Originally Posted by gsxr

The good stuff is the original #2 diesel fuel. The bad stuff has reduced BTU content, similar to winter blend, although I'm not exactly sure what the difference is... and we get it year-round, not just in winter.

The way I original figured it out was when I moved to Idaho in June 2002. When I lived in CA, all my cars got lower MPG, and I had years of data on their fuel economy. I saw a 10% boost in MPG across four different diesel vehicles using Idaho fuel (at the time). All four of them increased the same 10% at the same time, with no difference other than where the fuel was purchased. When any of those cars were filled in CA, the economy would drop (lose the 10%), same car filled in ID again, economy improved 10%.

Then we got the bad stuff in Idaho starting around 2006-2007 (approx) but it took me a couple years to figure it out... I thought something was wrong with my cars, replaced injectors, IP's, fiddled with timing, checked filters/brakes/etc to no avail. Then I went on a trip to Utah and got a 10%+ gain over several tanks of fuel, and the gain vanished when returning to ID and filling with local fuel. After I figured it out, it was fairly easy to determine... but it took YEARS, over many hundreds of gallons, and tens of thousands of miles.

On a related note... since power output is directly related to fuel quantity and BTU content, I believe that the original Bosch IP specs from the 1980's are all too low for the "bad" fuel... meaning there is reduced power to go along with the reduced economy. That is getting into a separate discussion though.