Fuel pump?
 

I have a 91 300SEL with 113000 miles. Recently (1 month or so) I have noticed that the car makes a loud humming noise while running. The noise starts when I crank the engine over and is present on the last position of the ignition switch before the engine cranks. I tracked the noise and the noise seems to be coming from the passenger rear area. Fuel pump gone???

Should I replace ASAP or can I wait?
Other diagnostic checks?

Instructions on how to change the fuel pump if this is the culprit would greatly be appreciated!!!
Thanks.

Raghu
91 300SEL 113000mi

It sounds like A fuel pump to me, I believe there is more than one.

The fuel pump cycles for about 1-2seconds before starting the car when the key is in the last position before cranking. I've had them go in 2 of my cars, but I never had the luxury of any advanced notice.

It is indeed ,the fuel pump.I believe yours have two of them, one in front and the other at the back. It is a simple remove and refit job with basic tools. good luck
Zee

NO, the fuel pump is not gone, its there making the noise you hear. LOL

You might be able to replace the bearings in the pump and save a lot of bucks. If it doesn't hum and the car quits running, try tapping the pump with the hammer in the MB tool kit. That trick got me home from OH onetime in my 300SEL6.3. I guess that's why MB put the hammer in the tool kit, because I never used it for anything else.

If you are going to repair the pump, the quicker you do it the better because the pump is destroying itself and if you let it go too long, it may not be repairable.

P E H