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Old 12-07-2008, 05:39 PM
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91 300SE Secondary Fuel Pump

I was noticing a buzzing sound coming from the rear and thought the fuel filter must be clogged and the main pump working overtime to compensate. When I went under to replace the filter I noticed the secondary fuel pump and thought it might be the source of the sound instead, and indeed after starting up the car, a distinct high piched buzz could be heard coming from the secondary pump.

I gripped the pump tightly to make sure it wasn't just a vibration but the buzz persited from inside the pump itself. Does this sound indicative of a malfunctioning pump or could it be working overtime due to a bad fuel filter as I had suspected of the main pump?

Is there any other concerns I should have regarding the sound this pump is making, and how much of a risk is being taken driving the vehicle while this condition persists?

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Old 12-07-2008, 10:30 PM
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Would the secondary pump be stressed if the fuel filter was clogged? Is the secondary pump before or after the filter?
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Old 12-08-2008, 08:17 AM
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That car has only 1 pump & that is making the noise. Gravity feed system.
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Old 12-08-2008, 09:39 AM
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Wow, so there is no pump in the tank itself?
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Old 12-08-2008, 09:41 AM
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Originally Posted by OM618 View Post
Wow, so there is no pump in the tank itself?
Thats is correct
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Old 12-08-2008, 10:04 AM
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When I bought my MB in 1997, the fuel pump whined. It continued to do so and lasted for 9 more years.

The new one I installed in 2006 also whines occasionally.

I'm not saying to ignore it, just saying you may have a noisy fuel pump even after installing a brand new one.

I stuck with the Peirburg brand as that's what the car came with. Wiring/connections set to work with that pump.
Mike Murrell
1991 300-SEL - Model 126
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Old 12-08-2008, 02:00 PM
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Mine is audible....perhaps you could call it a whine....but it seems to work fine....I've heard lots of whiny gasser fuel pumps....seems to be fairly common.
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Old 12-09-2008, 03:01 AM
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Thumbs up

Yup, no dual pumps on the I6 in the W126.

Just replace it. It should be audible but not noisy.
I'm not a doctor, but I'll have a look.

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Old 06-17-2009, 06:24 PM
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Well, the pump finally failed.

Failed a couple of weeks ago, and I've just been driving Lead Angel (my silver 300SD beater), until she took a nosedive after loosing a couple of belts...

Anyhow, I'm looking for a good fuel pump replacement guide. I've searched around a bit but haven't found anything as good as the belt replacement guide for the 300SD.

I decided earlier to start taking parts off, and I pulled off the main hose going into the pump...I don't know why I thought the fuel would stop. Heh. Some other car I've worked on stopped leaking fuel once the pressure had let off...

I guess since this is a gravity feed it's not the same.

So, what do I have to do? Plug up the main hose after disconnection while I do the rest of the install?

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