Ever wondered what the funny fuel pre-filter was for?

[turboseize]

This is a diesel pre-filter on a 240d (w123) after only about 850km/530 miles.



This is the debris that was left on the tray after I tried to drain the pre-filter:


There was still plenty of stuff in the pre-filter that refused to leave.


I have save the main filter and will cut it open after it has drained completely. You will, of course, not be spared the pictures.


But how how does stuff like this happen? This is the first time I have ever seen this funny little thing do anything.

I resurrected a 240d (w123) after sitting still for at least 15 years. Tank was full to the brim, and there was not the slightest leak anywhere, so we were optimistic that it might have survided. A vain hope, as we now know.
All fluids (except fuel... fuel is expensive here in Germany!) have been changed, as well as all filters. (There also was "some" body and paint work necessary as well as some chassis components that were shot, but that would be another topic).
After 500km, the car slowly started to lose power. At the time of changing the filters at ~850km, it was barely able to keep up with city traffic (50km/h) - and only on a flat road. Hills slowed her down to about 30km/h.
With both the pre-filter and the main filter changed, she is now back up to her top speed of 155km/h again!

Poor car will definitely get a "new" tank - a friend of mine already secured one. As this tank is a couple hundred kilometres away, the question is: how many filter changes will I need to get there?

 

[bullwinkle]

Got some junk in your diesel-might be able to kill it with diesel biocide, but you’ll be changing filters constantly. 155 KPH (over 96 MPH)??Are you driving it off a cliff?? I’ve never seen a W123 240D make it over 100 KPH! It doesn’t take a lot of water in a fuel tank to cause issues-the water sits on the bottom, the black stuff (fungus) grows at the fuel/water interface. Then sits on the bottom of the tank to get sucked up by the fuel pickup.

 

[fantastic]

There is rust forming when moisture starts touching anything metal within the whole fuel system. This may need a few of these changes until the filter appears to "Flow" again. I don't think I've ever seen so many tags on a post before LOL.

 

[turboseize]

@bullwinkle Yes, I assume that there was not only rust, but also algae. Fuel in the fuel filter appeared black with a reddish hue - and still does, even after two fill-ups (about a quarter tank each).

The original tank filler cap gasket had disintegrated, so I had fuel spill out during the first drives. The fuel then found it's way back into the car and into the trunk, running down outside of the filler neck:



That is not how diesel is supposed to look like!

I have added a can of biocide (Inox Diesel-Bakterizid) to the tank about 150km ago. I still have a can of Liqui Moly Anti-Bakterien-Diesel-Additiv at hand to use after the tank gets replaced, just to be sure nothing survives and I won't contaminate the "new" tank. (New tank is an 80-litre tank from a petrol 250, so it should be algae/fungus-free.)




The reason that you have never seen a fast 240d is probably that with their 4-speed gearbox, the get really loud. 100km/h (60something mph) is just shy of 3000/min, that is acceptable. 120km/h is noticeably noisier!

 

[bullwinkle]

I would change the tank fuel line to a more modern, ULSD resistant rubber hose while I was at it, too-the hose on my ‘82 300D got soft & started seeping fuel, pretty sure I used Gates Barricade or something similar (not sure what is available there).

 

[turboseize]

Fuel lines still look good. Soft and pliable, without any cracks. They are also completely dry from the outside not a single trace of fuel.
What we had to replace, though, were the fuel return lines on top of the engine, Those were dead and porous.

But of course I will closely monitor the lines. 40 year old rubber parts are somewhat unpredictable anyways...

 

[eljefino]

What if you found some cheap universal fuel filters and an electric pump, then stuffed the pickup for the pump down into the tank. Rig the pump to push through a filter, discharge into the tank, and keep running this setup until you stop plugging filters?

I can get 10-packs of chinese filters on ebay for short money.

 

[vavavroom]

If only I could get to the fuel filter.

 

[turboseize]

If only I could get to the fuel filter.

