I'm trying to trace a slow leak that has created a large puddle of diesel fuel in my bilge over a period of 4 to 6 weeks. In tracing the fuel lines from the tank to the engine, I found a problem with one of the fuel hoses that might be the source of the leak. I'll fix this first, then see if the puddle reappears.

The tank is mounted at high level in the port cockpit locker. At the outlet, it has a turn-screw tap (to turn off the flow into the fuel lines) with an 11mm outlet nipple (measured at the thick end with calipers). This connects to a fuel hose, which connects to a fuel/water separator and is shown in the photo below. The fuel/water separator has a 7.9mm or 5/16" inlet nipple. The hose is marked 7.9mm 5/16" SEA 30R7, and fits nicely onto the fuel/water separator inlet nipple. However, when I tried to fit it onto the larger tank outlet nipple, I split the outer casing of the hose.

What I think has happened is that a previous owner had used the single piece of hose to connect the tank to the fuel/water separator. It worked previously, but it has now begun to split at the tank, leading to the slow leak.

I'm thinking of making up a new connection between the tank and fuel/water separator with two hoses joined end to end. The larger 11m hose will fit onto the tank outlet nipple and the smaller 7.9mm will fit onto the fuel/water separator inlet nipple.

Is there a joining piece or taper that will join the two hoses end to end? Can you get tapered fuel hoses?

woko said..
It would be easiest to install a larger or smaller hose barb at the cock or the filter I'd recon

Yep, or a reducing bush at the tank and then a hose barb of the same size as the filter.

Not entirely sure I understand the question but you can buy reducing barbs. Look on eBay for a brass reducer barb. Here is a link to an example.

I am about to use one as the diesel return on the Yanmar is 6mm and an 8mm hose has been used. It doesn't seem to leak but it slides off too easily for my liking even with a hose clamp on it.

www.ebay.com.au/itm/8mm-to-6mm-Hose-ID-Brass-Reducer-Barb-Fitting-Straight-Connector-Coupler-2pcs-/303326386080?_trksid=p2349624.m46890.l49292

I think hose tail is the term for the barb bit, a reducing bush will screw in where the tail/ barb was and the new tail /barb screws into the bush. One size hose, no extra bits to contend with, stainless is good.

CAV fuel filter.
NOT recommended

garymalmgren said..
CAV fuel filter.
NOT recommended

Interesting.
So if you own a stack of CAV filter cartridges, is there any other housing type/brand that they can be used in, that are not prone to these air leaks? Or is the only option to bin them?

woko said..
A bit harsh on the cav type filters, I've had em on many machines & ive used many brands of cartridges. A bit fiddley yes but if you replace All
the rubbers supplied they're ok, not as simple an install as a screw on job admittedly

I too don't have a problem with them, I have installed hundreds of them on all different types of equipment and as long as your careful they will fit together and seal up . I use one on my boat

All of those observations seem right to me.
Reliable, cheap but you have to be careful to get the seals right.

I totally agree Gary if I was starting from scratch I would go for spin on job, with a built in priming pump. For comparison sake I'm just now servicing my bt 50 Mazda the lovely spin on fuel filter is $24

The one I just changed has a float mechanism that sends a signal to command central then to a light on the dashboard. All very nice but if I installed one on a boat I would just put a water trap in the line pre filter

Apologies to the OP, but I'd like to extract useful info from these gents while the subject is hot.

UncleBob said..
..The best mod I made to the fuel system was a sedimenter/water separator for each tank prior to the primary filter.

Nice idea; any chance of pointing to a product(s) readily available in Australia, that is a dedicated water trap?

Karsten said..
Apologies to the OP, but I'd like to extract useful info from these gents while the subject is hot.

UncleBob said..
..The best mod I made to the fuel system was a sedimenter/water separator for each tank prior to the primary filter.

Nice idea; any chance of pointing to a product(s) readily available in Australia, that is a dedicated water trap?

Hi, I got mine from western filters in Sydney.
5836B120 Delphi CAV Watertrap Filter Sedimenter Unit.

Achernar said..
Here is the repair. You can see the stainless steel tank, with a screw-down isolation valve/tap. Then, there is is a short section of 9.6mm hose with a hose clamp at either end. Then the stainless steel pipe reducer (only the central portion is visible). Then 7.9 mm hose with a hose clamp at either end, which joins to the Ryco water separator. The parts were less than $20. The engine runs, so fuel is getting through (made sure I bled out the air before trying to start it, this time). I'll have to wait a few days to see if it has fixed my slow leak.

that tight bend were the hose rubs on the ply is going to give you trouble and possibly pinch and close ofalso not good to be rubbing on the ply ,better to change the fitting on the tap to a elbow

Going to start a war here now
you have the fuel hose connected to the wrong end
better connected to the outlet for marine use
getting my coat!

Now that's an interesting observation. I can't wait for the theory behind it

That fuel tank looks like it's aluminium.