Fault Code P0300

[605Jake123]

Hey guys i received a fault code P0300 today and looked it up and it lists as a "random/multiple cylinder misfire detected" I also noticed that when at stop lights or idling it had a slight shake to it like it isn't running quite right, nothing horrible but noticeable. Has anyone else had this or have any advice? I was thinking maybe check my spark plugs. Also for info my Tahoe has 87k miles and the temp outside has been in the negatives for days, not sure if that would do anything or not.

 

[z0lt3c]

I would check or replace, in order

1) Spark Plugs
2) Spark Plug wires
3) Ingitiion Coils
4) Fuel Injectors

Hopefully you won't have to get to #3 or #4, at least not without a specific cylinder identified. But at 87K miles if you haven't changed plugs or wires yet you are due. You can also try using seafoam and/or fuel injector cleaner as well, lots of good writeups here on using those products. If you clear the misfire codes but it still idles rougth you can also clean your throttle body and MAF.

 

[605Jake123]

Thanks for the reply. hopefully I can take a look at the spark plugs sometime tonight.

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[605Jake123]

So shortly after I filled up today my rough idling issue went completely away. Possible bad gas maybe? I had put in a few gallons at a gas station sunday night to get me by until i could get to the grocery store where i get gas at a discount and starting monday morning i had the idling issues up until i filled up this morning. So i think i'm going to hold off on changing the plugs and wait and see if the issues come back.

 

[afpj]

If it was bad gas run some techron fuel injector cleaner. If it comes back and you ruled out the above items, consider the intake manifold gasket. IIRC THATS the code I got years ago and it would always idle rough when cold was the worst.

 

[605Jake123]

Thanks for the advice i'll pick up some fuel injector cleaner this weekend. Any particular brand or do they all work about the same? I ordered new plugs and wires anyway since it has almost 90k miles and has probably never been changed and i plan on installing them this weekend since its going to be a little warmer.

 

[afpj]

Techron is the best. O'Reillys or auto on usually alternate on their buy one get one free specials so check with them...since these beasts hold 26 gal minimum, you'll need 2 bottles of the cleaner

 

[605Jake123]

Well the rough idle came back soon after my last post so I changed out my plugs and wires and then took it for a little drive. The rough idle is still there. I am going to fill up tomorrow with some injector cleaner because I already have it and it couldn't hurt. When I changed the plugs I noticed a pattern, the four on the drivers side were black and the four on the passenger side were white. Any thoughts on this?

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[605Jake123]

Here's a picture of the plugs

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[605Jake123]

after talking to some people at work and showing them a picture of the plugs and they said the intake gasket is probably leaking. One of them has had the same issue and that was the problem. so i guess that is the next thing to look at, they said to check it spray some brake cleaner around the intake gasket seal and it will start idling really rough indicating there is a leak in the gasket letting in more air creating a leaner mixture.

 

[z0lt3c]

Those just look like 80K mile spark plugs to me. Did you replace the plugs and the wires already??

Was their oil on the plugs or in the plug sockets? Are you loosing/consuming any oil between changes?

 

[605Jake123]

There wasn't any oil on them or is there any issues with the oil. There is still the rough idling issue after changing plugs and wires and every once in a while along with the random miss fire code I get the code bank 1 too lean. Which would make sense since half the plugs are white and half are black.

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[donut slayer]

Classic intake gasket problem. They have updated intake gaskets that fix the problem. Happens more often during cold weather.

 

[605Jake123]

That is what I have been told from some guys at work that have had this issue with gm trucks around the same year. I have an appointment for Monday to have it fixed and got a quote for the gasket installed for about $300 and if I want to have them put in two new knock sensors while they have the intake manifold off it will be an extra $200. I wouldn't mind trying to do it myself since I'm all about saving the money but I just don't have the time right now.

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[afpj]

I hope that extra 200 bucks includes changing the valley cover gasket while they're there. One less oil leak source to worry about in the future

 

[605Jake123]

Well had it looked at and all these issues were caused by a silly o2 sensor. I have never had an o2 sensor make it run so bad.

 

[afpj]

were there any other codes besides 0300? Wondering how they determined it was O2 sensor...Only way I can figure is by looking at the voltage wave forms while it's running. Which sensor, pre or post cat?

 

[bermudablueZ71]

those plugs definitely indicate an issue on one bank only. the 4 plugs being clean and a light brown would be burning perfectly, the other bank are dark and sooty appearing which screams too rich on that bank. Intake gaskets can do some weird drivability concerns, but they will usually affect all cylinders somewhat equally. being only on the one bank would have to be something that only controls or affects that bank. Glad you got it figured out, makes sense if the o2 sensor was always reading lean, (pre-cat) fuel trims would be high on the + side if you could see the data.

 

[Pachanga02]

Might need to change an 02 sensor as I've been having the same code, had my knock sensors replaced, and intake manifold gasket changed. Used FI cleaner and still get it. Which sensor was it?

 

[605Jake123]

I'm not really sure which one was changed and I can't seem to find my receipt right now. If it shows up I'll let you know

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