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I have an 05 Cadillac STS v8 northstar. 112,000 miles. I started yesterday got a cylinder 5 misfire. I pulled cylinder 5 and on the spark plug there is a little oil. I haven’t pulled rest. Should I? What is the cause of this also noticed a rotten smell from exhaust. What should I do first and cheapest. Thank you.

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Couple of questions.....

How did you determine #5 was misfiring? .....and....which cylinder do you think #5 is under the hood? 

 

Anyway.....likely not a plug problem. #1 suspect is a bad coil. They go bad all the time. 

On the 2005....it appears there is a early design version.....and a latter design version. I don't know what the difference is between the early and the later coils.

If it is a bad coil....it's easy fix. Just swap it for a new one.

2005coil.JPG

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First guess for rotten smell from exhaust would be catalytic converter.

Cylinder misfire does have a diagnostic chart but it is not automatically the plugs.  I think the diagnostic table is similar to this rough idle table:

http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/rough-unstable-or-incorrect-idle-and-stalling-2/

Here are threads to research:

 

Bruce

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1 hour ago, Logan said:

Couple of questions.....

How did you determine #5 was misfiring? .....and....which cylinder do you think #5 is under the hood? 

 

Anyway.....likely not a plug problem. #1 suspect is a bad coil. They go bad all the time. 

On the 2005....it appears there is a early design version.....and a latter design version. I don't know what the difference is between the early and the later coils.

If it is a bad coil....it's easy fix. Just swap it for a new one.

2005coil.JPG

I have a smart ride device that communicates w car it’s basically among other things an obd2 reader. It pulled up cylinder 5 misfire code otherwise know As p0305. I pulled when facing the car on your right/drivers side/bank 1(I believe) 1,3,5,7. Please advice lil ol me if I’m mistaken. Past that regardless of the number more so is the issue of oil in ANY of the spark plugs...right? I am figuring valve cover gaskets?....

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Ok.....the reason I asked......some folks have posted....."my buddies buick did the same thing..... his was #5.....he said it must be the same problem." kind of thing. 

Sounds like your on the right track if you have something that can read the codes. 

Part of your reply is confusing.....there should be no or just a little oil in any of the spark plug wells. It's kind of like ear wax....some is ok and should be considered normal.

That said.....I don't think you are looking at the cylinder #s correctly. I believe #5 is passenger side....3rd plug back from the front of car. 2005 STS.....front engine RWD car.

2005northstar.JPG

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"there should be no or just a little oil in any of the spark plug wells. It's kind of like ear wax....some is ok and should be considered normal."

"#1 suspect is a bad coil. They go bad all the time."

"#5 is passenger side....3rd plug back from the front of car."

 

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