What causes a missfire CEL with lightweight flywheels? No one is really sure. There are theories though. One theory is that the Crank Position Sensor senses the rotational speed of your crankshaft. Since a lightweight flywheel reduces the rotation mass of your engine, your crankshaft accelerates and decelerates quicker than OEM specifications. Another theory is that there is a missmatch of information from your Crank Position Sensor and the Cam Position Sensor during acceleration and deceleration. This may or may not be caused by the slow reaction of the belt tensioner which will cause enough belt slop to give erroneous readings to the ECU. Since missfire CELs (due to lightweight flywheels) do not occur as often on the STi and the STi has two Cam Position Sensors, this adds further evidence that the missfire CEL has something to do with the Cam Position Sensor since there is only one on the WRX.
Which lightweight flywheel will not throw a missfire CEL? In truth, there is no lightweight flywheel that will not throw a mimisfire CEL. The odds of you throwing a misfire CEL are higher the lower in flywheel weight you go. Even with a “heavier” lightweight flywheel, you may throw a misfire CEL.
Will the use of a lightweight pulley increase my chances of a missfire CEL with a lightweight flywheel? Yes. To a smaller degree, lightweight pulleys also decrease rotation mass, adding their quicker acceleration and deceleration into the equation.
How do I fix a missfire CEL with a lightweight flywheel? The safest course of action is to use a portable OBDII code instrument and clear the codes frequently. Once you know the frequency of the missfire CELs, it will aid you in determining if this is an actual code or perhaps something to investigate further. While the missfire CEL code is a “safe code” (just an indicator and won’t throw your car into safe or limp mode), it is never a good idea to drive around with the CEL on for extended periods of time as another, more serious code can be present without your knowledge. Additionally, it is not a wise idea to remove the missfire CELs via engine management software. Missfire codes can be an indication of a problem and removing their presence will remove possible symptoms of an actual problem.
Who is a good or a bad candidate for a lightweight flywheel? Arguably, the users whose driving technique is most highlighted by the benefits of a lightweight flywheel are people who autocross frequently. Many people are also genuinely happy with the performance increase in their daily driven vehicles as well. The only group that is generally dissatisfied are people who competitively drag race as the reduced rotational mass does not lend itself to their severe launch techniques.