clogged paper filter

I just went to service a customer's Yanmar diesel that was dying above idle. The secondary fuel filter looked absolutely clean but was kind of harder than a paper filter usually is. The fuel would not go through it. I was wondering what kind of chemical reaction would do that.

 

Not a chemical reaction but particles too large to pass through the filter.
There is a small amount of water on the bottom of the fuel tank, maybe just a few drops. Anaerobic bacteria are present, feeding on the traces of sulfur in the fuel. They build large gossamer structures that are harmless for the fuel system but may clog very small apertures like the pores of a paper filter.

This does only occur when some water is present and the tank has been completely at rest for some time. I have observed it several times in the glass water separator of my 350 gallon storage tank.

 

Primary filters often do the donkey work leaving secondary filters reasonably clean to appearance. Cladesporim resinea ( not sure of spelling ) live in the membranous between the water and fuel and yet can not live in either.

Often found in the warm filters of the engine room which is why the outlet of the tank should be at the absolute bottom to take water out to the filter and be caught in the water/ seperator filter as designed and discarded or drained.

 

The interesting thing was that at first sight the filter looks absolutely clean.

 

How do you know it was blocked. Did you not suggest the primary was clogged.

 

Fuel didn't go through it.

 

Maybe it wasnt diesel and was palm oil bio. I know that stuff gets white wax floaters in cool temps.

 

No, it was #2 diesel. Also, it happened in warm weather. I kept the filter and fuel will not pass through it. The paper is stiff like if it had been soaked in varnish.