.

Over the years someone did replace the return lines on the injectors w/ some clear lines. Shouldn't there be a good "flow" of fuel through these lines?? There doesn't look to be very much.[/QUOTE]


I think a large return flow is not a requirement. After the injector pressure from the injection pump element has dropped and the injector closed off. The remaining pressure bleeds down I suspect into the return lines. I am just not sure of the correct sequence. There is not much fuel compressed by volume remember as each injector only sprays a very small amount of fuel on each alternate piston stroke.

I will remove the line and see if I have any fuel flow through the return and if so I will replace the old hardened line then clamp and recheck.

Funola, you should do all your testing basically on the thinner viscosity diesel fuel. There is a chance the higher viscosity and other factors may slew things. You can go back to burning wvo later. For example a lot of wvo cars benifited from a booster pump back by the fuel tank.

So first thing this morning I removed the return line from the banjo bolt at the secondary filter while the car was idling. I had NO fuel coming out of the return line from the injection pump, there was only fuel coming from the banjo bolt.

So you think you have an out of spec lift pump? I would suggest confirmation by measuring lift pump pressure directly at the lift pump output by cranking the starter and watching the gauge.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I agree with testing on diesel but not the booster pump at the tank. Most booster pumps cannot match the volumne and pressure output of the Mercedes lift pump and will act as a restriction and starve it of incoming fuel

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

The lift pump is not providing enough pressure to open the relief valve. So the lift pump kit is in order if the fuel will flow from the tank line in the engine compartment by gravity somewhat. Remember the fuel tank has to be pretty full.

Everything so far does point to the lift pump. You should try to find out the normal output pressure of the lift pump on a 616. I know it has to be far higher than 12 pounds. At least 30 pounds sounds reasonable. Some 617 five cylinder pumps make 40-50 pounds. With the obstructed return line test..

Thats what I am thinking...if the tank screen was partially clogged I think I would have seen more drop during hard pulls.

I also considered that as well. It is just that the flow test from the fuel line is so easy to do why ignore it? The pre filter input line has to be disconnected anyways to remove the lift pump for reconditioning. I expect fuel from it to flow reasonably easy.

Your logic is correct. Any major restriction of your system would have dropped the fuel pressure at cruise speed. That will always be the usual indication that one or more of the filters are starting to obstruct..Posing the potential of fuel milage also gradually dropping off.

Unless you take on a load of really dirty fuel. Or the filter is partially frozen in the wintertime from too much water being present in the filter. So generally speaking the gauge will act as a serious amount of water is present warning device to some extent in freezing climates during the winter.

A noticeable large drop in pressure on a cold morning for example beyond a little caused normally by the heavier viscosity of the fuel getting through the filters. You can deal with the issue then before the filter has too much water accumulated when it freezes to shut the engine down or reduce performance

If absolutly nothing else the gauge being a permanent installation will mean filters only get changed out on the real need to do so. Plus people will not drive to the point that their filters are killing off available milage. You can pay quite a price in many ways to drive these cars with a dirty fuel filter long term

This was in fact the most important clue. Actually a piece of the puzzle that ultimatly led me to originally develop the possible concept or theory. Poor injection pump base fuel pressure destroys engines over time. The actual real causitive of the all too frequent number one rod bearing failures. Not absolutly proven but too many parts of the puzzle fit too well to discount it.

Your mentioning things on site as you go along is great by the way. We all constanly continue to learn from others experiences. One thing seems to always lead to another. I will add a little information in the related area to each of my postings on your thread for newcomers to think about. Without stealing it. Some of it possibly is worth repeating.