Hey all, pretty new and fresh so please forgive my lack of knowledge..

Fuel system from filter has been rerouted on my used 1987 300SDL - was converted to run on veggie so that may have been the cause, but veggie system is currently down and would like to fix any mishaps with the lines

The fuel inlet runs to what looks like an aftermarket fuel pump to the filter

From the filter fuel line runs directly into injection pump bypassing fuel pump below.

The fuel thermostat and heating system are completely bypassed and the fuel pump has a plugged line on the top of it. No pre-filter in sight, but perhaps there could be a filter in the aftermarket pump?

My guess is that the lines have been rerouted due to the veggie oil conversion, but I am also perplexed. Why bypass the fuel pump unless it was faulty? Why add an unnecessary add-on fuel pump? There is also the chance that whoever made the change didn't fully know what was going on since the car had multiple owners.

The car runs what seems to be fine - I don't have much to compare against as it is my first Benz and diesel.

I'm curious to reconnect to the fuel pump rather than straight to the injection pump, but worry that I might to more harm than good. Since the engine is self-priming through its pressure I wonder if there would be enough to fully clear the lines without the fuel pump should it be faulty. Wishing she had a simple hand pump - cranking the engine with accelerator depressed seems much more rough on the car.


Any advice is very appreciated - I want to make headway, but am hesitant since I fear air stuck in lines and potential damage. Thanks

 

Where to start on this?

The original Fuel Supply/Lift Pump has a Piston to move the Fuel that is sealed by the good fit of metal to metal and is lubricated by the Diesel Fuel. Unless the Former Owner when in and removed the Piston and Piston Spring it is likely that Pump is now ruined. However, you can pull it of and check the innards to be sure.

The Fuel Supply System would normally have a Plastic Primary Filter (to give the Fuel Supply/Lift Pump some protection, it is about 20 nominal microns) and the Secondary Spin-on Filter that is about 10 nominal microns.

What to do?
That depends on how badly you want the to have the Original Fuel Supply/Lift Pump working and if you intend to run Diesel Fuel only in it.

If it was Me I would run only Diesel Fuel in it to prevent any chance of damage and if it is running OK I would leave the Electric Fuel Pump alone with the exception I would add a Primary Filter; what you called a Pre-filter.

The stock Fuel Supply System is supposed to be able to bleed the Air out. However, in practice it takes a lot of cranking to do that. And, if said Air gets trapped in the Fuel Injection Lines between the Fuel Injection Pump and the Injectors you can easily kill an old Battery trying to crank the Air out (to bleed the Air out of there loosen the Line Nuts at the Injectors and crank the Engine till Fuel comes out).

So if you had some sort of Push Button Switch to activate the Electric Fuel Pump or if it goes on when you first turn on the Key the Electric Fuel Pump will prime you system faster. So that is a small advantage.

The Fuel Pressure is controlled by a Valve in a Banjo Bolt on the Engine Side of the Fuel Injection Pump. What is unknown is if the Electric Fuel Pump is able to put out enough Volume to keep the pressure correct.
But, you said it seems to be running OK.

I don't have any specs to give you on what the Fuel Supply Pressure or Flow is supposed to be.

 

If you are speaking of the Stock Fuel Heater I think it depends on where you live if that would be useful or not.
The other question is the Fuel Heater still trying to heat Fuel when you are running the Car or did the Past Owner pull the plug on it?

 

Thanks for the advice Diesel9112 that brought some clarity - I'll look into if the lift pump, but feel like it's most likely broken - may consider purchasing a new one.

as far as the pre-filter I assume install it in between metal line from tank and electric pump?

fuel heater isn't currently needed - looks like plug was pulled


Forced Induction I may consider selling the car, but also see the fixes as good opportunities to learn - would like to fix some problems rather than passing them down the line to someone else

 

"install it in between metal line from tank and electric pump"
Install it under the Hood in the Rubber Hose where the Fuel goes; gets sucked into the Electric Fuel Pump.
The older Modes use a somewhat clear Plastic inline Filter. This can be handy when you are having Fuel Supply issues as you can see inside it.
Diesel Fuel is not as clean as Gasoline is it is not unknown to get a tank full of contaminated Fuel. If the contamination has solids in it that often shows up in the Primary/pre-filter and you know what happened.
If you look up the Filters form My 84 300D you can see what the Plastic Filters look like.
Fram makes some Clear Plastic inline Filters sold at regular Auto Part Stores; but you would have to go to their website to find out if they are OK with Diesel Fuel.