The main difference between a tank from a petrol W123 and a diesel is the petrol cars had the swirl pot at the bottom of the tank.   The swirl pot allows fuel to continue to flow even during hard cornering, as the return line not only keeps the pot filled, but draws fuel in from the rest of the tank.  Presumably the diesels don’t need this as they flow a lot less fuel and are less likely to do hard cornering.   In addition, an algae outbreak could clog up the little hole in the bottom of the pot.   I’ll be using the biocide additive on my car regularly, so I’m not concerned about this.

When the tank arrived, I was really impressed with how clean it was.  I couldn’t see any rust at all in the tank. The petrol fuel strainer was also quite clean.  Since I don’t want to do this job again, I gave it a clean and was able to get some of the normal residue out of the tank before installing it.   I used my pressure washer to clean inside the tank as best I could, then dried it with my cordless blower.   After that, I used some methylated spirits to make sure I got the last of the water out.

I came back the next day to prepare the tank for installation. I installed a new diesel fuel strainer.   The diesel strainer has a coarse mesh, to allow minor algae to get through and be caught in the pre-filter which is easier to change.   This is part 124 470 01 06.   I also re-installed my fuel sender.   Mercedes thoughtfully made both of these items use the same 46mm hex.   I also had to glue on the rubber ‘foot’ that sits around the strainer and between the tank and the body of the car.

Re-installing the tank was easier than getting it out.   The only slight hiccup was the rubber ‘foot’ had been somewhat dislodged and I had to use screwdrivers to reposition it from under the car.

Once the tank was in place, it was a fairly simple matter to install a new fuel supply hose.  The car was using the fitting from the original hose, with a normal fuel hose clamped to it.  There probably isn’t anything wrong with this, but I bought the proper cohline hose.  It is for multiple cars, so I had to trim a few cm off for the W123 diesel.

Next was the return line and breather hoses.   The EPC states that these hoses are both 120mm.  I cut 120mm hoses, but I found the return hose was too long and I trimmed a few cm off it.   The breather hose was also thicker.   I didn’t actually install the breather hose at first, I waited until I had the car running so I could move it to the hoist under its own power.   I had bought the proper hose clamps for the engine bay and the supply hose, but forgot these two, so had to use the regular hose clamps here.

While I was at it, I also replaced all the hoses and filters in the engine bay.   The pre-filter was quite dirty, and needed replacement.   The main filter probably wasn’t, but I replaced it anyway.  I also used the proper hose clamps that don’t bite into the hoses.   According to the EPC, the hose from the supply line to the pre-filter is 140mm and the hose from the pre-filter to the lift pump is 40mm.

The EPC also lists a special return hose some people refer to as the ‘Cigar’ hose.   This is part number 000 997 51 52.  A lot of people don’t seem to bother with it, but since I was ordering new parts I got one.  I found it to be very long, and I had to push it a long way onto the return hard line so it didn’t kink.

At this point I was ready to refuel the car.  I had 30 liters of diesel I had pumped out of the car through a filter, so I added this to the tank.   I was happy to see there were no leaks and I began to prime the system.

The primer pump felt very odd, and I couldn’t work out why it didn’t seem to be moving much fuel.   After about 30 minutes of trying various things, I noticed that one of my hose clamps was stopping it from going all the way in.   Simply rotating this clamp restored full travel to the primer and I was able to get quite a lot of fuel moving.   I think my issues priming the system last time were due to my blocked strainer, as I was now getting quite a lot of flow.

It still took quite a few times of priming the system, trying to start the car, priming the system, and trying the start the car until it finally roared into life.   It ran well and seemed to be idling better than before.  I wonder if it wasn’t getting enough fuel with the blocked strainer.

A quick test drive around the factory complex showed no leaks and the car running well.   The downside was that the A/C had stopped working, which I will need to attend to.

Since I was happy with how the car was running and there were no leaks. I re-installed the boot trim, and left an Easter egg for a future owner on the fuel tank.

I will check the condition of the fuel pre-filter over time, but I expect that with all this work my fuel delivery problems should now be behind me.

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Previously, I had determined that the fuel strainer was blocked and pumped out 40 liters of diesel into fuel cans.  I assumed there was not much left.   My next task was to see if I could remove the strainer.

A couple of weeks ago, it was a rainy Sunday and my three kids were bored.  They were keen to come with me to my little warehouse unit and help me out on the cars.   The first step was to get the 240D out and up on the hoist.   The three of them were quite pleased that they were able to push the car out on its own.  Then I showed them how to use the positioning jacks to spin it and position it properly for the hoist lifting arms.

