I have a '97 Seville STS (140K Miles) with the 4.6L Northstar that is getting OBD code => P0304 - Cylinder 4 Misfire Detected. In fact, my car is misfire at idle and throughout the entire range of driving. I can't seem to figure it out. So far I have done the following with no results:

1)Replaced Spark Plugs (Boch Iridium)
2)Replaced Spark Plug Wires (Magnecore)
3)Replaced all 4 coils (ACCEL)
4)Checked vacuum (17Hg at idle fluctuating normally when engine is accelerated and dropped off)
5)Replaced Air Filter
6)Replaced Fuel Filter
7)All Fuel injectors clicking and resistance check ok

Not sure how to go about the diagnostic – should I just go off replacing the cam/crank position sensors? or Ignition Control Module? Even though I get no error codes for these. Since I am relatively competent home mechanic (do all work on all our cars) - I have been baffled with this one and also tried the following (not sure of the affectthis car being a distributor-less ignition):

With engine running pulled spark plug wire off cylinder 4 and found no spark. However when I pull spark plug wire from all other cylinders I get a healthy spark. I attached an old spark plug to cylinder 4 lead and grounded this - still no spark.

(Moreover, last year the car was running rough and reported a misfire in Cylinder 2 but this only occurred at idle and would disappear after acceleration and driving. This came on and off for about three months then disappeared altogether (even with no maintenance done).

Help would really be appreciated - I am reluctant to take my pampered car into the shop.

Thanks!

 

Reading through other posts - reminded me of additional info:

8)Replaced Fuel Pressure Regulator

And while starting the engine has occasionally backfires (only since this problem arose) but it does start and will run. I am also the original owner of this car so I am aware of all maintenance and previous performance.

Thanks

 

Pulling plug wires off running DIS ignition is not advised and can be dangerous. Coil has unlimited voltage potential depending on source voltage.What does spark plug look like?

 

Looks like I got it. Turns out I replaced a bad coil with a brand new bad coil. What are the odds of that? Well I swapped in another coil and my car is now running great!

 

Hi spencer,

I have a Cad 2002 Sts and seems to have issue with coil pack. my cylinder 6 misfired and thought it may need tune up. got it done by shop by not cure engine light and still has slight choking while stopping or in park. per shop, coil pack has corosion on #6. since you worked on your cadillac, where is this coil pack located that attached to cylinder 6. is it in front or back? how much i have to remove to get it out? shop told me it's gonna cost around $500 replace just one coil pack. also, if i keep driving it, will it be any problem later on. also, where do i find good and better prices for these coil packs. my first time owner a cadillac for only 6 months and don't know much about this vehicle.

Any advice will be greatly appreciated.

Thanks,

Thor