Northstar missing at idle and while driving.

[brmurph]

1998 Deville Concours

Compression test shows between 180-205 psi

Throttle body cleaned less then 10,000 miles ago.

First symptom was a ping in the exhaust when engine was off (recently started, unburned fuel?), after about 400 miles of this (car was running great BTW) it suddenly started missing real bad, I wasn't sure I was going to make it home but I did. I also got the infamous PO300 (multiple misfire) from my code reader as well as the onboard diagnostics (been told if I had a better reader it might have told me which cylinder). When I pulled the plugs I found plug #5 was not only carboned up but also had 1/2 of the porcelain around the center electrode missing (I assume from being too hot?). I replace plugs and wires which has fixed po300 for me in the past, although the miss is much better it is still there at idle and while driving. The PO300 hasn't come back as of yet but I now have a "CODE P1376 IGNITION GROUND CIRCUIT" which of course I messed with while removing the coil pack. I cleared this code and so far has not come back but I will check it and make sure its tight (I did clean it some when re-installed).

So I am thinking I have fixed the symptom by replacing the bad plug and wires but the actual problem is still around :-(, I am thinking either fuel injector or coil but it seems it could be a lot of other things as well. BTW the cylinder that had the broken porcelain on the plug also had the highest compression (205 psi). The bad plug has me confused as the broken porcelain tells me it ran too hot but the carbon tells me it wasn't firing (just my thoughts, could be wrong).

Thoughts? Thanks in advance Brian.

[BodybyFisher]

See if P1376 comes back, if it does.

Try resetting the connections at the ignition control module. The ignition control module grounds through IC C2 to G102 via circuit 450, make sure you plugged that back in. See black wire to make sure its grounded. This ground problem could be causing your misfire.

[barczy01]

Do you have the heavy gauge ground wire that bolts on the back of the cylinder head to the bottom of the module bracket? If not, get some heavy gauge wire and eyelet terminals. Bolt one side to the side of the rear cylinder head near the power steering pump and the other end to the ICM bracket.

[brmurph]

Thanks guys. Problem fixed. I did check the ground to the module bracket again but also at the same time hooked up an old inductive timing light (do they even use these any more :-) and it was pretty obvious I had a problem with fire on both #5 and #2 and luckily they came back to the same coil. I swapped the coil with another one and the problem moved so I replaced the coil with a new one and all is running fine now.

BBF good to hear from you, is it just me or where you MIA for a while :-).

Thanks again!

[BodybyFisher]

Thanks guys. Problem fixed. I did check the ground to the module bracket again but also at the same time hooked up an old inductive timing light (do they even use these any more :-) and it was pretty obvious I had a problem with fire on both #5 and #2 and luckily they came back to the same coil. I swapped the coil with another one and the problem moved so I replaced the coil with a new one and all is running fine now.

BBF good to hear from you, is it just me or where you MIA for a while :-).

Thanks again!

Good to see you also, yep I was MIA, glad you got it fixed, bad cool, nice!