I recently had two vehicles where I back probed the MAP sensor to diagnose the problems. The first was a Dodge with a 4.7 with code P0306. The coil was OK, the injector was good and it even had good compression on no. 6 cylinder. I back probed the MAP sensor and triggered off the no.6 injector and disabled all the coils to get a longer cranking time. The results below speak for them selves. You can see a sharp rise in manifold pressure after the no.6 injector is fired. Removed the valve cover and found the no.6 cylinder exhaust rocker arm had fallen out from under the cam.
The second vehicle was an 01 cavalier 2.2 no start with a backfire during crank, no. 2 and 3 plugs would not come out so I did the same crank test, disabled the ignition and back probed the MAP and the no. 1 injector first. Got a similar result, but when I did a leak down test found the intake valve on no.1 leaking badly. I plan on adding this simple test to more vehicles that have misfires or rough idles in the future, I think it could help find things like plugged exhaust and jumped timing.
The second vehicle was an 01 cavalier 2.2 no start with a backfire during crank, no. 2 and 3 plugs would not come out so I did the same crank test, disabled the ignition and back probed the MAP and the no. 1 injector first. Got a similar result, but when I did a leak down test found the intake valve on no.1 leaking badly. I plan on adding this simple test to more vehicles that have misfires or rough idles in the future, I think it could help find things like plugged exhaust and jumped timing.
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