P0304 Fault in 2001 Jetta 1.8T: Engine Stumbles on Acceleration, Spark Plugs & Coil Pack Diagnosis
I'm experiencing a P0304 diagnostic trouble code on my 2001 Jetta 1.8T, which causes the engine to stumble during acceleration. The issue does not occur when the vehicle is in neutral or out of gear. Previously, I had a catalytic converter-related fault (code not recalled), and I've already replaced all four spark plugs—after clearing the P0304 code, it returned upon restart. I then swapped the coil packs for cylinders 3 and 4 and cleared the code again; however, the P030-4 code reappeared. When I removed each cylinder's ignition wire while idling, the engine stumbles significantly, but returns to normal when each plug is individually unplugged and reconnected. There is no noticeable sulfuric (rotten egg) odor from the exhaust. Given that the issue appears specific to cylinder 4 and responds to individual spark plug disconnection, I suspect a faulty injector or misfire in cylinder 4. I'm seeking advice on next steps—such as performing a compression test or checking for a clogged fuel injector—to confirm the root cause.
Perform a compression test to evaluate cylinder pressure, and inspect each fuel injector for potential blockages. A low compression reading or a faulty injector in cylinder 4 could be causing the P0304 misfire, especially given that the symptom resolves when individual spark plugs are disconnected.