'm planning to replace injectors in cylinders 5 and 6. I've shaped my own injectors for better fit and performance—some users report that poorly fitted or worn injectors don't function correctly. Let me know how your injector replacement goes—I’d be interested in hearing the results.

Anonymous 2016-12-22 12:11

Verify spark at each cylinder using a proper spark tester. Fuel pressure and KOEO pressure drop should both be checked—any significant drop within five minutes may indicate fuel system issues. Compression and vacuum readings are essential to rule out mechanical faults.

Anonymous 2016-12-22 22:33

I need to replace my current compression tester for this engine, as the one I have only detected one vacuum leak—yet the truck still runs poorly. I plan to test with a more accurate spark tester tomorrow to get reliable readings on ignition performance.

Anonymous 2016-12-23 08:34

A diagnostic scanner with live data monitoring would be extremely helpful in diagnosing this issue—it allows real-time observation of sensor values such as fuel trims, knock sensors, and engine load, which can point to specific root causes beyond basic codes.