Long story short... I had all bolt ons running e85 with all supporting mods on stock turbo. Car ran amazing! However e85 is very impracticle for work and just in general. Like 8 stations within a 50mile radius. Anyways. So I started looking at new setups. Trans took a dump on me and the Diff Pin knocked a hole in trans. So got a new trans and also MHI 18k turbo, Kelford 214b cams with valve springs. My goal was to be around 400awhp on PUMP gas. I did keep e85 as a 2nd map. Got the car back from the shop after a tune and it ran great for 24hrs. Then I popped a SES light. Pulled code and it was p0304. Cylinder 4 misfire. Ordered new plugs from Lancershop and installed. Car ran like even more ****. Lacking power, rough idle, acceleration sucks. So swapped out the 4th plug with an old good one still ****ty. Swapped coil packs still ****ty. Any ideas or anyone else with similiar issue? Next step is to empty mu tank and try e85 map see how she runs. Then I guess check injectors?
Btw heres mod list...
-MHI TF06 18k Turbo
-ID 1000cc Injectors
-Walboro 255lb Fuel Pump
-Cobb IC Upper and Lower
-Cobb Intake
-Kelford 214b Cams w/ Valve Springs
-ETS Intercooler
-ETS Downpipe
-Cobb Exhaust
-Forge BoV
-Grimmspeed 3port Boost Controller

Focus on the changes you made as the source of your problems--
First--did you gap your new plugs or just drop them in?
Next what were the condition of the old plugs you pulled out?

I had/have same issue.. My X was on full boltons tuned on 91 & e85.. When I run it on E85 it runs like a champ no CEL or anything.. I switch to 91 map when I travel and I always get rough idle and P0304.. I tried swapping everything, coils, plugs, injectors, etcc and I still get the p0304 on 91.. Honestly I gave up looking for the solution and I just run it on e85.. Also I had 2 different tuners look at my logs and they're all fine..

Who tuned your car? I would definately be getting a check on the 91 tune and checking the source of ur fuel.

I checked all of the new plugs and they were within the .020-.022 specs. So dropped in with the proper lubes and installed. Then car ran worse on new plugs than it did on old plugs. Old plugs were pretty dark and tiny amount of oil on 2 of them. Nothing major.

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Same mods on your car? Well I went ahead and pumped e85 last night and car runs and pulls real hard. CEL light still stayed on and for the same reason. However I still hear some backfire and popping from the exhaust here and there. When I was on the Dyno last week the tuner said AFRs and everything seems perfect. (before I put in new plugs)

Have you tried to clear the code aka unplug your ecu or battery for 30secs and then hook it back up and fire it up still get a cel?

How much E85 was in the tank when you filled with pump gas? Make sure you have it as low as possible. Go to the basics and check for any boost leaks. Make sure there are no other codes present. Verify the correct map is loaded.

Once you have done that, post a couple of logs. For now start with cruise and idle logs. If you have an OS tune, get the logs and map up here and I will gladly look it over for you.

We can figure this out.

tHx.