2000 Seville northstar cyl 4 misfire

[don0102]

Oil in plugs. replaced plugs valve covers and both coil packs. Engine cover gaskets also replaced. Misfire comes back within 10 miles gradually. Diagnostics done at shop compression is good, injectors are good. Cylinder 4 is the one continuosly misfiring.
Will appreciate relevant help. Please advise if I can conduct any tests to pin point the problem.
Please also point out any possible causes.

[Ranger]

What did the tip of that #4 plug look like? Any different from the rest?

[derone]

You can test the coil 'cassette' by swapping them, front and rear, and see if the problem follows the suspected bad one.

[don0102]

Originally Posted by derone

You can test the coil 'cassette' by swapping them, front and rear, and see if the problem follows the suspected bad one.

Yes. They were also switched around. One at the time was bad and got replaced. Ran the test later. Both cassettes good still cyl 4 misfire

[don0102]

Originally Posted by Ranger

What did the tip of that #4 plug look like? Any different from the rest?

2 different mechanics told me that there was oil in more than 1 plugs. They both replaced all. While hooked to diagnostics computer I saw myself that there is slight misfire in cyl 1 but not as bad as 4 at a point when it stopped firing at all. To answer it, I did not see if the oil was sitting on tip or around.
What do advise next ?

[Ranger]

If you are talking about oil down in the plug well, that is somewhat normal. just a little seepage form the cam cover O rings.

Get a stethoscope or piece of hose and listen to #4 injector. Does it have a rhythmic tic tic like the others?

[johnny kannapo]

Its called carbon tracking. moisture, water or oil in the plug boot has caused the spark to track down the porcelin to ground out leaving a black carbon track that you can see on the porcelin. The same carbon track is tracking down the inside of the wire boot and will be visible as a gray track. if you use the same contaminated plug wire boot it will quickly find a path down the inside of the boot to ground out. The plug porcelin can be cleaned but the boot is tatooed with a carbon track burned in and has to be replaced.

[kckranz]

don0102 - What kind of plugs were you using and what were they replaced with?

[submariner409]

The plugs needed are ACDelco #41-987, pregapped to .050". Use 13 ft/lb of torque and maybe a tiny dab of anti-seize on the first 4 threads. Get a new set of plug boots while you're at it...........www.rockauto.com

johnny kannapo is exactly correct. Those plug boots will carbon track just like an old distributor cap and cause a nagging miss that's a bear to track down.

[don0102]

Great advice indeed. So it's the grounding through that causes this. How do I take care of the oil in plugs so it won't provide a ground again ?
Plugs replaced were Bosch platinum. The other times don't know, really at the mercy of mechanics.
Please advise

[submariner409]

Oil does not cause grounding: oil is an insulator, as in: PCB-based oils used inside transformers. Don't worry about a bit of oil in the plug wells - it harms nothing.

The ground is actually a "carbon track" created when the high voltage takes a path of least resistance in the plug-boot assembly - the spark which runs down the outside of the plug insulator actually carbonizes the plug boot in a hairline and creates a very effective ground wire. Same principle as carbon tracking the inside of a distributor cap between the plug contact posts.

If you cannot track down the miss outside of the coil cassette, then you may have to go back to square 1 and change to the recommended ACDelco plugs, a couple of posts back, and replace the plug boots also.

[chubbyranger]

Originally Posted by don0102

Plugs replaced were Bosch platinum.

N* hates Bosch plugs. I used them in my 4.5 CDV and they were fine. After 10,000 miles in my STS they were burned out carboned up junk. Switch back to AC Delco when you change the boots.