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Discussion Starter · #1 · (Edited)
Hi All,

I previously asked for help with a fuel system issue and with help from this forum and other peoples similar issues I have fixed the issue. It was really not obvious what the problem was and so to give something back to the community I have summarised the whole process from end to end here in the hope it will help someone else out.

E90 (2008) 320 Diesel. N47 Engine. ~120k miles. UK (Right hand drive)
Much of the information I found in relation to this were from US posts and as diesels are much rarer in the US, mostly referred to petrol cars, which have a different fuel system setup.

In this post I refer to left and right side of the car, rather than driver / passenger as that changes depending on your market.

The problem:

It started with slightly longer cranking times, 2 to 5 seconds instead of the usual 1 to 2 seconds. I should have investigated right away, but as the car still started and ran, I just ignored this at first.

Then the crank times started getting silly long, 30 seconds, sometimes 3 or 4 attempts at 30 seconds to get started but it would eventually start.

At this time, the fuel gauge was showing about 1/4 of a tank remaining, I have never had any issues with fuel senders or the gauge and so had no reason to suspect anything was up with that.

Eventually the car just gave up and wouldnt run, I managed to get it going just long enough to get home (about 3 miles) and then started trying to figure out what was wrong. At the same time as this "last" journey I noticed a new noise coming from around the rear-right wheel arch.

Important thing to note, there were no malfunction lights from the car (apart from when it completely died on me) and I didnt have a OBD reader with me at the time.

I started to do a lot of reading up on forums etc. There are other threads that relate to this, this one in particular was VERY helpful, but also very long.

The ultimate solution in this thread turned out to be a fuel pump (mine is a different issue) but if you have a similar problem, this thread is worth a read (Thanks to Tice03 for the work he did on diagnosing his problem)

I concluded, incorrectly, (at this point) that my fuel pump was at fault. The fuel tank sits underneath the rear seats. If you remove the seat (very easy to do, it just unclips witha sharp pull) you will find two access ports to the tank, one on each side of the car).

The right side contains the low pressure fuel pump and a tank level sender.

The left side contains the "siphon pump" and a tank level sender.

The fuel tank itself sits over the top of the transmission tunnel joined at the top. I have heard it referred to as "saddle" shaped by a number of people.

This is a diagram of the tank.

So the low pressure pump is on the right and as I mentioned, my car was making a new noise which was coming from the pump. It sounds like this:

So, given the noise the pump was making and the fact the car had stopped, I concluded the pump was faulty. I read that these pumps rarely go wrong and so was quite happy buying a replacement from eBay which cost £20 (much, much cheaper than a new one).

Fitting the pump is actually quite an easy DIY job. There are some good instructions on pelican parts if you want to do it yourself.

It took me about an hour to do, but I went slowly and carefully, if you are a bit more confident its probably a 30 minute job.

Now, I noticed something interesting at this point. The instructions talk about changing the pump only when the fuel level is very low or the tank is drained. This makes sense and is what you should do, but I didnt. Firstly, I was working with diesel, which is much less likely to explode than petrol and secondly I considered that 1/4 of a tank was probably low enough anyway.

What I noticed though, was that the right side of the tank was pretty much dry, almost no fuel in there at all. Strange, because the fuel gauge reads 1/4 tank. I made a mental note of this and put everything back together.

When I started the car, it still took loads of cranking, but as I had replaced the pump i expected that the fuel line may have drained and this was expected. What I also expected was after it had started once, that it should be fixed after that. It wasnt. Still very long crank times and also, the bloody pump was still making the same noise. The "new" pump. Very unlikely that both pumps had the same issue, so it much be something else.

Listening more closely to the pump noise, I noticed there were two distinct noises (they dont come out very well in that youtube video) one is the mechanical whining and the other is the sort of rattly noise. That is actually a noise more like the sound when you suck the dregs of a drink through a straw. Which combined with an dry tank on the right, makes sense. The pump was starved of fuel.

Lots more forum reading...

The issue

So basically, for those that dont know, the fuel system works like this.

The low pressure pump in the right side of the tank, pumps fuel to the high pressure pump where it goes to the injectors. Excess fuel is sent back to the left hand tank where a "siphon" pump (its not an actual pump, just a siphoning effect created by the pressure of the returning fuel) pushes fuel across to the right side and round it goes again.