That problem is easy to solve: just get an old Mercedes. w114, w123, w124, w201... Regarding fuel filter changes, they are all identical, even thogh the 201 and 12 use different engines.
Fuel filter can be changed in just a few minutes. Actually, priming the filters and bleeding the system is what takes the most time and effort.

This is the engine by of one of my previous cars (I should never have sold this one!)

This is a 300d automatic with air conditiong and central locking. That is about as crowded as a w123 engine bay can get... a four cylinder without any options has way more space to work in the engine bay.
But even here, in this worst case scenario, you have more space to work than in pretty much any other car model.
You can see how easy both fuel filters are to reach - the yellow cucumber is the pre-filter, the main filter is on a post, in the picture right below the red oil dipstick. It doesnt't get any easier.

Oil changes are equally easy. The oil filter is a cartridge style (housing can be seen on the far right) - loosen two bolts, bush vacuum lines two centimetres to the side, open housing, replace o-ring, exchange filter cartridge, reassemble. You do not even need to get under the car to drain the oil - MB designed dipstick and drain pan so that the oil can be sucked through the dipstick tube by a vacuum pump. Mind you that the main target group demographic for the diesels were taxi drivers (back then, often owner-operators). Reliability and ease (but also speed!) of service were main selling points. This is also shown by details like the spring-operated hood which has two opening positions - one traditional 45° for quick checks, and then a second, 90° position to allow easier access when actually performing work in the engine bay.
Ease of maintenance was the main reason why I was running w123 back then when I had weekly commute of 2x800km each weekend. There were certainly more comfortable ars with better seats, there were cars that were less noisy, and there were cars that had way less fuel consumption. And there were cars for which you opaid only a fraction of the registration/road tax. But ease of mind (legendary reliability) and the extremely low maintenance cost made it worthwile.

 

[turboseize]

What if you found some cheap universal fuel filters and an electric pump, then stuffed the pickup for the pump down into the tank. Rig the pump to push through a filter, discharge into the tank, and keep running this setup until you stop plugging filters?

I can get 10-packs of chinese filters on ebay for short money.

No power outet in the garage (I rent a flat - and a parking space in the buildings basement.)
If I live in the countryside or in the suburbs, that would have been a good idea though.

 

[turboseize]

Until I get the "new" tank, it seems like I will have to settle on a 500km filter change interval.
Took advantage of the good weather today to take the car to a car wash to get rid of the salt from when I brought it home from the workshop. Found that there was a long line of cars waiting, but as my job requires that I am agile*, I quickly decided to spend the afternoon burning some fuel instead. The leftovers from dead algae can be corrosive and I want that stuff out of my system.
So I managed to get the tank down to slightly over 1/2 and then I filled up - with the winter season, it may well be possible that she may have to sit for several weeks in the garage until another salt-free window of opportunity opens. If I can't get rid of everything, at least I can dilute it and make it less aggressive. (The biocide had been in the tank for several weeks - any living thing in fuel system now should be dead by now.)

After the fill-up, not a single air bubble or sign of vacuum to be seen in the pre-filter:



Good!

But that only lasted so long. After the stop at the gas station, I drove to the car wash again, found a free space, got the salt off and some wax on.
Then on the way home the engine started to lose power above 4000/min when accelerating uphill in 3rd gear at full throttle. Luckily, that is a part in the power band that is not necessary for doodling along, but that is exactly how a plugged filter starts to make itself noticeable. This was ~250km after yesterdays filter change and 50km after the fill-up. And coincidentally, we started to see the first symptoms of power loss at around the same km mark when we brought the car home after purchase. So we will probably have another 250km until the filter will impair driveability. My fuel filter change interval will be down to about 500km for the foreseeable future.

I am really looking forward to installing the "new" tank! In the meantime, I'd better stock up on filters...



*Another buzzword! Yeah!

 

[turboseize]

628km (390mi) later:




This one came sudden. Usually, with clogged fuel filters, there is a pre-warning. You slowy lose power and the rev band gets narrower.
Here, I was cruising smoothly on a country road at about 90-100 km/h (roughly 55 to 60something mph), when the engine stuttered. I was loosing power very quickly and barely made it off the main road onto a small access road, where the engine stalled. I was able to restart it, but it would barely hold idle and did not produce any power. Feathering the clutch, I was able to get coax the car onto a safe parking space - at about walking speed.
From running smoothly at 90km/h to stalling in about 30 to 45 seconds.

