My car is mainly a track car. Aem cold air intake, 2.5 cat-back single using stock catalytic converter. Compression test shows 140 psi cyl 1-3 cyl 4 205.
At Homestead 1/5-6 after coming off track i had misfireing at idle and error codes p0301,and more i forgot but stating lean condition, leading me to believe bad coil cyl 1. Driving home engine light went out. Now driving the street, no track events till 3/24, the engine light comes back on and i have these error codes.
Ideas??

 

I had reset the codes to clear them. Now engine light came on and the Actron reader says error and cannt link to read. Its not the reader i tryed both at Autozone and Advance auto. ??

 

Assuming the engine is stock 140 is on the low side for compression. The fsm states 225 on the high side and 135 as minimum. That might be an area of concern. Also the DLC is powered. If a scan tool fails to connect usually a fuse is blown. On a lot of cars it is the cig outlet fuse that is shared.

 

oops, typo there on the 140 that should have been 240 psi for cyl 1=3
I will check for a bad fuse.
Thanks for the reply,

 

So cylinders 1-3 are 240 and 4 is 204? Was your battery dieing at the end? If not I think there's a problem with cylinder 4 it's more than 10% off the others. How do you know it's running lean?

 

Battery is new so no problem there with cranking power. I took car to local recomended shop today. Only code showing today was p0172. Rich condition. Here's what the plugs look like. Obvious which one is cylinder 4. Car is running smooth and seems fine again, so it makes me think bad o2 sensor. I am guessing so any input i aprieciate.
Really worried that i have a broke ring on cyl 4 though.

 

redo compression test, add a cap of oil to cylinder 4, if the compression goes up your rings are bad. the black plug could also be a bad tube seal so check that too while you are at it.

you can buy your own compression tester from HF for under $25

 

Sorry i dont understand what is meant by "tube seal"?
I really like your idea of oil to test compression on cyl 4 though! I will do that today.

 

there are 4 spark plug tube seals under the valve cover that can get ripped and allow oil into the spark plug dwell. this causing your misfires is unlikely though.

 

I added about 20cc 10-30 to cyl 4 and the compression jumped from 205 to 300 psi! My guess is p0172, rich condition, is unburn gas's.