Next was to get the car up into the air and drain the remainder of the tank.  We used a tool to stop the flow of fuel as we disconnected the main fuel hose.  I wasn’t sure how much diesel could flow through the blocked strainer, and didn’t want a face full of it as I disconnected the hose.  It turns out I needn’t have bothered, as only a thin trickle of fuel was able to get past.

We let the fuel drain slowly into a bucket.   We were seeing some debris come out as we drained the fuel, although not a lot at this stage.  I had previously purchased a 46mm socket online.  It required a 3/4″ drive, and so I also purchased an adaptor.   Trouble is, I lost the adaptor and so my new socket was totally useless.  By then the kids were bored and we left the car to drain into the bucket.

A couple of days later, I went back and the bucket had just slightly overflowed.  There was more fuel than I figured in the tank, but I was lucky that it was only a tiny bit more, as there wasn’t a very big puddle around the bucket.   There wasn’t a whole lot of clearance around the strainer, so instead of buying another adaptor, I bought a different 46mm socket that could use a regular ratchet.

Even with that and a wobble extension, it was hard to get enough grip on the strainer.  Even when I could, and using a long breaker bar, I was unable to get the strainer to move, the socket would just come off.   I spent (or wasted) two evenings trying various ways of getting the strainer to move.

Eventually a friend told me that taking the fuel tank out of his W126 was surprisingly easy.   The W123 is quite similar to the W126 in that regard, so I decided that I would remove the tank.

Yesterday I went and started that process.  It really wasn’t so bad and I should have done this earlier.   I found this article from dieselgiant the best.   It is quite simple.  While the car was still up in the air, I removed the hoses for the return line and breather.  They were quite hard, so I will replace them.   according to the EPC, they are both supposed to be 120mm in length.

Next with the car on the ground, the divider panel comes out with an 8mm socket.  There are four bolts.   There is no need to remove the carpet backing.   Most of the guides tell you that you need to remove the first aid box and disconnect the sender from the top.   I didn’t, and it was quite easy to remove it from the boot.

It looks like the tank will clear the vacuum reservoir that sits above it, but it won’t.   It is held on by a plastic fastener that I removed with a trim removal tool.   There is enough slack in the vacuum line to put the reservoir to the side.   I also found it easier to remove the right hand side boot liner, which is moulded plastic.   There was one plastic fastener below the tail light.

Its a bit fiddly to get the tank out due to the filler neck.  I found using a pry bar on the right hand side of the tank, as I pull it out from the left was the easiest thing to do.

Once I got the tank out, I was able to use the breaker bar and my 46mm socket to get the strainer out.  It was not easy, I doubt I could have done it from below, had I kept trying.   The fuel sender unit was even harder.

It was also interesting to discover that a used fuel tank was installed some time after 2005.  The replacement tank was removed from a 300D 18/06/05.  It is in keeping with this being a fairly low mileage car that saw limited use.

On removal, it was clear why the car was not running well.   The strainer was coated in black gooey algae.   Its surprising it ran at all!   The sending unit on the other hand was very clean.  On inspection, the sides of the tank are also very clean, there is just a layer of gunk on the bottom.   I used a couple of small brushes I had on hand to clean around the area near the strainer, but I want to find something better before I re-install the tank.    I’m not too keen to use water, as I don’t want to contaminate the fuel or cause rust issues in the tank.

My next steps will be cleaning the tank more, and then starting the re-installation. This will include cleaning up the area where the tank goes.

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I figured I would be doing a fuel volume and pressure test next.   Before I got to that though, it seemed to make sense to install a new W116 fuel hose, strainer and filter.   These parts are all still available and not super expensive.   The hoses get very hard and brittle, and as they are cloth covered, it can be difficult to see their condition.

I probably could have just cleaned the strainer, but it was worth waiting a few weeks to change the part and do the job properly.    The Petrol strainer is different to the diesel one (finer mesh) and now carries a 201 part number.    This part comes with a new o-ring.   The old o-ring was almost non existent, another good reason to change this part.

The W116 fuel hose screws into the bottom of the strainer.    Its a W123 part number and actually the same part as required on my 1977 450SLC, so I had one on hand.   I’ve now ordered another one to have a spare for my 450SLC.   The new hose was nice and soft – the old one had become very hard.

After installing the W116 fuel hose, I replaced the main fuel filter.   The new filter helpfully came with some new copper washers.   These jobs are quite easy to do when the tank is empty.    Its also much easier to do with a hoist.   The first time I changed one of these fuel filters, I was using ramps and it was much more painful.   In particular the hoist allows the access to get everything tightened up properly first time.

