Problem: The vehicle came to the shop because the Malfunction Indicator Light (MIL) was on.
Details: The tech connected the scan tool and pulled the diagnostic trouble codes (DTCs) listed below. He test-drove the vehicle and verified that the engine was obviously misfiring. Next, he checked the fuel injector PIDs with a scan tool. The “ON” time seemed to be excessively high, so he inspected the spark plugs. They looked fouled.
P0301 – Cylinder 1 Misfire Detected
P0303 – Cylinder 3 Misfire Detected
The tech decided to test the engine vacuum and found that the engine only had 6 inHg of vacuum at idle. He checked for restrictions in the exhaust system and found that the crossover pipe under the oil pan was plugged. It was interesting that the PCM only recorded cylinders 1 and 3 misfires when the plugged exhaust should have cause multiple misfires on both engine banks.
Confirmed Repair: He replaced the Exhaust Y-Pipe and the spark plugs. After the repair, he test-drove the vehicle. The engine misfire was fixed, and the power restored. The MIL stayed off and the DTCs did not return.