MDH

A question....does anyone else hear their fuel pump humming away when they're inside the passenger compartment? I've read that a certain amount of "hum" is normal. Mine's definately audible - in fact it's been doing it awhile. When it started, I first thought it was fan noise but when you stick your head under the bonnet, it's pretty loud and coming from under the fuel tank. Car run's fine, but I'm trying to decide if it's time to replace the pump before it decides to completely fail.

Michael Delaney

Have you changed your fuel filter lately? A clogged filter can cause the fuel pump motor to labor excessively and hum. Good luck. Roy

MDH

Yeah, I changed it out a couple of months ago. It hummed before the change, but a new filter didn't seem to affect anything one way or the other.

Adrian

Dear MDH,
What kind of 964 do you have. The fuel pump is an intermittant pump. It does not run continuously. This is why I would like to know what kind of 964 you have. You may also have a pressure regulator problem (leaking injector normally) which is keeping the pump running all the time. In this situation it will be noisy.
Ciao,
Adrian
911C4

johnfm

Adrian

interesting to hear that the pumps are intermittent. What are their duty cycles?? I have noticed my pump humming continually - it stoips when I pull out the fuel pump fuse!!

Adrian

Dear John,
Foot on gas pedal determines the duty cycle. The fuel pump pressurises the the fuel pressure regulator. There is actually a check that can be done to check the fuel pressure. If you have a pressure leak you will have the fuel pump running all the time, non stop.
The fuel pump must be on and running continuously for start. After start fuel schedule demand will determine fuel pump running time.
Ciao,
Adrian
911C4

PS: As the fuel pump ages and maybe cannot deliver sufficient pressure it will run more often as compared to a new master blaster. I remember this comin gup before and I asked how many people had recently carried out a fuel pressure check. Never got an answer.

MDH

Thanks for the input. I have a 91 C2. The noise is funny because I think it's there all of the time but I'll admit there's been occaisions I've gotten in and drove and never noticed it. Either I've gotten used to it, it varies with the type of drive, or perhaps sometimes I just don't hear it (at higher speeds).

Anyway, so how do I check fuel pressure?

69COU

I have a 964 Turbo which has done the same sort of thing ever since i bought it over a year ago. Mine is more intermitent. Its a cross between a hum that you can feel through the fuel tank when lifting up the hood and that sort of whine you get from a cheap car stereo installation picking up interference. The tone even changes when switching on and off heating fan in car.
Seems to come more sitting in traffic after car has been running hard for a while.

Had me very concerned at first and typically was silent both times I went to JZ for a service who could not pick up any fault.

Mine seems to have quitened down a bit now as it not driving me as mad as before or it may be that since instaling some better front speakers(i usually have the stereo on) so have not noticed much. Maybe I even got used to it.

If it stops snowing and warms up I will take the car for a run I will have a good listen to see if its still doing it.

JZ machtech told me there were two fuel pumps and it was more likely to be from the one in the rear making noise. (I could have sworn it was coming from the front though)Typically though when i was at the garage it did not want to make the noise.

Cheers

maverick

I also have a '91 Turbo, and I hear the front pump come on at first ignition. once it has pressured the system it stops. However I hear the one in the rear constantly. Maybe this is part of the reason for only 10mpg!

shaker71

69COU sounds like you have an ignition suppressor problem...

69COU

Shaker71

How do i check this?
Never got to get the thing making the noise when i want it to. London is snoweed out so the car is garaged until it thaws

Herbie

Intersting post, but have I a problem ?

My fuel pump runs for a while before starting, I think it's normal.

But, when engin is running idle, fuel pump is always running (hearing near the fuel tank, with trunk open).

Is there a "fuel pressure sensor" informing DME that fuel pressure is enough and pump can be stopped ? Where is it located ?

MDH

I saw this thread came back recently so I thought I'd add a follow up for those interested. I checked my fuel pressure and it was definately too low. So I decided to go ahead and put in a new pump. The new one is MUCH quieter. In fact, crawling under the car to check for leaks after installation, I could barely hear it, even with my ear right next to it. If you put your head under the bonnet you can hear a barely audible hum, but that's about it. I swear, the car seems to drive better now too, but I could be imagining it.

Herbie

MDH, is your new pump working continuosly or intermittently ?

Did you re-measure fuel pressure after pump substitution ?

Thank you

Herbie

MDH

Herbie, sorry for not answering this - I've been away. I did re-measure the fuel pressure. The old pump was operating at 29 PSI. If I remember correctly, the new pump was around 40 to 42 PSI.

As to whether it operates intermittently or continuously? At idle, it seems to run all the time. Can't tell when I'm driving cause I can't hear it