1972 Low Pressure fule Pump 280 6 Cyl Gas line

[aluminum]

What is the correct*** " Gas Return Line " *** Hook up ?

Car is a 1972 MB 280 SE 6 Cil Gas Motor ( Mechanical Fuel injected )

I am getting Mixed Info and confused by such ?


The Low Pressure Pump mounted at rear of Car has several Gas Lines in and out > The Car I am working with was Wrecked in the rear so Bad I could not see the lines and had to pry out the tank with a crow bar lol and the lines where torn and damaged all apart . Car has been sitting at least 14 years ..
The Question is this :

(1) What is the correct way to hook up that low pressure Pump ? :
(2) where do I hook up the return Line ! ?


(I have looked at such cars and found them hooked up different ways ?)
Rather than the return line going to the return Line port on the Pump I see that Port blocked off with a Screw at the end of the port ( the Port is threaded for this screw ? leading me to think you have perhaps an option as to how to hook up the return line perhaps ? )

***
2 Cars I looked at have that Port Blocked off ?

( A ) And a T Fitting with the return line going into the T

( B ) Out of the T the other 2 line's go's to (B 1 ) the Pressure line going to the motor ? < and > ( B2 ) to the Gas Tank ?
( so 3 lines at the T return line feeding in and then going to the Motor and going to the tank ? seems very strange )


Found this Good Video online that show's that you hook up the return line to the return line pump inlet now that makes sense it seems but is unclear as to how the return line gets the gas returning to the gas Tank
( Note Gas Tank has 2 Fuel lines 1 large one about 3/4 inch to feed the Pump by gravity flow with Gas out of the tank / and one going into the tank for Fuel return ...
Link to Video

https://www.google.com/search?client=firefox-b-1-d&q=1970+Mercedes+280+SE+how+do+you+hook+up+the+rear+Fuel+Lines+correctly+%3F#fpstate=ive&vld=cid:d4886566,vid:WNRFZ5hBfq8

All of this is important for several reasons :

Example >(1) if you block off the return Line ( don't do this I discovered )
The Low Pressure Pump will get very Hot as the Gas going through it keeps it cool " at the right Pressure "
THIS HOT PUMP syndrome will happen in just a few Min's pump running ( about 5 Min's > pressure will build and perhaps Blow seals
and damage the Injector Pump and I am not sure what all > you need the return line and you need it hooked up right no doubt

(2) the Fuel I suspect from the Fuel return Line has increased pressure as it I think passed the Injector Pump ( not sure on that ? )
( I used a Clear Plastic Gas line to get the Car running with a Temporary Gas Tank at rear of Car and noted a LOT OF FUEL go's through the Fuel Return line not a small amount at all a lot ! .

So again whats the right way ? to hook the return line to the Pump ?
or to as I have seen leave that port blocked and hook it to the pressure line going to the motor and the Tank ? >

I Can not find any real Info on this ?

in fact for these W 108 Gas Fuel injected car's the 280 SE are simple left out of several manuals I have ?? they just leave these Cars out of the manuals for some reason > Yes I am sure you can but them somewhere etc but I all ready have several Mercedes Manuals but none of them cover This Car ? They cover all the others cars the same year but not the 280 SE 6 Cylinders ? They skip that one.

Wow these Pumps are Costly I am glad I grabbed 3 of them / The Motor Run's great BTW I guess I well Part out the rest.

Thanks if anyone has the correct info Lets keep these W 108 W109 W111 going

[aluminum]

Originally Posted by subforry

I was just under my w108 280SEL. The return fitting at the pump runs up to a T in the return line. The other connectors on the T run forward to engine and backward to the tank.

Thanks : SEL Most likely a V8 / ( mine is a280SE 6 Cylinder 1972 with MFI )

But anyway your Return line hooked up to the return port on the pump

Both I looked at have that port blocked off (with a screw the port is " threaded" and you can block it )
And the return line hooked up to (1) The Tank return port on the tank (2) the Pressure line to the Motor ? (3) The return Line itself
*** ( But Not to the Return port on the Pump ? ) *** I am hoping someone can enplane that to me ?

As 2 of those lines are under pressure , Different pressure I think seems strange > To return pressure to the Tank makes sense
to keep pressure from building to Excess at the Injection Pump , Injectors etc .

Very strange set up ? the return line is under fuel pressure when key is on
as Fuel Pump is always running ( Key On ) and they hook it to a Line going to the motor that also under pressure ?
On top of that the pressure is also going back to the Tank ?

This is hard to digest LOL ???
( if you watched the Video it seems to show the same Return line hooked to Pump Port and a T connection )

[280SE1972]

Hello, I am new to this forum and just bought a barely running 1972 280SE (6cyl). While replacing all the fuel lines I came across a very weird situation. There is a T in the fuel return line with one leg going back to the pump. Makes sense but when I remove the line there is a screw blocking the bypass port on the fuel pump so there is in effect no return to the fuel pump. Did somebody screw up when they installed a new pump (it is the new short style)? It sure seems like that screw should be removed. I have been researching this for days and can't find an answer. I would really appreciate any insight from the experts out there. Thanks.

