Random misfire/oil on spark plug, codes P0301 and P0300

RICKYSH11

MEMBER
2005 CHEVROLET TRAILBLAZER
4.2L
6 CYL
4WD
AUTOMATIC
210,000 MILES

I started getting some really bad misfiring on the way to work one morning. I used my code reader and picked up a P0301 and then P0300. So I went and got a spark plug to change the one on cylinder one. After doing so the engine still ran rough. I decided to change all of them and coming onto the third cylinder I found oil in the well. The plug was fairly saturated with oil. This was the only one like it. After reading about possible causes, it seems that I will have to remove the valve cover to replace a gasket for the spark plugs. Is there any information as to what exactly would need to be done? I usually do most of the work on my vehicles but do not really know the starting point for this job.

5 Replies

KEN L

MASTER CERTIFIED MECHANIC
47,643 POSTS

Hello,

It sounds like the valve cover gasket is causing the problem. Here is a guide to show you what you are in for when doing the job and diagrams with instruction below on how it is done on your car:

https://www.2carpros.com/articles/replace-valve-cover-gasket

After you are done I would check the MAP sensor which can be off a little causing a random misfire code.

Check out the diagrams (below). Please let us know what you find. We are interested to see what it is.

Cheers, Ken

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Monday, June 11th, 2018 AT 11:50 AM

RICKYSH11

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74 POSTS

Thanks, that was very helpful. I finally finished the job of replacing the gasket and I changed all the plugs as well. After doing so I started it up and still had some really rough idling along. I have not checked the MAP yet but will do so next. I did notice however the battery may be in need of replacing it is a few years old and is not the best quality and the alternator may need replacing as well it is original to the vehicle, I am sure if that would contribute to the issue or not.

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Thursday, June 14th, 2018 AT 9:26 PM

KEN L

MASTER CERTIFIED MECHANIC
47,643 POSTS

Oh yes, the battery and alternator must be in good condition because the ignition system depends on a clean (no turbulence) voltage supply. I would replace them and clean the battery cables. Make sure you use OEM spark plugs and coils also can you run the codes again?

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

and

https://www.2carpros.com/articles/everything-goes-dead-when-engine-is-cranked

Please run down this guide and report back.

Cheers, Ken

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Friday, June 15th, 2018 AT 10:25 AM

RICKYSH11

MEMBER
74 POSTS

Hey thanks again. I ended up finding the issue to be the ignition coil on cylinder one. I checked the battery and the alternator and ran the codes again, both were good. I had gotten a code for P0301 and since I already changed the plug I tried a coil and the misfire is gone and so far it is running normally.

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Friday, June 15th, 2018 AT 4:47 PM

KEN L

MASTER CERTIFIED MECHANIC
47,643 POSTS

Nice work. It sounds like a coil issue. Glad you could get it fixed, that kind of problem can be tough. Please use 2CarPros anytime we are here to help.

Cheers, Ken