First post of many to come I am sure. I'm having a poor mpg issue with my '78 300cd and just noticed that when I remove the return line at the filter housing, fuel squirts out of the connection at the housing when I pump with the primer and if the car is running.

Now how is the fuel in the return line supposed to get returned if there is pressure going in the other direction?

I have the old style filter housing if that makes any difference.

To reiterate, there is fuel flowing into my return line from the filter.

Thanks

That's how it works. There are basicly 3 sources of fuel that feed into the return line: The injector return line, the injection pump return line, and the air bleed on the top of the spin on filter. The banjo bolt on the top of the filter acts as a junction to join these three sources.

__________________
1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine

That is how it's supposed to work. The supply line has a small plasitc filter on it and then it goes into the injection pump. The is a rubber line that comes from all the injectors and goes into the spin-on filter, then the fuel goes into the return line that goes back to the tank. The spin-on filter cleans the soot and gunk out of the fuel before it goes back to the tank.

Adjusting your valves might help with mpg.

__________________

Clarification: the fuel is flowing into the injector return line from the banjo bolt on top of the fuel filter housing. Towards the injectors.

When everything is hooked up I have a feeling that there is enough pressure in the injector return line to make the flow go in the correct direction. If you just unplug the line from the banjo bolt the fuel will naturally take the easiest route (the open hole)

__________________
1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine

Thanks. You've convinced me that that is not the problem. Did I mention I was getting 16 mpg with mostly city driving using either B100 or petro deisel. I measured timing chain stretch by lining up the marks under the valve cover and it read 12 degrees BTDC. Could that be causing my poor mpg?
Valves have been recently adjusted, albeit by yours truly, but I don't believe that to be the problem.

bet there is a fuel leak somewhere. Have you looked at the rubber lines that lead from the tank to the metal fuel lines?

__________________
1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine

Are you 100% sure about that?

Here is how it works:

Fuel tank->pre-filter->lift pump->spin on filter->injection pump->injectors
There are return lines from the injection pump and from the injectors. There is also a small bleed hole in the bolt that is on top of the spin on filter (not the bolt that attaches the filter) that returns any air and a bit of fuel back to the tank. Both return lines and the air bleed junction at the banjo bolt and enter the cigar hose to return to the tank.

__________________
1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine

I have not worked on the 617 engine but in post #6 coconutking said that the cam mark lined up at 12 BTDC. Is that correct? I thought the cam should line up at 0 or ATDC if chain streched.
If it is at 12 BTDC I believe that would really hurt MPG.
Did I miss something and become confussed as usual?

__________________
84 500 SEL (307,xxx miles)

So I rechecked my chain stretch after reviewing this helpful link:

http://www.peachparts.com/Wikka/OM617TimingChainStretch

I don't know why I was so off before but I now read between 0 and 5 ATDC depending on where I eyeball it. This is within spec especially at 142K. My suspicion is that I was turning the engine the wrong way the first time I checked it, giving the skewed result. While I had the valve cover off I readjusted the valves since they all needed it. .004in for the intake and .014in on the exhaust. Seems to run a bit better but it ran fine before. We'll see if the MPG improves based purely on the valve adjustment. Thanks for the input guys. Of course, now the vacuum on the door locks isn't working. One thing after the other...

It's always something

Maybe try checking the timing of your injection pump.

__________________
1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine

bgcast has the fuel flow correct.

P E H