Rich

[aluminum]

Originally Posted by 280SE1972

Hello, I am new to this forum and just bought a barely running 1972 280SE (6cyl). While replacing all the fuel lines I came across a very weird situation. There is a T in the fuel return line with one leg going back to the pump. Makes sense but when I remove the line there is a screw blocking the bypass port on the fuel pump so there is in effect no return to the fuel pump. Did somebody screw up when they installed a new pump (it is the new short style)? It sure seems like that screw should be removed. I have been researching this for days and can't find an answer. I would really appreciate any insight from the experts out there. Thanks.

Rich

Yes the screw should not be their > That Line is the Fuel return Line and has Hi Pressure BTW : ( it returns Gas to both the Tank and the Fuel pump at about the same pressure as to the injection Pump 15 LBS + > The best set up is to have a 10 / 20 Micron ( a fine Filter ) >> AFTER the Rear electric Flue Pump in that short ( 3/8 th pressure line > to motor ) A Rubber Gas Line that hooks to the Metal Line to the injection pump .> The Outlet After the electric Fuel Pump is High Pressure and you need that Pressure to Pump through a Filter that fine > The Gas Line from the Gas Tank is Gravity fed & low pressure, as such will not push through a fine filter ( as it gets dirt / crud in it ) >>> OK You Have 2 Factory Filters to Clean (1) is the Fine Metal Mesh Filter in the Gas Tank Drain that you remove with a Pipe wrench or a upside down certain size Spark plug wrench ( you simply remove the Gas Drain and clean filter than replace it >> (2) At the Electric Fuel Pump Inlet where the large Gas Line from the Tank enters is a Large about 1 & 1/8 inch or so I forget socket fits it > You need a Impact Gun and can remove it right on the Car > than just Tap the Dirt out upside down and blow it all out with compressed air . < if its been sitting I suggest you remove the bottom electric Fuel Pump Cover Plate screws and clean out any scum / dirt etc Its simple and you can't miss install it only thing is the Large rubber O Ring may need replacing don't remove it unless its broken or vastly enlarged .With the Cover Plate off you can remove the Fuel Impeller comes right off simple clean and inspect it .
This inline Filter works at O'reilly's or any auto parts store > MicroGard Fuel Filter 33002 about $6 / You Can Check your Fuel Pressure at the CSV ( Cold Start Valve at the Bottom of it on the intake manifold is a Pressure Check Plug & with an adapter you can hook a Flue Pressure Gauge / Check it with Key on Pressure > Than Running at Idle > Than rev the motor up a Bit when right the Pressure should be about the same perhaps a small drop when RPM's up > Your looking for 15 to 17 Lbs.

I suggest you do not take the injection Pump apart > if the Motor Runs than just keep running it and often it will clean it self out > For the Gas Tank > remove it and use Vinegar about 3 Gallons & slosh it should Every few Hrs 4 about 3 Days one way than 3 Days the other way > Keep pouring it out through a Canvas Rag into 5 Gallon Bucket to collect the Crud and Rust from the Vinegar & Pore Back to tank . > Pressure wash the tank as well as you Can than pour the now filtered out Vinegar back into the tank > Note the MB Gas Cap needs to Have Grease > Tape on it as the Vinegar will eat it is aluminum in Part not Vinegar's Friend > Once Cleaning is done Use Baking Soda to Neutralize The Vinegar and Rinse Tank > pour in about 1/2 Gallon of Diesel with about 1/2 a Quart of ATF mixed in and coat the inside of the Tank , As it will Flash Rust if you don't > once done you can Drain it out and store the Tank or use it

[cth350]

Originally Posted by 280SE1972

Hello, I am new to this forum and just bought a barely running 1972 280SE (6cyl). While replacing all the fuel lines I came across a very weird situation. ...

Rich

In general, it's best to start a thread unique to your problem.

Having said that, the fuel pump outside the tank is designed to deliver a lot of fuel at low pressure. It does not have a pressure bypass to route fuel back to the tank or to receive it from the engine compartment.

The system is designed to keep the fuel moving through the plumbing and not stick around in any one place too long to get hot.

It is possible that somebody got confused between the 280SE and 280SE 3.5 or 4.5 (both are V8s) setup, which is a high pressure pump with a return line.

If your fuel pump is working correctly, it will delivery about 4 liters per minute of fuel at about 15psi. Measure that with a 1 liter container in a 15 second span. If the volume delivered is too low, check that the voltage at the pump is at least 9 volts (low voltage == low fuel delivery on a DC pump).

Common problems with the fuel pump, other than old age and varnish may be fine rust particles clogging it up.

Best to figure out if the pump is good or bad and move on from there. A poorly adjusted fuel distributor (also called a pump) under the hood is more often the culprit. Try not to make it worse.

-CTH