P0303 Fault Diagnosis and Repair for 1998 GMC Sonoma 2.2L SFI Engine
Hello everyone! I recently purchased a 1998 GMC Sonoma 2.2L SFI engine with 67,000 original miles at a great price, but it wouldn't start initially. After using a code reader, the only initial issue detected was low fuel pressure. The seller included a new fuel pump, but I had to perform all repairs myself due to budget constraints. I've completed several repairs: replaced the fuel pump, fuel filter, starter, NGK spark plugs, AC Delco ignition wires, coil packs, antifreeze (Dextron), oil, and oil filter. The truck started, but it ran poorly with severe shaking. After re-running the code reader, I received a P0303 diagnostic trouble code indicating a misfire in cylinder 3. With so many components replaced, I'm unsure what to check next or how to proceed. A local mechanic is coming tomorrow to inspect the engine and suggested performing a compression test—particularly noting that this engine model (2.2L SFI) has known issues with piston-to-valve contact. I’m seeking clear, specific guidance on proper plug wire routing for my 1998 GMC Sonoma 2.2L SFI engine. Many online diagrams are too vague and I fear I may have incorrectly connected the wires. Additionally, there appears to be one vacuum hose missing from the top of the throttle body—this looks like a plastic end of a vacuum line—and I’m unsure where it should connect. I own a Chilton manual for my truck but can’t find detailed information on these components or troubleshooting steps. Any additional advice or diagnostic tips would be greatly appreciated. Thanks in advance!
For parts, consider using AC Delco components to ensure compatibility. The coil wire routing for a four-cylinder engine should follow this order: cylinder 2 → 3 → 1 → 4 (top-to-bottom). If wires are crossed, you may see multiple misfire codes (e.g., P0300 or P0302). A misfire is not solely caused by spark issues—fuel injectors should also be checked. A simple fuel pressure test and leak-down test can help identify the root cause. Finally, a compression test is essential. The vacuum diagram for your engine is typically located on the engine emissions decal or on the radiator support bracket. There are two vacuum lines on top of the throttle body: one connects to the fuel pressure regulator (FPR), and another goes to the EVAP system. Which one is missing? This will help confirm whether it's a vacuum leak contributing to misfires.