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PO303 Fault on 1997 Ford F-150 4x4: Spark Plug Wire Issue and Misfire Diagnosis

Model: Ford F 150 4x4 Fault Code: P0303 Posted: 2006-08-24 10:39

I have a 1997 Ford F-150 4x4 with a 4.6L V8 engine. The service light is illuminated, and the truck idles roughly. At low speeds, it misfires and runs unevenly, especially under load. Occasionally, the service light flashes on and off while driving at low speeds, but the vehicle runs smoothly above 55 mph. A diagnostic scan revealed code PO303, indicating a misfire in cylinder #3. I replaced the spark plug for cylinder #3, and the truck ran well for two days. However, after that, the service light returned and the symptoms reappeared. I'm seeking advice on possible root causes. Could this be a faulty coil, fuel injector issue, or mechanical problem? Any insights would be greatly appreciated.

Related fault codes
P0303
Comments (9)
Anonymous 2006-08-24 11:43

Try swapping the coil for cylinder #3 with a coil from another cylinder. If the code changes, it indicates a failing coil. If the code remains PO303, then the issue may be mechanical, such as a faulty fuel injector or electrical connection, or possibly a vacuum leak. Note that some 1997 Ford F-150 models with 4.6L engines use a distributorless ignition system with individual coils on each cylinder—confirm your setup to ensure proper diagnosis.

Anonymous 2006-08-24 11:50

Thanks for the tip—I’ll try swapping the coil and update you on results.

Anonymous 2006-08-24 15:40

Make sure you're using the correct spark plugs for your engine. If the truck hasn't had a tune-up in years, it's worth replacing all spark plugs—old or degraded ones can contribute to misfires and increase coil failure risk. Good luck, and please keep me posted!

Anonymous 2006-08-24 15:48

I recently replaced both spark plug wires and plugs at a Ford dealership. Since then, I've driven over 20,000 miles with no issues. Thanks for the advice—my comment was meant to say 'thanks,' not 'that's in the above form.' Appreciate it!

Anonymous 2006-08-24 17:15

I live in Hamilton, Ontario. I'm curious about your ignition system setup—does your truck have a coil-on-plug (COP) configuration with one coil per cylinder, or is it an older style with coil packs and spark plug wires? I may have given incorrect advice earlier regarding coil swapping. I recall that some 4.6L F-150 models transitioned to COP systems around certain years—let me know your exact setup so I can provide better guidance. Also, vacuum leaks from a leaking intake gasket are known issues in older Fords and could cause misfires. Where are you located? Good luck!

Anonymous 2006-08-25 08:16

I live in Hamilton, Jeff. I’ll check the coil setup when possible—currently it’s rainy and I can’t test it today.

Anonymous 2006-08-25 10:43

Fred—I have a distributorless ignition system with two front coils and one coil on each side of the valve cover, with four spark plug wires per cylinder. This is a standard configuration for the 1997 4.6L V8.

Anonymous 2006-08-25 13:19

A resistance test using a digital multimeter (DVOM) across each coil can help confirm if any coils are failing or have high resistance.

Anonymous 2006-08-26 12:27

Jeff, thank you so much for your detailed and helpful advice—it was very appreciated. The issue turned out to be a worn spark plug wire for cylinder #3; the coil itself had failed as you suggested. I replaced both the wire and the coil, and now the truck runs smoothly with no misfires or service light issues. Thanks again!