Low mileage in TDI Jetta running on biodiesel

I have been driving a 2001 TDI Jetta since 2006, using 100% biodiesel processed by Pacific Biodiesel. The problem is the mileage. I’ve heard that biodiesel gets a few less mies per gallon than regular diesel but my car fluctuates between 28 and 23 mph. Other biodiesel Jettas here in Hawaii are getting 35 mph and more. I have asked for advice from a blog, from the VW mechanics and from Pacific Biodiesel but no one seems to have a clue. What do you all think?

This time of year with the temperatures plummeting the mixture of bio-diesel you are using may be having an impact on your mileage. Do you know what type (the ratio of bio-diesel to oil based-diesel) you are running now? For instance (to my knowledge) it may be suitable to run B75-B100 (which is 75%-100% bio-diesel) in the warm months where the higher viscosity fuel will not be much of an impact; however, in the colder months you would likely (ask around) want to run a much lower mixture (say B25 or lower) or even straight traditional clean (but oil-based) diesel as the fuel is less viscous. It is the same with using different weight oil in your crank case different times of year. This may be something you want to look into though I am starting to question this as the culprit given that you live in HI.

You may also want to consider having your fuel filter checked/changed if it has not been checked recently. Bio-diesel (more an issue with the post-fryalator DIY kind) is known to have more particular matter in the fuel than traditional diesel so you could have a partially blocked filter.

Did you know VW only recommends using B5 in their TDI’s? Otherwise engine/fuel system damage can occur. http://www.officialbiodiesel.com/fuel_policys.htm


I re-read those recommendations and actually VW only says it isn’t responsible for engine use that could occur at biodiesel percentages higher than B5. They are still testing higher percentages before changing their recommendation. This is a little different from saying biodiesel can cause engine damage.

Have you made any accommodations for the Bio? Most owner’s running more than 20% seem to have made some modifications to avoid problems.

I would suggest you stop by TDIClub.com The home of all things TDI. Lots of friends there who really know TDI and know Bio.

Yea, that seems low for what you are driving. I get closer to 50 mpg on my NB TDI using standard diesel.

You are right. Hawaii’s climate doesn’t demand these changes in the percent of biodiesel used. I had fuel filter changed after first tankful of B100 and again recently.

I did try TDIClub.com and got nothing useful from it but thanks for the suggestion. Maybe I should try them again to access different folks and maybe better explanations.

Here are a few things to consider. How are you calculating mileage? Are you using the fuel gauge or the gallons purchased? In a current post, someone in the Technical/diesel forum of VWVortex was using the fuel gauge instead of gallons purchased and getting poor results.

Also, you must check mileage over a few tankfuls or else inconsistent fills can make bad results. It’s best to fill at the same pump on the same parking pad with the car aimed in the same direction to contribute to consistent refills. It’s not impossible that the station’s pump is out of calibration but if so, it should not be by much.

An incorrect odometer will also contribute to poor mileage as well as injection timing out of specification.

It is not likely but enough errors all in disfavor plus driving habits can make poor results.

While you’re at it, I would research the engine life of TDI’s that have run on various products… diesel, biodiesel of various mixtures, and veggie oil. You can probably do this on various biodiesel boards.

Then carry on my wayward son.