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I did this on my 1991 W126 300SE (M103) with around 92,000 miles on it.

There are a few fuel filter threads on here, but I didn't find one for this exact model. Mind you, they are all pretty similar, but it can't hurt to have another one.

So, I decided to change my fuel filter, as part of many maintenance items I am doing at the moment. I checked the service manual, which seemed pretty straightforward:
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Looks pretty simple right? What could possibly go wrong? Everything - that's what!!

Actually it was a bit fiddly, but as long as your connections are not too badly corroded, it should be simple.

The fuel filter is located behind the right rear wheel, so I started out by reversing the car onto ramps. Once up there, I decided to try a suggestion that I had heard about online. To remove the fuel pump relay, run the engine, let it suck up all the fuel it can, and then when I go to change the filter it should be empty. The other suggstion I had heard, was to open and close the fuel filler cap, in order to release any pressure - I did that as well.

So, where is the fuel pump relay? I've seen different locations for this on different cars, so you'll have to hunt about a bit. I googled the part numbers that were visible, and the first one I tried came back as the fuel pump relay! Here is mine, right in front of the brake fluid reservior. Note: this is a RHD car.
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So, this was a bit of a problem for me. I tried starting the car without this relay, and it wouldn't start. In the videos I had seen online, the car should start and run for at least a few seconds, until the fuel that is already in the lines gets used up. I then tried pullilng the relay while the car was running, and it cut out immediately. Hopefully someone can advise on why this didn't work for me. Also, for anyone trying to remove this relay - it does need quite a lot of pulling! The relay has a small tab on the lower part of each of the long sides, so it is worth gently rocking it back and forth as you pull.

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Here's what the fuel package looks like on my car. The top one is the fuel filter, and I think the bottom one is the fuel pump? I had soaked the botls and connections in penetrating fluid the day before, in anticipation of this job. Actually it looks like the fuel filter is newer than the pump, so perhaps it has already been changed...

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The first step is to remove the protective cover by taking out the three 10mm bolts shown.

Bolts 1 & 2 were no problem, I used a long extension and a universal joint, with a 10mm socket. But bolt 3?! It was impossible to get a socket on there, and although it was just about possible to get a spanner onto it, there was no room to move the spanner once it was on.

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My box ends only have a 10° offset, and the ideal tool would have been one with a 60° Offset, like this:
(PNG Image, 2152 × 2098 pixels).jpg

Alternatively, if I had had a 10mm universal socket, there may have been enough room for that, as opposed to trying to fit a socket AND a universal joint on there.

Anyway, after trying and trying, I noticed that the shroud was cracked, right near the bolt:
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This made me realise that someone else had already been here for sure. Not only that, but they had also not been able to get a tool onto this bolt, and had probably pulled or twisted the cover, instead of doing the job properly and removing the bolt first. Still, it meant that the cover now pivoted quite easily on this bolt, so after turning it slightly, I was able to get the bolt off.

Next I should have disconnected the fuel lines, but for some reason I jumbled up the process, and tried to release the holder first. In hindsight, it didn't make a big difference, but probably it's better to tackle the fuel connections while the filter is securely in place.

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The manual says to remove the bolt in the middle, but as you can see, this cannot be accessed because of the thread of the other bolt. So, that one has to come out first. The other bolt also has a 10mm hex nut on the other end of the thread that is not visible in this photo. You'll need to feel for it behind and then hold onto it while you loosen the E8 bolt from the front.
 

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Discussion Starter #2
So, I realised my mistake halfway through that, and then had a go at the connections instead. I used an open 19mm spanner to hold the filter, and then a 17mm box end on the banjo bolt (a bolt with a hole through it, to allow fluids to pass). It required a lot more force than I had anticipated to loosen this connection. In fact, after a recent unpleasant experience with rounding a bolt off, I switched from a box end to a line wrench:
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This is the kind of thing that is annoying, and also the reason that I wanted to make a thread for others. If you look around the forum, you'll find people who have easily managed to get a socket onto the connection here. Then, when I go to MY car, to carry out the same job, there is a complication. In this case, it is not possible to get a socket onto the banjo bolt. In fact, I had to really press that pipe out of the way, even to get the line wrench on there.

So, the other end is a fuel line, and therefore a line wrench is compulsory, but the filter in there was not an original part, so the 19mm spanner was not big enough to hold it. In the end I found a 25/32" spanner that did the job, and the filter slid out to the left. Note: both ends are super tight, so beware.

Here are the filters side by side - MB Original on the left, and the one I removed on the right:
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Notice that the original part is slightly shorter, and both ends can be held using a 19mm spanner.

Even though there is probably little or no difference in this case, I always prefer to fit original parts where possible. The filter was £31.90 from MB (002 477 4401).

Note: My earlier attempts with the fuel pump relay apparently had no effect on clearing the filter. The filter was full of fuel (approximately one pint), so expect this to come out while you're working.

When putting the new filter in, two things are always stated as the most important. First make sure you install the filter the right way around - there are arrows showing the direction of travel for the fuel. The other thing is to make sure you have the metal washers on correctly on the right hand side (the banjo bolt end). here is how it looks with the filter off:

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Some people have mentioned that their new filter came with new washers, but mine did not. I reused the ones that were already there, and made sure to tighten them as much as I could. I checked for leaks afterwards and haven't seen anything yet. I expect that reusing them is ok if MB did not include new ones with the filter.

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Here's how it looked in the end.

It's pretty clear that this filter had been replaced at some point, so it is unlikely to affect the performance of my car. But, I do have another one to do on my 500SE, which should be a lot easier now that I have done this one. Then again, if it has never been changed, there might be other issues to look forward to! :grin
 

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Nicely done. You can get to bolt #3 (10mm) with a swivel head ratchet, or a flexible extension with a wobble head, or a flexible head ratcheting wrench, or combinations thereof...










 

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Nicely done. You can get to bolt #3 (10mm) with a swivel head ratchet, or a flexible extension with a wobble head, or a flexible head ratcheting wrench, or combinations thereof...
Thanks for these ideas, they're definitely useful for me! I particularly like the flex head ratcheting box wrench! That flexible extension seems the simplest and cheapest solution though.
 

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I need some help with the 500SE. The middle bolt on this car is a screw!? I cleaned it up and took a photo, to find out that it's rounded off already and will not move. It's not possible to reach the nut on the back, although I did try soaking both ends. Obviously I cannot use heat. I tried a screwdriver, and when that failed, a turbo socket, but it's not possible to push hard enough on the screw apparently.

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I have loosened the fuel connections, so I just need to free up the filter enough to slide out. I thought I would be able to do this from the two bolts that are mounted on rubber bushings. One of the nuts came off, but when I try to loosen the other one, the rubber bushing turns instead, loading my ratchet with a spring force. Is there another way of loosening the filter from its housing?
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I really appreciate any advice!
 
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