2000 Mitsubishi Eclipse 3.0L - P0300 Random Cylinder Misfire: ECT Sensor Role & Fuel Pressure Diagnosis

The car appears to run fine when cold but starts misfiring as it warms up. This pattern suggests a temperature-sensitive issue—possibly related to the ECT sensor or fuel delivery. I've also replaced the fuel pump and filter, checked the regulator, injectors, fuses, and relays—all without resolving the problem.

The ECT (Engine Coolant Temperature) sensor is a primary sensor that directly influences fuel injection timing and mixture calculations. A faulty or inaccurate ECT reading can cause misfires across multiple cylinders, especially under varying temperatures. To diagnose further: check fuel pressure with the key on engine off (KOEO), then monitor for pressure drop over time. Use a professional scan tool to read ECT voltage and compare it with IAT (Intake Air Temperature) or oil temperature readings—this helps verify sensor accuracy. The PCM may also store manufacturer-specific codes that provide more detailed insight.

I'm not familiar with 'KOEO,' but I understand the vehicle is complex for a standard fuel pressure test. I've been struggling to get it passed smog—during my 30-minute drive, the P0300 code lights up. The car runs fine most of the time, but once I stop at a light, it throws a P0301 and then returns to P0300. I’ve tried disconnecting the ECT sensor—this allowed the engine to start, which suggests temperature plays a key role in starting. However, I’m unsure if this was coincidental or indicative of a deeper issue. If fuel pressure were low, wouldn’t the engine still start when the relay is bypassed—even at cold temperatures? Once warmed up, it runs smoothly for the rest of the day. Any advice would be appreciated.

Replacing parts can become very expensive—especially with aftermarket components that may not meet OEM specifications. Replacing a good OEM sensor with a low-quality one could worsen diagnostics. Modern engines are computer-controlled; you need a proper scanner to interface with the vehicle's ECU. Note: a diagnostic code like P0300 only indicates a misfire—it does not specify which cylinder or component is at fault. This requires further analysis.

I understand that, but in the past I've paid for scanning only to be told they'd replace parts without guarantees of fixing the root cause. The smog shop offered a $50 scan but wouldn’t confirm whether they could identify what needs repair. Should I let them attempt diagnosis—or should I take it to a specialist with proper diagnostic tools?

I drove to the smog office and the car ran fine until I stopped at a light—then it triggered P0301, followed by P0300. After disconnecting the ECT sensor, the engine ran smoothly again. I’m now scheduling a professional diagnostic session for morning hours. Is this necessary?

With multiple cylinder misfires and significant spending already, I need to pass smog. Should I let the smog shop scan it without knowing what the diagnosis will be? I’m concerned about being misled by a basic scan.

A scanner helps with diagnostics—but for fuel-injected engines, fuel pressure is critical. Check the #1 spark plug: if it's black, wet, or dry—this can give clues about combustion efficiency and potential fuel delivery issues.

The car previously showed a P0306 code. I’m now planning to check fuel pressure first—especially since I recently purchased a discounted online fuel pump. Thanks for the advice—I’ll update with results once I complete this step.

The diagnostic scope test failed, and at $300 for a distributor replacement, I’m skeptical about trusting shops to guess or replace parts without clear evidence of failure. I’d prefer a more targeted, data-driven approach.