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2000 GMC Montana P0302 and P0171 - Misfire and Poor Fuel Economy Diagnosis

Model: 2000 Montana Fault Code: P0171 Posted: 2011-11-02 06:30

After driving 44 miles on 2 gallons of Chevron Regular gasoline (average fuel efficiency: 20 MPG), I added 1 gallon of Shell Regular. The vehicle exhibits a noticeable misfire at idle, but no diagnostic trouble codes have been detected. Using an OBD-II scanner, no fault codes were found during the check. This suggests either a minor ignition or fuel delivery issue, or possibly a deeper mechanical problem such as internal engine damage.

Related fault codes

P0171P0302

Comments (2)

Anonymous 2012-01-12 08:02

Hey Jeff, Just after Thanksgiving, I experienced a wrist pin failure in the piston, which caused a hole in the oil pan. I'm not sure if the connecting rod was bent—by the time I inspected it, both the rod and piston were severely damaged. The piston remained in the cylinder bore with its bottom missing. This explains the clacking noise; it wasn't a lifter issue. I've since replaced the engine with a new GM crate motor. -Bob

Anonymous 2012-01-12 08:45

A bent rod can cause a misfire DTC that may pass all standard tests like cylinder pressure and leakdown, because the cylinder remains sealed. The key point is that as the rod becomes shorter, it causes the piston to move faster in the firing order, leading to uneven crankshaft acceleration. The Powertrain Control Module (PCM) detects this imbalance and flags it as a misfire. My theory is that at some point, the affected cylinder ingested coolant while parked, and upon startup, it hydrolocked slightly—without causing catastrophic failure. Thanks for sharing your experience—it's valuable content for forum users facing similar issues.