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Originally Posted by bnaylor3400
I agree that misfire detection is important in that the primary purpose is to protect the CAT convertor.
All electronics components are heat sensitive and automotive is no exception. Until the engine warms up the PCM operates in open loop mode. It will switch to closed loop mode around 145 degrees referencing to the ECT temperature sensor and other pertinent sensor input parameters.
Also, when you replace a PCM module a case learn is recommended. I put in a re-programmed DHP PCM in my Grand Prix GTP over 2 years ago and did not have a case learn performed. It hasn't missed a beat nor have I encountered any misfire odb-ii DTC error codes. 
BTW - The crank and camshaft position sensors are Hall-Effects sensors. The best way to check the outputs and timing is with an oscilloscope. |
The reason for the CKP Variation Learn to to "customize" the PCM to the variations in crankshaft speeds as the engine is running..the crankshaft speeds up and slows down many times each revolution (has to do with if the piston is on a compression or combustion stroke).
This sets a threshold the PCM looks for to determine misfires.
If the CKP VL is not learned, the PCM COULD sense a false misfire and set a DTC..once the DTC is set and the code is current, the PCM freezes some of its control strategies such as ST fuel trim adjustments until the DTC passes.
True misfires will kill a converter, false misfires will not.
Performing the CKP VL prevents misdiagnosis of false misfires that could cause a DTC to be set.
Doesn't mean you have to perform the CKP VL, just means that you could misdiagnose a vehicle that hasn't had the procedure done.
The CKP VL is a procedure I perform on vehicles that have misfire codes set that I don't feel a misfire on..then I continue with verifying the concern.