Pulling Codes?

[jimmyjamed]

Originally posted by ChunkFunky
It's funny, I was reading those and I dont buy that the car does compression tests... there's no pressure sensors in the head that could do it... unless it's a calculation based on O2 and Air Intake... I thought that was odd.. maybe newer ones do it ... but I dont think my 89 does. I could be wrong... I have been many times before...
-as

Your right, theres no pressure sensors, but it is doing a compression check, so to speak.

Through playing with the Injector pulse and timing, the computer can determine if compression is low in a cylinder or an injector is clogged, by how much the RPM's vary or changes in O2 readings.

While it can't say #4 is 165psi, it can say "I varied something, and when I did it to #4, the RPM's didn't change as much as the other 7"