Yeah, but Forced hates bio, so there you go.

I too suffer a milage loss with AC on. 31 vs 33.5 mpg

BUT ULSD SUCKS for another reason. BTU's . For better or worse, I have at least one cylinder in this Benz that "pings" on straight USLD. Quite horribly I might add. Power Service Cetane boost will work if I put enough in, and then it cuts power dramatically. Hey, not for nothing, but I picked a good 6 cyl twin cam for a reason, not an ancient 4 cyl.

Best fuel has been bio. Even 15% bio in teh tank gives the fuel enough BTU's to run quiet and smooth in this engine. Cant wait until I get a good source of bio, petro diesel is horrible in my ride. I'm trying the blue bottle from Stanadyne now, hope it does better. Only WINGAS cures it though.

I'm sure someone will tell me this is wrong or that is wrong, or Forced will tell me how the engine should run fine on ULSD. FACT: Engine prefers bio.

But 8-9 mpg drop is hairy. Something else is amiss.