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Originally Posted by GMMerlin
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.
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I don't disagree with you and understand your explanation. Basically, it is a calibration procedure, otherwise you could get an anomally (false misfire) which could throw you for a loop during the course of troubleshooting. The problem is a lot of DIY types don't have a Tech II machine. A lot of us have nice odb-ii scanners that can read GM manufacturer specific codes in addition to generic but that doesn't help. And a lot of us are probably too cheap to take it to a dealer.