Diesel Return Line Volume Near Zero

[Mccuedan]

My ISUZU C-240 diesel has been totally reliable, till now. While addressing a fuel flow issue, I attached a transparent inspection hose to the return line outlet on the engine filter head. All my hard injector and return lines are clear, hoses good, filters new. Yet, regardless of engine speed - there is no apparent surplus fuel being returned to the tank - or in this case an inspection container. When engine is hand primed excess fuel gushes into the return line. I have replaced the mechanical lift pump just in case - and no difference. This problem occurred out of the blue - one day the engine refused to start. Bleeding and hand priming allows the engine to run. My concern is the lack of return fuel…as this suggests it may be seeping into the crankcase. Ideas / Recommendations will be appreciated.

 

[madabouttheboat]

If it’s leaking into your crank case your oil level will rise. Have you checked? If it gets too high your engine may run away and……bang.

 

[vas]

depends on engines, fe my generator yanmar 2GMF practically nothing returns.
OTOH, my 2 IVECO 6.7lt straight six turbodiesels, can flood the whole place if return line fails.
I guess the answer is, if it works fine, ignore...

 

[jwfrary]

More likely air is getting in the fuel feed an causing a loss of prime.

Leak off lines often have little or almost no fuel in them

 

[vyv_cox]

I have measured the return flow on my Yanmar 3GM30F. It is very low indeed, a few mls per minute. Some injection pumps utilise a weir system that creates a big flow, while others do not.