fuel pump humming
 

There seems to be a humming sound coming from from the fuel pump. I changed thefilter. What is your opinion. Is this normal? Do you think I need to be concerned? Thanks for any help.

Ewenjo

Replace it. Now.

Or tow it later:eek:. While you are at it you might consider replacing the accumulator.

I dunno, but mine has been humming for 2 years. It is supposed to make some sound. Maybe if there are irregularities in the sound or it is simply loud it might be of concern.

George Murphy, the tech person for MBCA, has "fixed" a number of fuel pumps like ours by removing it and reversing the electrical hook ups on the bench, thus running the pump backwards and flushing it all out. I've never tried it myself, but it sounds like it's worth a try if you're planning to replace it anyway.

It's a strange little device, unlike any other pump I have ever seen, and devilishly expensive. It can't be opened for repair, short of using a hacksaw. It can leave you stranded if it fails.

Ditto. There seems to be two schools- 1).replace it now and 2) Mine hums and it runs fine.
Mine is a hummer for over two years on two m117's- never had problems.

Strange Experience/Observation With Pumps
 

I went to pick up the hardtop belonging to my 380SL, this being the longest trip I've ever taken with it (it had been towed to my house), about 80 mi each way. I usually don't let the gas gauge go down more than 1/2 way, for two reasons: 1, I don't want to get a heart attack at the pump, and 2, that I know it's bad to let a pump run dry. But this time I've let it go down to 1/4. Between 1/2 and 1/4, the fuel pump (louder than the 560's twin pump already) started getting even louder. Then, the engine started to sputter but still was working to some degree. It felt like driving a car running out of gas. I very much doubt that I was, in fact running out of gas, possibly due to a faulty gauge, because I must have driven 10 miles.

I made it to a gas station (barely, the car was difficult to drive and did stall a few times) and put in about 8 gallons, putting it up at about 3/4. The car ran perfectly after that, and the fuel pump noise returned to what was normal for this car.

On both my 560SL and 380SL, if I remove the gas tank cap, I usually get a "woosh".

I have a new fuel pump (the one on the car is certainly original), and also a fuel filter to install (the one on the car is probably a replacement).

Is it POSSIBLE that a defective gas cap may cause a 107 to create a vacuum in the tank, make the fuel pump(s) work harder (and possibly damage it/them), cause it to draw more current (and possibly damage the fuel pump relay?).

Shouldn't a working gas cap allow a vacuum but not allow vapor escape?

Exactly
 

I think the point page62 and I were trying to make is that a noisy pump is more likely to fail when you're driving it. Although easier to access than a lot of parts on the car, a side of the road repair would not be something I would want to attempt. As strife mentioned, I've read of several pumps and/or relays failing after the humming gets louder. Before I replaced almost everything in the fuel delivery system I had a humming noise that seemed to be getting louder, as well as the "running out of gas" that strife described when the fuel guage got to 1/4 tank. I think the accumulator had something to do with that. I still have the old pump and accumulator and will try the suggestion about running the pump backwards on the bench. I may cut the old accumulator in half, lengthwise, just to see the function better.

Confused about fuel pump humming...
 

As a major newbie with both this forum and my 380SL, I'll proceed with timidity...

I too hear a humming noise from my '83 380SL fuel pump. 67k miles on the car. We put the car up on a rack yesterday with the motor running and listened to the pump. From underneath the car, the pump was making only a very SLIGHT humming noise - kust enough so you could tell it was running. But, from inside the car, it seems much noisier. I was all set to order a replacement pump, when my buddy (who knows a bit more than I do) said he thought it would be premature. So first thing, does anyone have any advice for me in that regard?

Secondly, there is a silver cylinder, just above and somewhat larger than the pump itself. What is this and could it have any impact on the pump's sound "amplification"?

Finally, behind and just above the pump, mounted on the floorboard, there is yet another cylindrical device that appears to be another filter of sorts that appears to never have been replaced. What is this devices' function and same question - any relationship to the noise factor?

Thanks very much for everyone's patience in my learning curve with these wonderful automobiles!

Steve Sidwell
Santa Barbara, CA

The mysterious cylinders are the filter and the accumulator.
BTW you can run the pump(s) in reverse while they are on the car.....disconnect the wires at the pump terminals and connect 12v from a separate source in reverse polarity. I tried this on my 560...one pump ran freely backward and forward; the other ran freely backward and very reluctantly forward. The current draw on the second pump is much higher in the reluctant direction, so I suspect it is defective and perhaps a cause of my rough-running problems. I have replacement pumps on the way.

Thanks! You mentioned two pumps - is that configuration unique to the 560 (or have I missed a pump on my 380?) Also, how often should the filter and/or accumulator be changed?

The 560 has two pumps. My early 450 had one. The 380, I don't know; I suppose what you see is what you got.

I thought a gas cap sealed your tank. I have not heard of gas creating a vacuum...rather the opposite...pressure. When it hits a certain pressure it opens a check valve which then vents to the charcoal cannister.

Some humming is normal.

Not an expert but I am pretty sure the 380 has one pump.

Due to the expense of a pump, I would not replace one that is not actually failed.

Sputtering could be due to some impurities in the fuel.

Tom W