'06 CTS Rough Idle / Misfire codes

[reyxlp]

After a long drive yesterday, when I stopped outside the garage door I noticed what felt like a rough idle.

 

When I started her up today, CEL came on.

 

Scanned for codes and got 2:

 

P0300 Random/Multiple Cylinder Misfire Detected

P0304 Cylinder 4 Misfire Detected

 

Don't think it's the battery- hooked it up to a trickle charger since I noticed the rough idle yesterday, and scan tool today showed 13.60min - 14.07 max.

 

I can't remember when (maybe several months ago, maybe a year? can't tell the passage of time anymore since the pandemic) I once before I noticed what felt like a rough idle (but can't remember how similar or not to this one now) and a bottle of Techron cleared it up.

 

Where should I start?

 

Techron again?

Coils?

Plugs?

 

Also, in case it might be related but I don't think so-- for a while now I have an occasional smell in the cabin of what I think is smells like burnt oil. It's mild and brief. It rarely happens. When it does happen, I *think* it's only in the winter and only when I first start her up or am not moving, and I feel like it goes away once the car is warmed up or has been driven a bit. Best of my memory, not too sure there's a pattern though. Just started tracking oil consumption recently, so I can't answer that yet. From what I've read online, this may be the valve cover gaskets?

 

Appreciate any advice on how to start troubleshooting the rough idle and misfire codes and welcome any input on the oil smell, too...

 

Thanks so much!

 

[JFP in PA]

Most likely a coil issue. Try clearing the codes and switching coils # 3 & 4, and see if the code comes back on the other cylinder. If it does, you will know what it is.

[reyxlp]

Update:

(Thanks JFP in PA for your reply!)

Decided to replace all the coils, spark plugs and engine air filters -- they were due, anyway.

Coils looked fine, no oil- though I had a minor panic attack when I pulled #6 coil only to realize its rubber spark plug socket stayed stuck in place. Would like to give credit but can't remember the name of another forum member here or elsewhere I came across in my panicked google search that had the same problem and successfully extracted using a pick tool, worked great. So thankful it happened in the easily accessible #6 instead of the dreaded hard to reach #4.

Spark plug #4 def had a little bit of oil but only toward the tip, nothing above the threads. Shined a flashlight down all the openings and didn't see any oil, even in #4.

With the new plugs, coils and air filters, the rough idle went away. Drove her a bit and definitely a big improvement.

I cleared the faults and CEL.

CEL hasn't come back yet but using the scan tool, the fault codes are back.

So the problem is improved but not resolved.

Still a relative newbie to DIY but I am positive the valve cover gaskets are leaking oil. I get the burning oil smell in the cabin on occasion and if I look carefully enough a tiny bit of white smoke on the outside sometimes, too.

Would that likely be the cause to the #4 misfire related codes?

I am thinking to do the valve cover gaskets next but not sure I want to take on that project if there isn't much chance it will solve this problem.

Appreciate anyone willing to share their experiences and knowledge on this.

Thanks!

[JFP in PA]

I would run a compression test on the cylinder in question, or a leak down if you have the correct equipment to do it.  If the oil is only on the tip of the plug, the problem may lie inside the combustion chamber or cylinder.