Shaken, but not stirred

Canal Boat - - Back Cabin: -

QI have bought a CAV ag­glom­er­a­tor for the Isuzu 42 en­gine on my boat. I as­sume that fit­ting it near or be­low the bot­tom of the fuel tank would avoid prim­ing prob­lems. But is it bet­ter mounted on the boat struc­ture near the fuel tap or on the same en­gine mount­ing as the fuel fil­ter? Is there is any­thing else I should con­sider about the in­stal­la­tion?

ROGER CREWS, via email

ATONY REPLIES: Firstly, an ag­glom­er­a­tor is a fil­ter. Nor­mally the en­gine fil­ter is used as the ag­glom­er­a­tor and the ex­tra ‘in­line’ unit is a sed­i­men­tor. The pores in fil­ters must present a de­gree of re­stric­tion to fuel flow; by con­trast a sed­i­men­tor uses an in­verted cone to cen­trifuge the larger dro­plets of wa­ter and dirt from the fuel, with per­haps a 2mm gap around the cone, so it presents no re­sis­tance to fuel flow. Apart from that, the parts of the CAV ag­glom­er­a­tors and sed­i­men­tors are com­mon to both.

Your unit has a metal bowl, so with no dan­ger of glass break­ing, the risk of fit­ting it low down is not great. A mis­fit­ted seal could start to leak, so I would fit it higher up so less diesel can leak into the bilge, but en­sure the bot­tom of the ag­glom­er­a­tor is be­low the fuel level when the tank is close to full.

An en­gine run­ning well should not shake too much, but if it starts mis­fir­ing it re­ally will shake about, so I would mount a sed­i­men­tor or an ag­glom­er­a­tor to the hull rather than en­gine.

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