Go to this link and read how to get the codes. If you get a PCM DTC that is a P0301 to P0308 that will tell you the cylinder. If it is just a P0300 you get nothing out of it.
www.cadillacforums.com/forums...look-here.html (Got codes? Look here!)

the day after i bought it and drove it home. im thinking it could be the engine cleaning they did. the engine compartment was pristine. so im almosty positive they cleaned it. you thinkn that could be a possability, just plug boot ?
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Did you just fill up?

yes but it did it before then

I have the engine out of an 00 Deville. It was wrecked bad at one time. The hood must have been off the car for some time. I pulled the coil cassettes off and the electrode had rust on all of them. I know the owners of the car for the last 3 years. It has not had a misfire condition, but The coils are bad.
Water can get down into the boots.

all i got was a P0300,