Underconverted biodiesel and the TDI

jonny mac

Well-known member
Aug 27, 2007
Denver, CO
2002 Jetta Wagon
I have made some biodiesel in the past and tested it using the 27/3 conversion test AFTER it was washed and dried only to find that the fuel was not completely converted to biodiesel. This means that some of the mono- and di-glycerides were not converted to methyl esthers. I chose to run the fuel in my 2002 Jetta TDI without re-processing the fuel and I ran it in the summer. Does anyone know what the reprocussions of doing this are? I mean, it's similar to mixing filtered like 5% WVO in with good fuel. So as long as it's warm out, this shouldn't be a problem....maybe more of a problem in a PD engine?? But what about the 2002 TDI's? Are they ok for this. I have heard that older mercedes cars can run on anything.


Oct 19, 2005
Passat, 2005, Silver Blue
I did the same thing. I only ran 2 tanks with it and have not had a problem so far. The 3/27 test showed a very tiny layer of oil in the bottom of the test vial. By the way, I have fixed this problem by using the 80/20 process method. Easy to do and it seems to work.


Veteran Member
Aug 1, 2005
Huntington, NY
'00 NB
Jonny, I had made underconverted fuel in the past due to weak (just old) lye. It left some greasy stuff that clogged my final filter before putting it in my tank. That filter was a 5 micron and clogged after processing about 30 gallons.

Now with well converted fuel, there is no build up in the final filter after filtering hundreds of gallons. The biodiesel just comes out of the processor and wash clean as can be.

By the way, the car ran fine with low conversion fuel. I would imagine that stuff could eventually muck up the car's fuel filter but it should melt as things warm up so perhaps not. I wouldn't recommend using low conversion fuel for long when it is easy to fix the process and cheaper than fixing the car.


Active member
Jul 16, 2003
Different methods?

Well converted
Low converted
80/20 process

Is there quick explaination of these terms?


Top Post Dawg
Sep 13, 2005
Round Rock, Texas
2015 Beetle tdi dsg
Well converted same as High conversion = greater than 98% or so conversion from oil. The others are variations of this. 27/3 is a test to try and tell if you got good conversion or not. 80/20 is a process where you use 80 % of the methoxide on the first pass and 20% on the second pass. Search those terms here:


for a much better understanding of what is happening.


Well-known member
May 19, 2007
Reston, VA
2002 jetta
I used some underconverted fuel last summer in my 02. In sept my fuel pump conked out. BUT:
my jetta had sat around outdoors in Michigan (cold) for at least a year, undriven, before I bought it, and then I put it on a BD and ULSD diet -I was smart enough to change the filters first. I don't know that the fuel pump wouldn't have conked in any case. I had to have other work done to the car also, related to the lack of care.

That said, I'm not putting a drop of underconverted fuel in the car this summer, even though my pump is warrantied and my dealer is BD friendly.