Fuel Filter analysis

Cutting open the fuel filter led to an unpleasant discovery. However, first we need to discuss the structure of a fuel filter.

There is an external metal shell inside of which the actual filter sits. At one end of the shell is the top and in the flat top is a series of holes – small ones in a circle surrounding a larger threaded one in the center.

Fuel enters the filter shell through the smaller holes. Inside they go through a corrugated filter (corrugated so there’s more surface area) into the center of the filter, which is hollow. The fuel goes up the center, hollow, section and out the bigger hole in the top and thence onwards through the fuel system. Marine diesels generally have two of these filters – the primary and the secondary. The secondary filters out what gets through the primary filter.

I brought the secondary filter home with me after it was changed during the Mechanical Inspection, and cut it open with a dremel tool.

What we found was that the inner core was severely rusted, which indicates that there was significant amounts of water over a long period of time in the center of the filter. Since there is nothing between this filter and the engine to prevent it, that means there’s been water in the engine too. Despite being a marine diesel, they do not like water in them. They want to stay dry.

The other bad news was that there was significant blow by on one of the cylinders. This means that the piston isn’t sealing against the cylinder wall correctly and stuff is getting past it, indicating damage to the piston, the piston rings, or the cylinders themselves. Unfortunately, without opening up the engine, we can’t tell how much damage has been done to it at all, meaning we’re taking some risk in buying the boat. The engine might work fine for years or it might die tomorrow. Fixing it could be a few hundred dollars, six thousand dollars, or need complete replacing at about twenty thousand dollars, plus labour.

We’ll wait for the results of the marine survey and then talk to the seller and see if we can come to an agreement about who shoulders how much risk. Worst case is that we walk away from the deal, though I’m hoping that doesn’t happen.