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1998 Mitsubishi Eclipse 2.0L - P0171 and P0301 Error Codes: Lean Mix & Misfire Diagnosis

Model: 98 2.0 Eclipse Fault Code: P0171 Posted: 2013-09-01 09:58

My daughter owns a 1998 Mitsubishi Eclipse 2.0L with approximately 129,000 miles. The vehicle has been experiencing persistent error codes P0171 (System Too Lean) and P0301 (Cylinder 1 Misfire). I've already replaced the spark plugs and wires, installed a new air filter, oxygen sensors, and a MAF sensor (though she claims it was recently installed), and used Lucas injector cleaner to clean the intake manifold. All fuel injectors show continuity at around 14 ohms. I performed a vacuum leak test using brake cleaner with no signs of leaks detected. The engine runs smoothly for the first 3-5 minutes after starting, then begins running roughly. If I turn it off and restart, it runs perfectly until about 3-5 minutes later when the roughness returns. This behavior occurs consistently on every restart. I've cleared the codes multiple times, but they reappear shortly after driving. The vehicle has a two-EGR system (one with airflow, one without), and I'm unsure if this is normal. Recently, I noticed that fuel comes out freely when using a gas can—indicating possible lack of fuel tank pressure. Is this a plausible cause? Any insight into diagnosing the P0301 misfire or addressing the lean condition would be greatly appreciated.

Related fault codes

P0171

Comments (5)

Anonymous 2013-09-01 13:51

P0171 indicates a lean air-fuel mixture, while P0301 points to a misfire in cylinder #1. Focus your troubleshooting on cylinder #1. Without a scanner, try using a stethoscope to listen for a clicking sound from the #1 injector—this is common during misfires. Use a noid light to check if the #1 cylinder's ignition signal is flashing. If possible, swap the #1 fuel injector with another cylinder and observe whether the error code changes to reflect the new cylinder. This test can confirm if the issue lies with the injector itself.

Anonymous 2013-09-01 17:20

I've tried spraying brake cleaner extensively around all connections—no leaks found. She replaced most vacuum lines, but I do hear a faint hiss near the firewall or front of the engine block. I'm not familiar with this sound and suspect it might be related to intake or manifold pressure. I plan to bring the car home to remove the intake manifold and inspect the injectors directly for better access. The EGR system has two lines—one shows airflow, one does not—though I’m unsure if that’s normal. A key observation today: during slow acceleration, it runs fine, but when using a gas can, fuel pours out of the tank neck—this suggests possible lack of fuel pressure in the tank. Could this be contributing to the lean condition? Thanks for the input!

Anonymous 2013-09-02 11:15

Focus on cylinder #1 as the primary suspect. Is there an EGR-related error code present? If not, that rules out a major EGR malfunction. Regarding fuel tank pressure—while no specific diagnostic code is reported, low fuel pressure could still cause lean conditions and should be investigated further.

Anonymous 2013-09-02 18:01

Not all vacuum leaks are detectable using traditional brake cleaner or carb spray methods—especially on older engines like the 1998 Eclipse. Intake gasket leaks are common in this model, particularly near cylinder #1. I recommend performing an intake manifold smoke test to accurately identify any hidden vacuum leaks. If necessary, remove and replace the intake manifold while the engine is off. In addition to checking for leaks, swapping the #1 injector with another cylinder can help determine if the misfire originates from that specific injector.

Anonymous 2013-09-06 20:53

Thank you all for the detailed input! I’ve also noticed cracks on the engine side of the catalytic converter—planning to repair it if feasible. After repairing the converter, I’ll remove the injectors and inspect the intake gasket (which appears new), though no one is ever truly certain until a full inspection is done. Appreciate everyone’s help!