The system is supposed to keep the right side filled with fuel whilst constantly emptying the left side to the right side.

I want back under the back seat and accessed the left side of the tank and to my surprise the left tank had loads of fuel in it, whilst the right was pretty much dry. Ok, so the fuel is not transferring - why?

Whilst I had the tank open again I checked each of the fuel senders was working properly (I didnt think this was the cause, but others reported problems with these and since the tank was open I check them)

You can use a multi meter to check the resistance of each sender, or the car can actually do that itself. Put the OBC into its hidden menu functions (loads of info on this elsewhere if you need to know how to do this) and go to menu 6. This shows the fuel levels and resistances from each sender and you can check them from there. You will still need to remove the senders from the tank so you can move them through their full range of movement to check the resistance range.

So these were working fine, this is what my fuel levels look like:

fuel levels.JPG

As you can see, left side 17.6 litres, right side 1.6 litres, total 19.


So now I know what the problem is, what is the solution and there seems to be a few possibilities.

1. The low pressure fuel pump. If this is failing, then (I think) it does not pump enough fuel to engine, so not enough is returned and there is not enough flow to move fuel across. Since I had already replaced my pump, I ruled this one out for now.

2. The low pressure pump control unit. This sits up near the right side rear wheel arch, access if from behind the rear seat back (requires removal). It's a bit of a pain to get to, but once you get access, its very easy to replace and if you buy from ebay these cost about £20. Make sure you get one from exactly the same car as you have though, otherwise the unit will need to be re-programmed so it knows you are int a 320D not a 335i for example. If you cant find one for your car, you can re-code them, but you need to get into coding your car to do this. Lots of info about this is you want to look it up, it is quite involved though.

3. A blocked / split fuel line preventing the transfer. The pipes are strong plastic and they are encased inside the fuel tank. I think its very unlikely one of these would split, but its quite easy to do a visual check. I also took a long piece of stiff wire and carefully threaded it down the pipes that run from left to right to check for blockages. I found nothing.

4. The fuel filter. This one surprised me, but yes the filter can cause the problem. Filters are cheap and easy to fit (even easier on a hydraulic lift - wish I had one of those!) but if the filter is old and is restricting fuel flow, then it can cause enough reduction in flow to prevent the siphon action from working.

I didnt know at this time if it was the control unit or the filter, but seeing as they are about the same price and the filter is easier to fit, I tried that first. Swapping the filter took about 40 minutes and would be much quicker with a car lift or pit.

After that was done I put 20 litres of fuel in the car from a can, this took me up to just over a half a tank, about 20 litres ish on each side.

I started the car (long crank again, but the fuel lines were drained, so expected). After starting, the second start was much better, almost instant but the fuel pump was STILL making the noise.

Then over the course of the next few minutes, with the engine running the pump stopped making the noise. I still dont know what caused the noise, the only thing I can think of was their were pockets of air in some of the pipes and it took a while to get out of the system. It doesnt make total sense, so its probably wrong, but the noise hasn't come back again.

Test Drive
Putting the OBC on menu 6 again, I did some driving. With over 1/2 a tank of fuel, it's difficult to tell as every corner you go around sloshes fuel from side to side. So working out if fuel is moving from sloshing or the fuel system is difficult. A number of (problem free) miles later, the tank is under 1/2. Driving gently and monitoring carefully I can see fuel is definitely moving across again.

In fact the fuel seems to transfer at its fastest at around 1500 to 1800 RPM, at this RPM fuel moves across at about a litre every few seconds. Whilst driving along my friend took the following picture:

fuel levels 3.JPG

Left tank empty, all fuel on the right. :thumbup:

The one thing I have yet to do is run the fuel right back down. I don't think its going to be an issue, but I will provide a final update when I do (fingers crossed!).

In conclusion - if you have this problem, I would change the fuel filter first (unless you know its not that) since it is cheap and easy to do and hopefully saves you all the grief I had to go through!


101 Posts
Its old post but I hope you see my post. I have exactly same issue. I have no fuel right side which is fuel pump side. I have all the fuel left side which is filter side. My car is 525i 2007. How did you fix the issue? I read this post but I couldnt figure out.
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