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So last night coming home the check engine light comes on and the car begins running really rough. I plug in my code scanner and I get a code P0300 for Multiple Random Cylinder Miss-Fire. (Insert your favorite expletive here...I probably used them all)

Here's the background info:

2009 5.7 with 9900 miles and a 6-speed. The only mods to the car are a catch-can, 180 T-Stat, and Diablosport 91 octane tune with adjustments for fan temp points and tire size.

Initially I thought it probably sucked up a little oil from the catch can and it was time to dump. It was about 1/3 full and there was no blue/white smoke or burning oil smell. I dumped, cleared codes and took the car out. Same symptoms in about a block.

Ok...now what. I did just recently fuel up at a Metro-Detriot BP station with 93 octane less than 100 miles ago. I've never experienced "bad gas", Well....at least my car hasn't, but that's the only thing different between running well and running bad.

I really don't have time to try to fix so I make up my mind that I'm going to take it into a dealer to have it looked at in the morning. I plug in the Diablo and reset the factory tune. I take the car out to see if that helped some how. NO CEL, and NO rough engine. GREAT! but that doesn't explain what the problem may have been.

My notion is that the fuel really is bad or I really didn't get 93 octane, even though that is what I pumped, and I have been experiencing really bad spark knock since the CEL came on when the RPMs were at full advance.

Any input would be much appreciated or anyone has had similar experiences?
 

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I'd run the factory tune for a bit then try the 91.
 
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