How much fuel is returned to tank on old Mercedes? - PeachParts Mercedes-Benz Forum

MisterSalty · #1 01-05-2017, 01:32 PM

Hi there!

Does anyone know roughly what volume of fuel an old Mercedes diesel (specifically a 240D from '79) returns per minute/hour to the fuel tank? (i.e the fuel that the pump sucks from the engine, but doesn't actually get injected into the cylinders and instead returns to the fuel tank via the fuel return line). I can't seem to find specs for that on the internet, and discussions of diesel engines in general mention very different amounts.

I'm also interested in how much it varies depending on whether the engine is at idle or running hard, etc. From the Youtube videos I've seen of people doing diesel purges, it looks like the flow is at least 10 GPH even at idle? (but that's just from eyeballing it).

Context: I've got a '79 Mercedes 240D engine in my sailboat (lucky me!) and a 300 gallon fuel tank. It would be good to know how many gallons/hr get circulated in the tank for a couple reasons: 1) it'll let me know how large of an external fuel filter I need to install (boats need a water-separating fuel filter in front of the engine's own spin-on filter--diesel absorbs more water on a boat); 2) it'll also let me know how long I need to run the motor to have (on average) sucked all the fuel through the filter (again, in a marine environment you need to do that periodically to clear out algae that grow in the tank).

Anyone have any concrete knowledge here?

Thanks.

MisterSalty · #2 01-05-2017, 01:40 PM

Here's the Youtube video I used to guess the flow rate is at least 10 GPH:

https://www.youtube.com/watch?v=KVDGTArYz3w

(goto 2:30 to see the best shot of the flow).

oldsinner111 · #3 01-05-2017, 01:52 PM

when I use to run cooking oils,my tank would heat up,in a hours time.So I looped myyy fuel return so I would not heat tank,and cause moisture.

meltedpanda · #4 01-06-2017, 09:10 AM

I am no expert here, but I remember when I purged my ol diesels I was amazed at how much fuel returned. I theorized at the time that whatever came out of the pump was essentially returned as the pump is designed to keep all lines full and somewhat under pressure . The opening of the injectors reduced overall flow but was also replaced as quickly. If you really need to be exact , set up a temporary 1.5 gallon fuel supply , have the exit hose into another container , run the engine at idle until it used a gallon ( that way you wont have to worry about air, oh and get a good book) then do it again at increased RPM.

Idle · #5 01-06-2017, 06:41 PM

I don't have a source for this but I remember reading some 30 or so years ago that one reason the fuel returns is because it helps to cool the fuel system.

The pump puts up more than the engine could ever use so some fuel will be forced to return. Just what part of the system this is supposed to cool is something I cannot remember.

It seems to me that friction causes heat and the fuel running through the lines would, through friction, cause the lines to heat up. But apparently this is not the case, or at least not in this application.

Diseasel300 · #6 01-06-2017, 07:30 PM

How much fuel returns to the tank is a function of how many RPM's the engine is doing and what kind of load it is under. The lift pump always supplies more than the engine can use at full load and RPM by design to keep the injection elements from starving for fuel, it also helps to purge any air that collects in the tank.

The fuel does help cool the injection pump. Friction heats up the elements and the pump body, and lots of heat is transferred from the oil, especially on the (70s?) later pumps that used engine lubricating oil. Cooling is not the primary function of the overflow/recirculation though, it is to ensure ample supply and help keep the system purged of air/vapor.