Engine is missing
Within the last 1000 miles, I had a new O2 sensor, gaskets replaced on the intake manifold to pass smog. The car was clean, but the "check engine" light would stay on.
The other day, When I started the car, the light came on and there was a noticible missing, shudder.
The OBD indicated PO 300, 304 & 308. This indicates that cylinders 4 & 8 are misfiring. I changed the plugs, but did not help. Is there a way to troubleshoot this problem, perhaps checking if coils are good, etc? what else?
Check your coils. Take off the valve-cover plastic covers that say BMW on them. Remove those 10mm nuts holding on your coils. Start the car and pull the coils out slowly. You should hear sparks as you pull them out, if you don't than you have dead coils. Cylinder 4 is the cylinder closest to the fire-wall on passenger side. And Cylinder-8 is again closest to fire-wall on driver-side. Are your valve-covers leaking? Cam position sensors cause misfiring too
Any danger from the voltage when removing colis?
No, there is no danger when removing the coils, as long as the car is not on.
The way to determine if bad ignition coils are causing your misfire codes is to swap coils on cylinders that are misfiring to cylinders that are not. In your case, you might swap the coils on cylinders 3 & 4, and on cylinders 7 & 8. Then start the car, and check the codes again. If the codes now indicate misfire on cylinders 3 & 7, then you know the coils are faulty.
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