After I put the car together, I bought 25 liters of new fuel.   I didn’t want to re-use the old fuel as I had a suspicion it wasn’t any good.   I couldn’t really explain why the car ran so much better when I added fuel to the tank previously.   The state of the fuel strainer, and the rate in which the tank drained made it clear the swirl pot wasn’t all blocked up.    One theory was that the old fuel was so bad that adding new fuel diluted the old and allowed the car to run properly.    It did pain me to discard $60 worth of fuel given current prices.

I started the car up and it ran the same as before.   That is, it would idle ok, but the car would stutter if I gave it any throttle.   It did seem pretty clear that the filters were not the cause as had it been a blockage here, it would have run a lot better from the outset.   I still wanted to rule out bad fuel, so I ran the car for a bit longer.   At this point I was pretty confident that I hadn’t fixed the problem, but felt I should rule out the bad fuel on the off chance.

I’m glad I did.   The car started running better and better.   I ended up taking the car for a 40 minute drive around the local roads.   By the end it was running pretty well.   Not perfectly, but quite well.   Idle was still a bit high, but this may be to do with the cracked exhaust y-piece.   I live in a very hilly area and I was soon able to drive up hills, kicking the car down and not having any stuttering or hesitation.   The throttle linkages and kick down are adjusted properly on this car.   Its quite common to find they are not.

I didn’t push the car too hard as it was raining, I don’t know the car yet, and the tyres are a bit old.  My test drive allowed me to drive on a couple of 80km/h roads to put the car through its paces.   I am going to have to fix the steering coupling before I drive this car too much, as its quite bad.   The exhaust leak is also something that will need attending to.    Next step though is to do an oil/filter change and change the shift bushings.

So far at least, It looks like the running issues for the car were caused by bad fuel.   I have two theories as to why that is.   First one is that the car had been filled up with e10 and then not driven much.   The ethanol attracts water and doesn’t store well.   The other is that somebody wanted to buy the car cheap at the auction and sabotaged it.    I’ve heard of this happening, although I think the first theory is the most likely.   E10 is pretty nasty stuff and has no place in anything but a rental car.

On the test drive, I noticed that the design of the W116 allows you to drive in the rain with the windows down, and have little to no water get into the car.   Its quite a nice feature that modern cars don’t have.

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The two fuel strainers are rarely inspected and replaced.    I decided that I would drain the fuel, and replace the two rear filters.    I have been wanting to buy a waste oil drain tank.   I’m always making a mess doing oil and other fluid changes, and I was able to find a good deal on one.    This felt like a good opportunity to use it to drain the petrol before I ever use it for oil.    It’s not designed for this use, but it felt like the cleanest and safest way of dealing with 30L of petrol.

Draining the fuel was pretty easy.  I disconnected the hose from the exit point of the tank.   It connects to a device that I think is some kind of vibration damper.   I used a hose clamp so I could control the volume exiting the tank and avoid splashing.    As can be seen from the photo above, I used a cloth to see how much debris I was catching, but overall the fuel was very clean.

The fuel came out very fast at first, although it slowed down after a while.   I suspect the slow down was related to when the level went below the swirl pot.   The flow even stopped for a couple of seconds at one point.    Regardless, I didn’t see anything that would make me think the pump can’t draw enough fuel from the tank.

To get the fuel strainer out, first the hose needs to be removed from the bottom.    This isn’t particularly difficult, but there is very little room to get the spanner in, so you’re turning the hose 1/8 of a turn each time.   Once that hose is off, then the strainer can be removed.    The same large hex used for the fuel sender is used for the strainer.   With the right tool,  it is pretty simple to remove.

As can be seen above, the strainer was dirty, but not all that bad.   At least from inspection, and the volume coming out of the tank, it doesn’t appear that this was the reason why the car was not getting enough fuel.

Next step was the main fuel filter.    Since the tank was already drained, there was no need to block off any fuel hoses.    Surprisingly, when I got it out, there was not much debris coming out.   When I have changed this filter on the 300SE, it has been much dirtier and that car runs well.    To do a proper inspection, I cut the filter housing open to check the element.   Obviously, being a fuel filter I used hand tools only!

As can be seen, there was some debris in the housing and in the element, but no more than you would find with a routine filter change.   The filter looked fairly new, so I am not sure how long it has taken to build up this amount of debris.

I have not yet put the car back together as I ran out of time, and my new fuel strainer has not arrived yet.   At least from visual inspection I don’t think I have solved my problem.     I’ll put the car back together with a new fuel strainer, new hose from the tank, new filter and then perform a fuel volume test, followed by a fuel pressure test.    The fuel volume test will also allow me to swap the small strainer at the fuel distributor too.

I borrowed the large hex to remove the sender and strainer.   Its been a very useful tool, I may want to consider getting one for myself.

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