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Adam, ASE Certified Technician
Category: Cadillac
Satisfied Customers: 58
Experience:  ASE Master, L1, >10 years
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2000 Cadillac Deville: northstar..warming..Misfires..operating temp

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2000 Cadillac Deville, Northstar. Run great when cold and warming up. Misfires when full operating temp is achieved. Already changed out all three O2 sensors. Code 0303 comes up on computer, misfire cylinder 3.

Hello Chief,


An engine that is warmed up will be "harder" to fire than when it is cold. Has anything been done with the spark plugs/ coils yet?

Customer: replied 7 years ago.
Plugs were all changed 2 months ago. not the coils. O2 sensors changed. Also fuel filter just for GP.
Customer: replied 7 years ago.
Plugs were all changed 2 months ago. Not the coils. All O2 sensors changed. Also fuel filter just for GP.
  • OK, thanks for the info. I am going to assume that the engine was misfiring before the spark plugs were replaced. When diagnosing a an engine misfire, there are a few different possibilities, but I will show you how you narrow it down.
  • Normally with a coil over plug setup, you can swap the coil to another cylinder to check if the misfire moves(Instead of a P0303, you will get a p0304). With Cadillacs' Ignition coil cassette however, is not interchangeable between banks.
  • You can, however confirm that the compression of cyl #3 is within 10% of the others. ( Check with engine warm, as this is when the problem occurs)
  • You can swap fuel injectors to see if the misfire moves

FYI: I never like to guess, however failing coils are a common problem and that is the first thing that comes to my mind when reading your description.

Good luck! If I can do anything else for you, please let me know.

Customer: replied 7 years ago.
Actually, there was no problem prior to changing plugs. It was just time with about 50K miles on them. Why would temperature affect the coils function? Its like I leave the house and in about 10 minutes, I get the misfire/Check engine flashing/low power thing. Your suggesting changing the bank of coils?

If there was no problem prior to changing the plugs, then the first thing I would do is swap the plug from the misfiring cylinder with another to confirm that the plug is not the issue.


The temp of the engine will not affect the performance of the coil. Rather, an under performing coil could be masked by a cooler, easier to fire cylinder. I would suspect the coil, if the spark plug tests passes.


Good Luck!

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