sounds like a single cyl misfire maybe increasing to multiple.

Unfortunately, nothing is easy to test without a competant scanner. with a scanner you could verify which cylinder was working either through misfire counts or through injector activations. In injector activations the injection is cut one cylinder at a time to see the resulting roughness occuring. With a 4cyl its is significant on each cylinder except the one that was misfiring.

Once the light is on then the cyclinder will be IDed by an OBDII scanner. The above functions won't be available with only an OBDII scanner.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician