P0303 Fault in 1998 Dodge Ram 1500 5.9L 4x4: Diagnosis and Repair Steps
I recently replaced the spark plugs, wires, distributor cap, and the front O2 sensor (the rear one was replaced a few years ago). The P0303 misfire code started appearing only after a recent tune-up. I've already replaced the #3 cylinder spark plug twice with no improvement. My next step is to replace the fuel injector for cylinder 3. However, I'm seeking additional diagnostic advice before investing in a new injector. Any insights or troubleshooting steps would be greatly appreciated. Lynda
Before replacing the #3 fuel injector, remove it and swap it with an injector from another cylinder. Then retest for the P0303 code. If the misfire remains on cylinder #3, the issue is likely not the injector. Over nearly a decade working at a dealership, I replaced only about 12 fuel injectors that were actually causing cylinder misfires in these trucks—most of the time, the root cause was either a blown intake plenum pan creating an internal vacuum leak or a failed valve seat with a burnt and sunken valve. You can often detect this by removing the valve cover and placing a straight edge across the valve stems—the one that's higher than the others indicates a faulty valve seat. Additionally, remove the throttle body and inspect the intake manifold using an inspection camera. If you see engine oil or signs of a blown gasket, it may indicate a failed plenum gasket. There are known TSBs (Technical Service Bulletins) available online for these specific issues on the 1998 Dodge Ram 1500 5.9L model.