Fuel feed by suction from tank

[chewi]

I helped a friend start his 1gm10 this weekend.
I found the feed pipe from the tank to the primary was to suck over the top and down to the primary filter. No fuel was flowing and I did not want to draw lots of air into the system, so I syphoned the supply bedfore I reconnected it to the filter. ( I am accustomed to a gravity feed ffrom the bottom of the tank on my own BukhDV10

I wondered what the norm is here. Do folks use the engne pump to prime the system, then bleed all the air out?
What stops the syphon breaking and introducing air again if the boat bounces around in heavy weather on a low tank and exposes the pickup end?

 

[Slowtack]

on mine the inlet is in a small sump below tank floor level, has never been a problem even when fuel level is low

 

[prv]

I think suction feed is the norm these days. In theory you can use the engine lift pump to prime it, but it takes a bloody long time! In both my boats I've fitted an outboard motor style bulb pump which gives a good squirt of positive pressure right through the system for bleeding, filling new filters, etc.

Pete

 

[chewi]

on mine the inlet is in a small sump below tank floor level, has never been a problem even when fuel level is low

So thats a feed from the bottom of the tank? So there is no syphon to break is there?

 

[chewi]

I think suction feed is the norm these days. In theory you can use the engine lift pump to prime it, but it takes a bloody long time! In both my boats I've fitted an outboard motor style bulb pump which gives a good squirt of positive pressure right through the system for bleeding, filling new filters, etc.

Pete

I thought of that , which would help with priming. but can it not still break the syphon or collect air if the pickup pipe is briefly exposed to air in the tank?

 

[oldvarnish]

but can it not still break the syphon or collect air if the pickup pipe is briefly exposed to air in the tank?

Of course it can. When you start to suck air you're out of fuel - simple as that.

 

[SHUG]

This may solve your problem:
http://www.asap-supplies.com/marine/cav-type-filters-and-elements/cav-type-oem-fuel-filter-with-primer-302051

 

[prv]

Of course it can. When you start to suck air you're out of fuel - simple as that.

Exactly.

It's not really any different with a tap in the bottom of the tank (or rather, in the side wall an inch or so above the bottom, to keep the crud out). If you heel the nearly-empty tank away from the tap, you'll get air into the pipe.

Pete

 

[rob2]

It's not just a question of whether you have a tap at the bottom of the tank (ideally from a sump, with a tube to clear the debris) or a dip tube from the top. The position of the tank vertically relative to all the other plumbing and engineis also significant. Many tanks are at least in part above the engine, but the hose then runs up to the filter, so you still have to establish a syphon unless the actual fuel level is higher than any other part of the system.

The filter c/w pump suggested above is very helpful. Even on my old Citroen ZX, the fuel had to be pumped up from the hose beneath the floor and it had this type of filter. Without the complication of a diaphragm operated by a rocker arm on the camshaft which needs to be correctly positioned for manual operation, priming and bleeding was quick and easy with the integral filter pump.

Rob.

 

[chewi]

Obviuosly if completely run out, no diesel is no run,
However a brief disturbance on my bottom feed tank (in heavy weather) would fix itself once the feed pipe is submerged again, the bubble will float back up, but I think the top feed one would not recover without bleeding, would it?

I'm trying to understand whether or not that is a vulnerability in a suction feed arrangement.

 

[chewi]

That looks like a useful bit of kit, thanks.

 

[KellysEye]

> When you start to suck air you're out of fuel - simple as that.

Or you have leaking fitting that is leaking diesel and sucking air in.

 

[chewi]

> When you start to suck air you're out of fuel - simple as that.

Or you have leaking fitting that is leaking diesel and sucking air in.

Maybe I'm not explaining myself well enough.

I'm not so stupid as to think it can run on air.
I'm trying to understand/know whether a suction feed arrangement recovers from a _brief_exposure when the boat is bouncing around in heavy weather, or is motorsailed on the wrong tack and if so how would it be reprimed.

nobody so far seems to see this as a problem, so perhaps the answer to my question is it either doesn't happen, or it does recover.

 

[rogerthebodger]

You are right in theory but it depends on the detail design of the fuel pickup arrangement. IMHO the correct arrangement is to have baffles around the fuel pickup so that in a low fuel situation the sloshing of the fuel when the boat is bouncing around reduces the possibility of the end of the fuel pickup becoming open to the air.

Well designed fuel have a sump that the fuel pickup goes into that in in effect not only creates a baffle arrangement but also a sump that will collect any crud in the tank. The design must also prevent this crud getting any crud being sucked up into the fuel pickup and block the fuel flow.

 

[chewi]

You are right in theory but it depends on the detail design of the fuel pickup arrangement. IMHO the correct arrangement is to have baffles around the fuel pickup so that in a low fuel situation the sloshing of the fuel when the boat is bouncing around reduces the possibility of the end of the fuel pickup becoming open to the air.

Well designed fuel have a sump that the fuel pickup goes into that in in effect not only creates a baffle arrangement but also a sump that will collect any crud in the tank. The design must also prevent this crud getting any crud being sucked up into the fuel pickup and block the fuel flow.

So it is a risk, that might be avoided by the design of the tank, but if not could well cause a problem if the engine does not recover.
I will look at the tank design next time I get there. Thankyou.