Anyways. here is the prefilter:

I did only change the pre-filter this time and let the main filter in place. Working with care and reasonable attention (and with the help of a few rags that I keep in a closed metal box in the trunk), I can change the pre-filter without spilling fuel on the ground, which is why I will do this "on the side of the road" if necessary. However, I'ver never succeeded in changing the main filter without making a mess - it's full to the brim and there's a good chance you'll spill more than the rag can soak up. As diesel fuel has no business in the environment, I'll change the main filter in a workshop or garage only.

Luckily, changing the pre-filter solved the issue and the car is driving fine again - for now.

 

[turboseize]

I changed both filters today - car was starting to lose power while driving on country roads. City traffic was still fine.
So I drove to a self-help garage, booked a 30min timeslot (6€) and changed both filters.
The pre-filter lasted only 139km. (Main filter lasted 767km, will be cut and photographed and get it's own thread.)






What I found remarkable is that the car was still totally driveable in city conditions. Under low load, a diesel uses very little fuel indeed...

 

[53' Stude]

I changed both filters today - car was starting to lose power while driving on country roads. City traffic was still fine.
So I drove to a self-help garage, booked a 30min timeslot (6€) and changed both filters.
The pre-filter lasted only 139km. (Main filter lasted 767km, will be cut and photographed and get it's own thread.)



View attachment 195419View attachment 195420


What I found remarkable is that the car was still totally driveable in city conditions. Under low load, a diesel uses very little fuel indeed...

Big issue if you can’t take it out on highway, but runs fine in city

 

[bullwinkle]

That looks more like rust than algae-algae is usually black.

 

[turboseize]

Probably both. That stuff is smooth and slimy. There are no hard particles - at least not in a size that you can feel between your fingertips.
But it's colour could well be from rust.

 

[JHZR2]

Probably both. That stuff is smooth and slimy. There are no hard particles - at least not in a size that you can feel between your fingertips.
But it's colour could well be from rust.

I have my doubts that it is rust. I’ve had rusty tanks. On my one very high mileage car, the tank had a lot of rust even on the cap and filler neck. But the fuel in the prefilter was still clean, the rust particles are too big. For red rust to color the fuel Id have to think that there’s a ton of water in the tank, and then the engine wouldn’t run properly.

I’d install a racor filter underhood and use that as a means of having more holding capacity. They make knockoff filters that are sold on Amazon. For this use case I’d get a cheap disposable knockoff racor media, biggest size (25um I think), then plumb the OE prefilter, and run a quality Hengst, Mann, or mahle secondary.

 

[JHZR2]

What I found remarkable is that the car was still totally driveable in city conditions. Under low load, a diesel uses very little fuel indeed...

That’s because the lift pump is run off the internal cam in the injection pump. There is fuel in the system at a base fuel pressure. At lower speed, the base pressure can be maintained and enough flow, including back to the tank, can be preserved.

At higher rpms, the lift pump probably can’t get enough fuel due to the restriction, and then base fuel pressure drops. It starts to consume out and the higher pressure fuel from the return lines gets past the overflow valve.

I’d check two things:

- overflow valve spring length. Just because you want to keep more base pressure in the IP.

- if the actual lines are correct coming from the tank. Sometimes folks with tank issues will swap the to-engine and from-engine lines. This allows unstrained fuel from the tank to go to the prefilter and lift pump. This is because if the tank is bad, the tank strainer screen can clog. You should know if this is the case, as ultimately it needs to be remediated.

 

[JHZR2]

Probably both. That stuff is smooth and slimy. There are no hard particles - at least not in a size that you can feel between your fingertips.
But it's colour could well be from rust.

Are you running biocide in this car?

While you’re plugging filters so rapidly, now is the time to be killing the bugs and capturing them with the slime.