Hole in fuel return line

[santa clara]

I am thinking of buying a small yacht. The owner reports that the engine starts better when the manual fuel lift pump is operated before cranking. The hypothesis is that there is a tiny hole in the fuel return line from the pump to the tank and that the fuel is draining back if the engine is left for a period after use.
But if this where true, if the pump wasn't operated before cranking wouldn't the air be drawn into the injectors on cranking so that the engine wouldn't start at all until it had been bled?
The compression has been tested and is good, the engine is a Bukh.

Would really like to hear any thoughts on this.
Thanks
Martin

[Pete7]

Martin, surely its more likely to be a hole in the supply fuel line or a fitting is leaking perhaps a fuel filter seal or o ring etc.

Pete

[santa clara]

I was on thinking that a hole before the fuel pump would suck air in and ultimately stop the engine,. This doesn't happen. Once running it goes like a charm.
But a hole after the pump might allow the fuel to drain back. What I'm not getting is why if this is the case the engine starts at all.

Thanks martin

[Pete7]

A hole after the pump would be pressurised by the fuel pump on cranking and eventually once any air pocket has either leaked out of the hole or past a gasket / o ring or worked its way through the injectors. When we change the fuel filter even topping up the new filter there is always an air pocket. Doesn't seem to be a problem though.

The question though is what to do about it. Me? I would replace the return fuel pipe with good quality flexible fuel lines. Ours are a mix of copper and flexible pipe with joints. I will get round to it one day.

Pete

[SailingHarmonie]

The idea of a hole in the fuel return line causing this issue makes no sense and can be dismissed.

Return lines all go to the fuel tank, and end in the air space above the fuel level. A hole in the line would not increase the air drawback at all, and it would be easy to find, because fuel would leak OUT when the engine was running. Besides, the idea of fuel running BACKWARDS through the injection pump and the lift pump is just bogus.

Something else is wrong.

[boatpoker]

Originally Posted by SailingHarmonie

The idea of a hole in the fuel return line causing this issue makes no sense and can be dismissed.

Return lines all go to the fuel tank, and end in the air space above the fuel level. A hole in the line would not increase the air drawback at all, and it would be easy to find, because fuel would leak OUT when the engine was running. Besides, the idea of fuel running BACKWARDS through the injection pump and the lift pump is just bogus.

Something else is wrong.

THIS !

[Kelkara]

Originally Posted by santa clara

I was on thinking that a hole before the fuel pump would suck air in and ultimately stop the engine,. This doesn't happen. Once running it goes like a charm.

An anecdote to encourage you to look at your fuel line before the pump ...

This happened to me last year, exactly the same symptoms ... I blamed it on my recent intsallation of a diesel heater which I tapped off the engine fuel line ... and all summer the engine was a brute to start, but otherwise ran perfectly.

I spent ages looking for leaks around the heater installation, because I thought that was the only change I had made to anything ... after about 2 months I remembered that I had also changed the fuel filter at the same time, and discovered that I hadn't tightened the bleed nut quite enough, and there was a tiny air leak there. After that engine started just fine again.

[skipperpete]

Originally Posted by santa clara

I was on thinking that a hole before the fuel pump would suck air in and ultimately stop the engine,. This doesn't happen. Once running it goes like a charm.
But a hole after the pump might allow the fuel to drain back. What I'm not getting is why if this is the case the engine starts at all.

Thanks martin

Hi Martin, an engine can lose its prime for many reasons particularly if the tank is lower than the engine. The mechanical lift pump valves normally stop fuel drainback but they often make an imperfect seal and then the fuel can run back into the tank. Most return lines go to the top of the invariably vented fuel tank so a “ pinhole” in the line would be of little consequence other than creating a very small external leak. Some return lines go only to the fuel filter head and a pinhole in that case would possibly allow air into the suction side. There are too many variables to dismiss the owners starting technique theory, if it works, just do it.

[santa clara]

Thanks all for the advice. I've got a trusted marine engineer having a look soon...Im putting my money on the lift pump I think. I wonder how much that'll set me back

Regards
Martin

[boatpoker]

Originally Posted by santa clara

Thanks all for the advice. I've got a trusted marine engineer having a look soon...Im putting my money on the lift pump I think. I wonder how much that'll set me back

Regards
Martin

$149 on E-bay and 15minutes to change it out or ... take the old pump into a farm equipment store and they'll probably find you one for $60.

Have a go at it yourself it's two mounting bolts and two fuel line nuts with two rubber grommets.