Been doing a bit of searching on this subject and there appears to be very little info/discussion on the net.
There were suggestions on the Australian forum that it may be caused by biological activity. A couple of people using a biocide claimed to have never suffered from the problem (but that's not really very conclusive)
I then found this from a report on running buses on B20 in Canada ...
http://www.nrel.gov/docs/fy07osti/40128.pdfHowever, in April 2005 two buses reported road calls for engine misfiring and stalling caused by plugged fuel filters. The first incident happened with Bus 2210 on April 8, 2005. The plugged fuel filter was removed from the vehicle and cut open for examination. A brown “grease-like” material was found in the filter pleats and was the suspected cause of the filter plugging (Figure 9).
The second incident with Bus 2208 occurred three days later on April 11, 2005. The plugged filter from this bus also contained the brown material. The filters on the other three B20 buses were changed as a precautionary measure, and inspection of these used filters also revealed signs of the brown material, but not of the quantity and consistency of the plugged filters.
Fuel was removed from the vehicle fuel tanks and tested for several properties as shown in Table 8. None of the fuel samples exhibited excessively high levels of biodiesel or cold filter plugging point (CFPP, determined by ASTM D6371). Water determined by Karl Fischer method (ASTM D6304) indicated higher levels than typical of a No. 2 diesel fuel but not excessively high. The Bug Alert™ ATP test for microbial growth does not indicate that microbial contamination is an issue.
The dark brown gelatinous residue coating RTD fuel filter No. 2210 was analyzed by gas chromatography mass spectrometry (GC-MS) (Agilent 6890 GC equipped with a 5890 MSD mass selective detector). The sample was prepared by scraping 23 mg of residue from a filter pleat and dissolving this in 1.0 mL of toluene. 1.0 μL of the solution was injected into the GC-MS using a split injection (100:1) onto a 30m x 0.25mm column, (0.50 μm DB-5 film).
The resulting chromatographic data are shown as the total ion current (TIC) signal from the MSD, as a function of component elution time in Figure 10. The multiple peaks in the 8-17 minute region are identified as diesel hydrocarbons. The larger peaks in the 18-23 minute region are fatty acid methyl esters (FAME) from the soybean derived biodiesel. The presence of these components is due to the fact that no attempt was made to extract them from the sampled filter residue.
Plant (or phyto) sterols were detected in the 39-45 minute region. The compounds were identified by matching mass spectra of the peaks with library spectra.
Campesterol, stigmasterol, sitosterol and stigmast-4-en-3-one were the major species identified at retention times 39.59, 40.06, 41.30 and 44.34 minutes, respectively. The relative amounts are in rough agreement with those reported in soybean oil [10]. While this analysis is semi-quantitative, the total level of sterols is significantly higher than expected for B20. Thus, this analysis suggests that high levels of plant sterols might be responsible for the filter plugging. These sterols are much higher molecular weight (≥400 amu) and would have a higher freezing point than typical of FAME or diesel fuel components. However, based on this analysis alone we cannot rule out other potential causes.
Two other filter plugging events occurred during the remaining study period. One happened about two months later on June 14, 2005. Bus 2209 filled from the B20 dispenser with a report of slow fueling by the dispenser operator. The fuel filter on Bus 2209 plugged shortly thereafter. It was later discovered that the B20 tank was nearly empty. Bus 2209 required several fuel filter changes and vehicle tank drainage to correct the plugging problem. The remaining B20 was also drained from the storage tank, the tank cleaned, and then refilled for continued B20 service.
A final plugging event happened during the last month of the study period. Two buses, 2207 and 2211, were road called for plugged fuel filters on July 7, 2006. The fuel storage tank was again near empty in anticipation of completion of the study and removal of the tank. Because the tank is drawn from the bottom, this implies that a material less dense than biodiesel was floating on top and was pumped into the vehicles as the tank became nearly empty. One well known quality issue with biodiesel is the presence of soap, which in large enough concentration will float on top of a fuel tank.
As mentioned previously, fuel filters are not included in the analysis of engine and fuel system labor and parts costs because they are considered preventative maintenance. Due to fuel filter plugging events fuel filters were replaced on the B20 buses in excess of their preventative maintenance schedule. Table 9 lists the number of extra fuel filters replaced, and indicates the associated labor and parts cost. In addition, labor for diagnosis and related work (draining and refilling fuel tanks) amounted to $712.50 for a total cost of $1,054.81. These additional fuel filter replacements were not significant additions to the maintenance analysis, adding an additional maintenance cost per mile to the B20 group of only $0.002. However, disruptions in transit service and related costs (bus substitution, affected ridership) are not captured in the maintenance costs, and were considered significant to RTD.
The above suggests that the brown residue may be plant sterols. Wikipedia states that these are soluble in water ...
http://en.wikipedia.org/wiki/Phytosterol , but from my experience, my drop out isn't soluble ... hmmm!
I next found this ...
http://www.biodiesel.org/reports/19970612_gen-234.pdf which implies incomparable materials could be a cause ...
A review of the results of material compatibility studies indicated that nitrile rubber and
polyurethane foams undergo deterioration upon contact with biodiesel and biodiesel blends (90).
Korbitz suggested that components made from these materials be replaced with fluorine rubber (89).
Bessee et al. indicated also that nitrile rubber, nylon 6/6 and high-density polypropylene were
incompatibie with biodiesel and biodiesel blends, but concluded that Teflon and Viton materials were
the least affected of the elastomers they tested (96). Reed et al. observed no degradation of
aluminum, brass, steel or phosphatized tiei tanks in their material compatibility study (90). Bessee
et al. however noted heavy gum formation in samples of biodiesel or biodiesel blends with copper containing
materials during their metals compatibility study (96). They also observed a sharp increase
in acid number over a six month period for these fuels when a steel or aluminum coupon was added.
‘Brown gum” and “light brown film” formation were noted for biodiesel blends with bronze and steel.
So ... biological action and material comparability could be avenues of exploration.
