Random/Multiple Cylinder Misfire Detected - 2002 GMC Yukon XL Diagnosis and Solutions
2002 GMC Yukon XL 4WD The service engine light came on a few months ago. An auto parts store initially diagnosed a faulty O2 sensor, which I replaced—this resolved the issue temporarily. However, the check engine light has returned, and the new code read: 'Random/Multiple Cylinder Misfire Detected'. I recently replaced all spark plugs, which appeared in good condition with consistent gap settings and a slightly white color indicating potential lean fuel conditions. The vehicle currently has 90,000 miles and is exhibiting rough engine performance—unable to exceed 45 mph under acceleration. When pressing the accelerator, there's a noticeable intake sensation, but the engine does not respond as expected. This may point to either an air leak or a fuel delivery issue. Given that spark plugs appear normal, what should I investigate next? Should I check for vacuum leaks in the intake system? Where are common locations for such leaks? Could it be a fuel injector problem—such as injectors stuck closed—or insufficient fuel pressure? What is the recommended diagnostic and repair path to resolve this issue?
Since you've already replaced the spark plugs and ignition wires, the next step should be inspecting the distributor cap and rotor. Remove the cap—look for white corrosion or tracking around the terminals. Corrosion can cause weak spark delivery and result in misfires across multiple cylinders. Additionally, a faulty coil pack could also contribute to random misfiring. Check each individual 'prong' on the coil pack for signs of wear or damage. I recommend starting with a new distributor cap and rotor, preferably from an OEM dealer source, before moving on to more complex diagnostics.