How to re-haul the rear fuel delivery system on w123 (gasoline)

Fuel pump assembly w123

The fuel delivery system on w123 consists the fuel tank and a collection of components until it reaches the combustion chamber. This article will focus on the rear components until it reaches the CIS fuel injector assembly. The CIS system is pretty much a black box for me still. I will however go through how you change the fuel injectors and gaskets in another article.

I had to do a re-haul of the entire rear fuel delivery system when an entire tank worth of gasoline (~80 liters) had leaked out after my 280CE was parked for a while, totally delayed my plans for cruising in the summer by almost a week. This made me furious and is the reason for why I wrote this article.

Preparations

It is up to you how many part you want to change. I would recommend to change all the rubber fuel hoses and fuel filters as a minimum!

Parts

  • New fuel tank filter (the first one)
  • New main fuel tank delivery hose
  • New fuel delivery filter (the second one)
  • New high pressure flexible fuel hose
  • Two other fuel hoses and clamps (see pics)
  • 4 fuel assembly rubber mounts
  • [New fuel accumulator?]
  • [New electric fuel pump?]

Tools

  • Spanners
  • 45mm socket
  • screwdriwer
Common questions

So why bother changing the fuel accumulator? Well it can lead to problems such as warm start problems (yeah the M110 usually have warm start problems more than cold starts), this is due to the inability to hold fuel pressure after shutting down the engine and starting it before it gets cold. There are separate start up procedures either if engine is cold or warm. I thought a broken fuel accumulator was the problem with my car’s warm start problems, but it was in fact the old injectors and brittle gaskets that was the cause since they could not maintain fuel pressure.

Why bother changing the electric fuel pump? This has to be the number one reason why people are stranded in their cars with M110 engine. After some time (around 7 years if in daily use) it will with certainty stop working. The part is however quite expensive and I would suggest to not change it if is still working and not obviously very old. A maintenance tip instead of investing in a new part is to remove the corrosion on the electrical connections.

Procedure

Pretty straight forward, just change all the fuel hoses and refit… OK I will guide you through it like Gandalf in the mines of Moria. Smelling fuel is usually an indication of leaking hoses.

Before you even start you have to disconnect the battery to avoid explosions. Then completely drain the tank, either by sucking up the fuel from the gas filler tube or by unloosening the main tank outlet hose from under the tank. I would suggest you drive the car nearly empty before undertaking this task. Make sure the area you are working in is very well ventilated since gasoline fume is highly explosive!

Main fuel hose 280CE

After the tank is drained you can remove the plastic cover start by removing the thick fuel hose connected to the tank. Just have a look at this one in the picture under which caused all the fuel in my car to drain out.

broken fuel hose
Broken main fuel hose

A massive 45mm socket is needed to remove and fit the fuel tank filter. Remember to replace the gasket as well.

fuel tank filter 280CE
Fuel tank filter.

Now you can remove the flexible hoses connecting the high pressure and the return hard lines. If you see rust on the connection then you have to be extremely careful not to round off the relatively soft metal on the hard line unless you want to go through with replacing the entire line. Believe me this is a shit job (my old hard lines were rusted and leaked fuel).

High pressure fuel line W123
High pressure hose

Disconnect the two electrical wires to the fuel pump and when all the fuel lines are disconnected from the body, you can go on and loosen the 4 bolts holding the bracket for the pump, fuel filter and accumulator. Beware this bracket can be pretty rusted and you need either to get hold of a new one or remove the rust and repaint the one you have. The rubber mount bushings are probably dried up or broken so you should replace all 4 of them.

On a work bench you can now go on with the job of replacing the components and the old fuel hoses. Look at the picture below on comparing new and old hoses.

fuel assembly hoses 280ce
Old vs new fuel assembly hoses.

Assembly is just opposite of removing. The main thing you have to consider is to make sure there are not any leaks before you start driving. Look at the picture below to see how the components should fit together in the assembly.

Fuel pump assembly 280CE
Fuel pump assembly 280CE. From top to bottom: Fuel accumulator, fuel filter, fuel pump.

The best thing is that there is no need of special fuel bleeding or start up procedure when starting up the car after fitting all the lines together. Just refill the tank and crank the engine and it will start right up. Nothing difficult as with the diesel.

Then mount the assembly and fasten the hose to the fuel filter. Don’t forget to fit the electrical wires to the fuel pump and mount check for leaks when starting the car. When no leaks are detected, mount back the plastic mud cover to protect the components. Hopefully you will have no leaky days or being stranded by a malfunctioning fuel pump.

Cheers!

Author: robs

Passionate about older Mercedes Benz where driving, maintaining and learning new things is what keeps me doing this. Enjoying writing about cars and teaching others on the black arts of car maintenance. Have a major in space technology engineering and currently doing IT consulting for a living.

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