I ran into a new issue a few days ago. Cel po300. Only result is that I feel some extra vibrations and a rougher idle. I looked around for anything out of the ordinary and assumed it may be the coils so I made a DIY cop kit. That didn't seem to help. Actually when I was just testing the new DIY cop kit, I took out the cyl1 coil for a few seconds and there was no change at all, no extra vibrations, sounds, or change in idle speed. Spark plugs are 300 miles old. What am I missing here? Where do I go from here?

 

Have you tested for spark on cyl 1? Also could check mechanical timing. It could be igniton timing too. The only time I've seen that code in my own time was on an Evo 7 running like 12+ degrees of timing at idle. Pulled it back to around 7 and that fixed the issue.

 

For one, I would have found the issue before modding the car.
was anything changed prior to this happening?

 

What were you doing when the light came on?

 

Nothing was changed, kinda popped out of nowhere. And I take back the cylinder 1 issue, I tried taking the coil out again and I did notice a change in idle/engine noise, don't know what when wrong the first time I tried. I'll change the coils back to stock and go from there. What should I look for? Since it's random could it be a clogged fuel filter/injector? I got new plugs laying around, I could try installing those. I wanted to try seafoam but I'm skeptical about it.

 

Be careful pulling coils, if you are taking them off the plugs. I'm told when the car is running and they aren't grounded, they can burn up.

I've run across a lot of random things that have caused a p0300. The last bout with a p0300 turned out to be the lower timing pulley mounting bolt that wasn't tight. It allowed the trigger plate to move around enough to destroy the sensor.
You can test the sensor with a voltmeter, from right in the engine bay.
Since you haven't changed any parts, I don't think your problem is any of the others.
I'd double check the plugs and their gaps, swap out coils, if you have a friend that has some, and test that sensor.
I'm distracted at work, right now, so that's all that's coming to mind for the moment LOL.

Hang tight and see if the experts chime in with ideas and/or corrections.

 

Old thread but I've come across this issue P 0300 code after doing an autocross event last weekend. My situation is rough idle and misfire. A friend cleared the code and I drove home with no issues which was about 15 miles. My misfire only happens at idle and the car seems to pull hard without issue anywhere else. From other posts I've read it likely is a vacuum leak but have no idea where to look. I've done new spark plugs stock ones as recommended gapped to .020. Ever since that was done 500 miles ago I felt the slight misfire or sputter after getting on the gas or spirited run. I've cleaned the MAF and show no improvement. Any thoughts where to look ?

 

this might be extreme and you may not have this but then that code came-on on my car .. cylinder 4 piston cracked lol

 

That would be some tough news if that was my issue.

 

Crank angle sensor.

 

Before loading up the parts cannon i would elminate any mechanical possibilities.

By starting with compression test and a cylinder leak down test.

 

I think the limit is no more than 10 seconds when you are testing them.