Since i bought this car i've noticed it had a hum when running that sounded like a higher frequency than an engine would generate.

Today i hopped in the back seat and low and behold i can hear the hum coming from under the seat. I can even feel it if i put my hand on the seat.

I doubt its the fuel pump, so what could it be? An air pump for the shocks? For the seats? It sounds like its constantly on and its a bit annoying.

Sounds like it is definitely the fuel pump to me. Most likely have a dirty clogged up fuel filter and the pump is struggling due to the restrictive filter making the sound you hear.

The fuel pump you say? Well i guess i should look into the last time the filter was replaced.

A rule of thumb for cars is if driven the typical 12-15k a year you should change them once a year but that almost never happens and official recommendations are higher, I bet some cars go well over 50k miles without changing them.

Please tell me these filters are in the engine bay and NOT in the tank?

In the tank. Filter pump all are one piece which mb calls fuel sending unit. Under the back seats.

And let me guess, its not the kind of pump assembly that pulls out through the top. YOu have to drop the tank, right?

Dang, the previous owner of my car replaced the pumps so I haven't even thought about doing filter yet since I haven't driven it that much. I didn't even realize these things don't have an inline fuel filter like other cars under the car or in the bay. Did some quick searcing and found people talking about filters being on the pumps, kinda weird that is the only filters and not a main one inline somewhere.

As for replacing, you pop the seat bottom off and there are caps to the top of the tank on both sides of the car. You open those and pull the fuel pump units out to replace the filter.

I just checked the EPC for my 2005 E55 and it does not even show a replaceable filter at the pumps ("Delivery Unit with Fluid Level Sensor"), so how does one obtain such a filter?

I believe my pumps were changed earlier this year by the previous owner as many of them leak (causing much angst among many of us), but since I'm looking to replace the ring seal (A 211 471 05 79) and possibly lock nut (A 001 990 80 54) soon, I'd like to change the filter(s) while I'm in there.

The filters are integral. If the pumps were changed, you're good-to-go.

See my post #8 above yours. It has not come time to do a change for me so I have not looked in to it and just assumed. Most cars have a fuel filter that should changed like a basic maintance item yearly since they are cheap and you never know how good/clean the fuel is from hitting so many different gas stations.

With the E55 apparently they don't have a prefilter like nearly every other car out there, and only have the filter on the actual pump/sending unit which is not cheap and is in the around $500 range online.

Yes it's only on 1 side, although I caution anyone doing it on their own, that these are very very prone to leak and cause the dreaded fuel smell, so be careful, even dealerships make terrible mistakes

At this point with the ongoing nvestigation on the fuel leaks unless I had a problem I wouldn't even think of messing with pulling the unit to replace it. My 03 is on the original setup with 56K miles now. No leaks, no record of any work for a leak. Till NHTSA is done and issues a ruling I see no point in replacing it and opening up the can of leak worms. Worst case you kill a fuel pump from a clogged filter, you have to replace the pump with the filter anyway. Only issue is how is the motor gonna react to starting to go lean from low fuel pressure when under load/boost. Will the computer see what's up and save the motor or will it go lean and pop the motor? haven't heard of one dying this way yet though so again I'm sticking to not messing with it if it's not broke.

Add this junk setup to the list of problems with the fuel tank setup.

Yup BB

There's a lot of confusion here about our fuel system setup, so to clear things up:

The tank under the rear seat is a saddle type tank, with a "hump" in the middle to allow clearance for the driveshaft and exhaust.

The car has 2 fuel pumps. Both are located together in the RH (passenger) side of the tank.

The car has 2 fuel level senders, one for each side of the saddle tank. The RH sender is attached to the fuel pump assemblies.

The fuel filter, pressure regulator, and LH sender are all integrated into the LH sender assembly. This assembly has hoses and wires that run inside the tank across to the RH fuel pump assembly.

To replace the filter, you have to change the LH sender assembly, however you also have to remove the RH sender ring/seal/cover to connect and disconnect those hoses/wires inside the tank.

The replacement interval from MB varies depending on the model, and I believe they have changed the spec. several times, from 50k to 100k miles, I would have to check.

The fuel filter sending unit part number is A211 470 49 94 for anyone wanting it.

You are correct, it's 100K miles.

It really is a shame they used such an unconventional setup for the fuel filter. I understand you have to pay to play with these cars, but this filter just bugs me.

Year, model, and VIN# will allow me to give a solid answer..

roy@pelicanparts.com

Got the VIN#.

Interesting FYI:
The vacuum pump has been superseded
MB# 0008002748
+ MB# 0008000318
+ MB# 0008003515


The fuel tank item #10, and pump item #55, have been superseded.

I would suspect it is the fuel pumps.
I have replaced more of these pumps item #40 and #55 than I want to remember.
* Running the fuel tank on empty all the time = BAD idea = the fuel cools the pump (no fuel = ruined pump)..
* Contaminated fuel.
* Garage queen = setting 3-6 months per year = the gasoline turns to varnish/sludge and ruins the pump.
* Defective unit.
* Road damage.


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roy@pelicanparts.com

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