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Fuel Filtration

Installing new Barrus 50 hp engine in 57 foot new build. Looking to install extra filters to lessen chance of future problems.Advise please!

Asked by: Michael Clement  | 3.28pm, Thursday 26 March

WW says:

It is a good idea to fit an additional fuel filter and a water trap to ensure clean fuel is fed to the engine. The combined unit often seen on boats is the CAV unit that combines both functions into one unit with a cartridge filter in the top with a space below for any water. The marine version of this has a fire resistant aluminium bowl, but do make sure that the drain plug is metal and not plastic otherwise it will fall foul of the BSS.

Rupert Smedley  | 6.36PM, Thursday 26 March

CAV filters are good, but they can struggle to deal with gross contamination- the paper filter can become saturated in some cheaper types. Although commonly used on inland craft, they are rarely fitted to other marine installations, as they are not as effective in dealing with gross contamination.
Also the only way to check for any contamination is to drain the filter directly, and this is infrequently done by many owners, which leads to water levels (and major sediment) becoming too high which then cuts off the fuel supply.
Another possibility would be to include a water seperator with a clear bowl, such as the FuelGuard filter- which passes the essential ISO 10088 (2009) standard and is marked as such. This uses a permanent cleanable filter membrane which stops water passing through and most solids (down to around 20 microns). however, a big plus is the fire-rated transparent bowl, which allows you to see contamination, and drain the filter before the level is too high.
In an ideal world, you would fit a unit such as the FuelGuard, follwed by a good paper based filter, which reduces to 10 microns or so, then the final filtering by the engine-mounted filters, which could be set to 5 micron or less.
Many people forget to read the important part of their manuals, in that failing to fit a primary fuel filter would invalidate the engine warranty for any fuel issues.

Mark Langley  | 11.04AM, Friday 27 March

Readers say:

Thank you for your help Gentlemen.
Very useful
Mike C.

Michael Clement  | 12.45PM, Friday 27 March

When I bought my boat a couple of years ago, the surveyor noted that no primary fuel filter had been fitted, and advised that I must fit one as a matter of urgency. He was only able to run the engine with the boat out of the water (i.e. not under load), and it ran fine. We had no trouble taking the boat through a flight of locks on the first day, but once we increased revs above 1200 it kept cutting out. Despite having run less than 300 hours from new (in 5 years), we found that the fuel filter on the lift pump (which had probably never been replaced) was almost completely blocked. Replacing this filter completely cured the problem.
This raises three points:
1. You really must fit a large pre-filter!
2. On Vetus Mitsubshi engines and several others (mine is an M4.17), the first filter reached by the fuel (on the end of the lift pump) is very small, and must be replaced whenever the engine is serviced. The fuel only reaches the larger filter on the engine after it's passed through the lift pump. The small filter is easy to overlook, so check your manual.
3. The symptoms of fuel starvation caused by a blocked filter are quite clear - above a certain number of revs the engine cuts out, but you can just about stop it stalling by throttling back quickly. This rev limit will gradually reduce as the filter gets more blocked! For this reason I always rev the engine to maximum for a second or two occasionally just to check it can sustain a high fuel flow.

Robert Wood  | 4.37PM, Saturday 15 August

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