: misfire at idle

12-19-07, 06:19 PM
Just swapped in a rebuilt northstar into a customer's '97 Deville. Car runs great except for a rough idle. Scan tool shows code P0300 and misfire on cylinders 2,4,6,and 8. Only those cylinders. Swapped coils, ICM, wires from a known good Cadillac. New plugs...but no help. Any thoughts?

12-19-07, 06:24 PM
One of my cars occasionally makes same codes too . I just clear it and it idles perfect after . You can also see 0304 besides 300 .

12-19-07, 06:29 PM
Forgot to mention that I've also swapped intakes from a known good motor, trying to eliminate a vaccuum leak, if it had one. No luck.

12-19-07, 06:51 PM
I think you did everything ... But hey there is another thing just comes in my mind . Well, it might be o2 sensors ,cats or anything emission related thing but dunno for sure

12-19-07, 07:01 PM
O2 sensors switch normally and heat up normally. The misfire only happens at an idle or on deceleration (times of high vaccuum, like a vaccuum leak would), but there are no vaccuum leaks.

12-19-07, 07:02 PM
Have you tried cleaning the TB & EGR?

12-19-07, 07:13 PM
No, I haven't tried cleaning either of them. But if they are the problem, then why only misfires on clyinders 2,4,6, and 8? Am I correct to assume that they are the suspect cylinders and not 1,3,5,7...as this is a waste-spark system?

12-19-07, 07:25 PM
Well I was just shooting in the dark. It is a waste spark system. Have you ruled out the injectors on that side? Wiring harness or fuse maybe? Something common to one bank.

12-19-07, 07:32 PM
I haven't ruled out anything...but why does this occur ONLY at an idle or on decel. and run perfectly fine any other time? Makes me come back to the vaccuum leak that never seems to exist.

12-19-07, 07:49 PM
One last interesting bit of information is that long term fuel trim on the bank with the misfiring 2,4,6,8 is around 25% to the rich. The other bank (1,3,5,7) is around zero.

12-19-07, 07:57 PM
ONLY at an idle or on decel. :hmm: Now that IS baffling.

12-19-07, 08:19 PM
This guy has EXACTLY my problem. Is there any way I can be put in touch with him?


12-19-07, 08:51 PM
WOW, that is an ancient thread. He does not have an email listed, but his last visit was 10/28/07. You do not have enough posts to send a PM yet, but if you post your email, I will forward it to him. Keep in ming he will only see it when and if he checks in here. If you do not whant to post your Email address, just post back and I will PM you with mine and you can send it to me that way.

EDIT: I PM'd him the link to this thread. Hopefully he will see it and respond here.

Crown Vic Owner
12-20-07, 12:54 AM
IS this a ld8 or l37?

Stupid question, could part of the injector harness get damaged during the install?

12-20-07, 01:07 AM
I would check the fuel injectors/rail for flow/pressure issues

12-20-07, 01:08 AM
I also wouldn't rule out o2 sensors

12-20-07, 10:49 PM
My email address is stangman@rocketmail.com. I'm not sure about the injectors...I don't have another with the new fuel rail recall to try. And as far as the 02 sensors, how could it only cause a problem at idle? They're responsive at idle, not at all lazy in switching.