• False Misfire Detection and CELs: The stock ECU measures crankshaft acceleration to detect engine misfires. Because the lighter flywheel accelerates and decelerates faster than the ECU expects, it can trick the computer into registering a false misfire, causing the Check Engine Light (CEL) to flash, or dropping the car into a safety/limp mode

The 2G Mitsubishi Eclipse 4G63 engine's ECU detects misfires by monitoring minor fluctuations in crankshaft rotational speed. When you install a lightweight flywheel (like an ACT), the reduced inertia allows the crankshaft to accelerate faster, which the stock ECU can misinterpret as a misfire, triggering a ghost P0300 code.

1 https://www.dsmtuners.com/threads/p0300-error-code.530621/
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You can determine if the flywheel is causing the issue through the following diagnostic steps:

1. The Real-World Test
• Notice the symptoms: A true misfire is almost always accompanied by stumbling, hesitating, or a rough, vibrating idle. If the car pulls hard, accelerates smoothly, and idles fine but you only get the Check Engine Light, it is highly likely a false code caused by the flywheel.
1 https://www.go-parts.com/garage/obd...wk2hrUJxNiOA-ztLPw4W2U5jhMJnSodMdSlfDzLly3KjK
2 https://www.dsmtuners.com/threads/p0300-error-code.530621/
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• Monitor at different RPMs: Flywheel-induced ghost misfires typically occur during deceleration, coasting, or low-load cruising. If the misfire only occurs under these conditions and is completely absent under heavy load, it is likely the flywheel.

2. Check for Real Ignition/Mechanical Issues
If the car actually stumbles or hesitates, you must rule out actual misfires before blaming the flywheel.
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• Ignition System: Check for cracked ignition coils, worn-out spark plugs, or damaged plug wires. The stock 2G ignition system is notorious for breaking down under boost.
1 https://www.identifix.com/blogs/p0300-code-the-guide-to-diagnostics-and-repair/
2 https://www.jegs.com/tech-articles/...A1px71j3fzBQPyXBALWb088uS3EPb5wVDgumOEW7F8wiz

• Boost/Vacuum Leaks: Test for unmetered air escaping the system or unmeasured air entering it, which will drastically lean out your air/fuel mixture and cause actual misfires.
1234 https://www.jegs.com/tech-articles/...URSda-Z5-1hY3fS9GHFPsuxzKbs2Y0cG0RPcfnaozbPLS
5 https://www.go-parts.com/garage/obd...KcS2oe6OYJMX9csJC8uikxJCVFfqwnrRHc2yJGYAhgVFR

• Valve Train: A worn camshaft or collapsed lifter (common on high-mileage 4G63s) can cause an actual misfire.
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3. Use an Advanced OBD-II Scanner
• Access a diagnostic tool (like an advanced scan tool or ECMLink) that can read misfire counters per cylinder.
• If the scanner shows random, scattered misfires spread evenly across all cylinders simultaneously (e.g., 20 misfires on Cyl 1, 25 on Cyl 2, etc.), it is almost certainly a false-positive caused by the ACT flywheel.
1 https://www.dsmtuners.com/threads/multiple-cylinder-misfire-p0300.260269/
23 https://www.go-parts.com/garage/obd...YshtLQUfM1qQUOLHxxGziE_3wwaQAR9RPOtykQLnCzsYZ
4 https://www.solopcms.com/blog/fix-p...YGZ4fCCtjPi2Ii0pTGPfu3eHMLXJHPAwW2Rc2k88kKT7-

How to Fix the Flywheel Issue
If you verify that the flywheel is the sole cause of the P0300 code and your engine is otherwise mechanically healthy, you have two options:
1 ECU Adjustments: Using ECMLink or a custom flash, you can disable the ECU's misfire detection routine entirely, permanently fixing the ghost code.
1 https://actionclutch.com/blogs/news...jpEeMxHFO-4z79hZP5oC8i8SyBnen7SaXVmT3VE7q9CcW)
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3 Driveability: Some drivers simply live with the intermittent light or clear the code, as the car is perfectly safe to drive as long as there is no true engine hesitation or stumbling.
12 https://skanyx.com/codes/p0300

Issue resolved thanks to my good friends at DSMTuners I wouldn’t have been able to do it without you!