New Fuel Pumps...low volume buzzzzing noise...
 

Is it typical for new fuel pumps to be loud until they break in? I just installed two new Bosh replacments in my '91 300E alsong with a filter and they seem much more powerful than what I had in there before. Now the car starts with more authority. However, when I sit at idle, I can hear the humming noise they make. Do I need to insulate these with some thin foam from where the cover goes over them and where they contact the top? The old ones were weak but you didn't hear them at idle. Also, these are a bit larger even though they matched up to replacement specs on several websites.

I believe there should be rubber sleeves around the pump casings, maybe yours are gone? Yes i am betting it's just an insulation issue. Or maybe the rubber cusions on the brackets are hardened??
Gilly

In my 400E I have always heard mine. Maybe bosch doesnt care if they make noise. Sounds like 2 beehives.

Did you replace the fuel filter?????

Yes, I replaced everything down there. The only part that was on the pumps was a plastic seperator that goes in between the two on this car. I suppose I can get some insulation foam rubber and squeeze it in there. You can really hear them buzz when you start 'em. My daughter's 300E is even louder but at least her starts on the first crank!

Foam rubber won't last under there, and I think you would be masking the real issue. It is quite normal for those pumps to make a buzzing noise that you can hear from outside the car, but if you are hearing it inside the car then I suspect your fuel accumulator may be shot. It serves two purposes - keeping fuel system pressure when the car is off or fuel demand suddenly increases, and also dampening the hydraulic noise that the fuel system makes. One symptom of a shot accumulator is the car starts fine, but takes a bit longer to crank and may even need some accelerator pedal to start if the car has been off for a long time (overnight) and the fuel pressure bleeds off through a tear in the accumulator's diaphragm.

Fuel accumulator is also brand new. It is the pumps that are buzzing. I took off the bottom and matched the sound by feeling them. They are just loud and touching the bottom protective panel. I'll just install a spacer and it will be fine. Thanks for the help and suggestions.

Update on buzzing fuel pumps...
 

I had to remove protective cover and retighten connections due to small leaks at the filter and lower pump. The pumps have since broken in a little and I don't notice them much at all when driving now.

As mentioned in the Service manual the plastic insulator is a vital component between the pump and the bracket otherwise rapid corrosion occurs due to cathodic flow.
mak

FYI...The plastic insulator/spacer was reinstalled when I did the job. I guess I did not explain that it was. I rechecked it's position but was not the cause of the humming noise but thank you for the feedback.