Wednesday, 21 August 2013

Pre-filter foibles

I love it when mechanics say "I've never seen anything like this."  It's happened to us a couple of times.  This time, it was in response to finding a small hole in the diesel pre-filter cannister assembly, which was leaking diesel and letting in air.  We were bleeding out the air bubbles but after awhile they were getting in faster than we could get them out.  The mechanic said it was just dumb luck that we were able to get the motor started after the first stall coming out of the Glenan archipelago.

I call that dumb luck the thick slimy grey mayonnaise that formed in the bottom of the pre-filter cannister, which actually plugged the hole from time to time.  "But", you retort, "there's not supposed to be thick slimy grey mayonnaise in the bottom of the pre-filter cannister," and you would be right.


The pin hole in the cannister.  Veeeery difficult to see.

Eeeuuuuwwwww.  Inside a neglected marine diesel pre-filter.

Turns out you're supposed to drain the water out of the bottom of the pre-filter every-so-often, where every-so-often means every fill-up according to the mechanic, or every 15 days according to my book on the care and feeding of the marine diesel engine.  We knew that one of the functions of the cannister was to seperate out any water than may have gotten into the diesel from condensation in the tank, but we thought, well, you know, draining off the water once every year or so would be enough.  And so now we know better.

The hole?  That one's still got me stumped.  The mechanic suggested that the water lead to rust which led to a hole, but the cannister is aluminum and there was no sign of rust on the cannister itself.  I suspect the hole started when we tried tightening the cannister with some inappropriate implement (wrench or other) and it just got worse over time.

And while we're on the subject of tightening things inappropriately, the mechanic also broke off the air bleed screw when he unscrewed it to chase out the air introduced during the filter maintenance.  Those screws are very fragile and are actually hollow inside.  They are only meant to be tightened finger tight.  Admittedly, the hex-head on it makes it ever-so-tempting to slap a socket wrench on it and hunker it down good and tight, but don't - that's just a tease.  (*confession:  I figure finger-tight depends on the fingers... I used a teeeny weeeny bit of socket wrench anyway.)


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