Part 2 covers overview and cautions about used ECMs
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2017 Hyundai Elantra 2L non turbo. The car has had this problem since it was almost new. It's been to the dealership countless times where it's had many parts changed. The customer was told the next step was a fuel injector. This car sets a P0303 - Cylinder 3 Misfire. No other codes.

I looked at this car at least a half dozen times. Never once felt a misfire. I even went on a 30 minute test ride and nothing. Customer has brought it to me right away when it was acting up. Drives it right to our shop, shuts it off, I get in it within 5 minutes, fire it up and ... nothing!

Eventually I was able to make it have a slow, constant surging idle by inducing a vacuum leak. The customer said it would do that for a while then start misfiring constantly. I saw the ignition timing was going all over the place on the scan tool. It does have a two wire crank position sensor. The kind that makes its own AC voltage. When those sensors fail they produce less amplitude. At slower engine speeds the amplitude can get low enough that the ECM has a hard time detecting it. Doesn't explain why it's only ever a P0303 but gotta try something eventually...

I go to bring it in the next day and HALLELUJAH! It's misfiring! I was all nervous it would stop so I drove it in the shop and started testing as fast as I could! It turned out to be a neat problem and a quick catch with the lab scope. This would have been quite difficult without a labscope.


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