w126 Fuel Pumps
 

Why are there two? Do they both operate in tandem. Is one considered the "high demand" pump and turns on when appropriate?

And how common is it for them to leak? My forward one is not really leaking per say, but the underside is varnished up pretty good. the other looks clean as can be.

For replacement on a 560 SEL, what is recommend? (I am sure a search will say, but as long as I am making a post.)

those pumps are in series so the output of one goes to the input of the other.they both run together ALWAYS.

And should they be audible? They are not in the car, but outside it sounds like steam under great pressure or like a micro-turbine pump.

In a word.....No.
They or one of them will fail - generally leaving you stranded far from home.
Budget to replace both units - and filter.

Filter I just did. I think there is a check valve bad because the car takes a few turns to fire up. It has new injectors, dist, etc, etc.

I will look into some new pumps. (not the shoes)

And should they be audible? They are not in the car, but outside it sounds like steam under great pressure or like a micro-turbine pump.


I thought we should hear that sound coming from them. I have a 1990 300SE and I can hear the pump hiss when I jump the # 1 and 2 pins on the MAS relay. I was told to listen for that sound which verifies the fuel pump is working.

Depends how loud you are talking about. You should be able to hear them, but they should not scream.

Like HMX says, you can hear them run, an efficient whjirring, not rough. They run in series to make the needed pressure, seems like its eighty-something PSI. I replaced both the pumps on my car cause it wasn't running when I bought it. Be sure all copper washers are in place and tight, also hoses are good. With a slow leak you can smell gas but it evaporates before it puddles. Pump relay can keep the pumps off, too. Good luck!