Engine Misfire and Fuel Trim Diagnosis
A 2012 Holden Commodore (3.0L V6, 180,000 km) has a rough idle, decreased power, and the check engine light on. The owner says the misfire started after refueling at a remote station. A scan tool shows DTC P0302 (cylinder 2 misfire) and fuel trims: STFT +25% on bank 1, LTFT +20%. The spark plugs were replaced 10,000 km ago. You perform a compression test: all cylinders within 10% of 150 psi. A fuel pressure test shows 55 psi (spec 50-60 psi). You notice the fuel smells slightly like diesel.
1. What is the most likely cause of the misfire and high fuel trims, and how would you verify it?
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The high positive fuel trims (+25% STFT, +20% LTFT) indicate a lean condition, meaning the ECM is adding fuel to compensate. The misfire on cylinder 2 could be due to a lean condition affecting that cylinder, possibly from a clogged injector or intake leak. However, the fuel smell like diesel suggests contamination. Diesel in petrol can cause poor combustion, misfires, and damage sensors. To verify: 1) Check fuel sample for contamination (diesel has higher viscosity and distinct odor); 2) Test fuel injectors for flow and spray pattern; 3) Perform a cylinder balance test; 4) Inspect spark plugs for fouling (diesel contamination may leave oily deposits). If diesel is present, the fuel system must be drained and flushed, and components inspected.
2. What are the potential consequences of continuing to drive with this condition, and what repair steps would you recommend?
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Continuing to drive with diesel-contaminated petrol can cause: 1) Damage to catalytic converter from unburned fuel; 2) Fouled spark plugs and oxygen sensors; 3) Carbon buildup on valves and pistons; 4) Potential fuel pump and injector damage due to poor lubrication. Recommend: 1) Drain and flush fuel tank and lines; 2) Replace fuel filter; 3) Clean or replace injectors as needed; 4) Replace spark plugs if fouled; 5) Perform an oil change (diesel can dilute oil); 6) Clear codes and re-test. Advise customer to keep receipts for fuel purchase to claim against the station if possible.