Is my fuel pump dying?

porschenut · 05-13-2011, 04:41 PM

After sitting dormant for 4 months in the garage, I've been taking the car out to wake her up from hibernation. I'm hearing a fairly loud hissing sound that appears to be coming from the front half of the car, low in the area of the trunk. it's not a whine, just a smooth hiss noise. I can't think of anything else in the front half of the car that would make a noise like that....is the fuel pump about to let go? It's not the fresh air blower, which is turned off.

edit: it can only be heard with the engine running.

howard freeman · howard freeman's Garage

Try replacing the fuel filter 1st. When they get stopped up from this lousy "AL Gore" gas we have now,the pumps get to singing.
Howard

porschenut · 05-13-2011, 08:27 PM

That's a good suggestion Howard, except I just put in a new filter about 2 months ago. I need to get under the car and really find out exactly where the noise is coming from.

Evans, Marv · 05-13-2011, 09:59 PM

Getting down where you can hear & hopefully locate the sound is the best idea. The only time I had a fuel pump go out, it just stopped pumping & I had to coast into a parking lot. I got another fuel pump and changed it in th paking lot & went home. The fuel pump that went out didn't make any noises or give any warning, so I can't comment for sure what your noise is.

gregwils · gregwils's Garage

I had a pump on a mid-80's Audi that failed without warning and pump on early-80's Scirroco that was amazingly noisy it's entire life and never failed during my ownership (100k miles).

I took a totally different route on the 911. No noise and working fine, but it had a little undercoat on it and the paint was faded, so I assumed it was original. This past winter I drained the tank and replaced the fuel lines the entire length of the car and decided to swap the pump too. I figured 25 years was long enough to count on it reliably.

E Sully · E Sully's Garage

Vacuum leak at the brake booster?

Laneco · 05-14-2011, 07:14 AM

+1 on the vacuum leak. That would be exactly my first guess.

I've had fuel pumps warn me on the way out the door. The pump in my 81 got VERY loud and was hot to the touch before I replaced it. Prior to that, it was nearly silent. But it never made any sound that I would identify as a "hissing" sound. It was a slightly metallic tapping/rattling sound.

Open the trunk and use the broomstick method. With the car running, take a broom stick, put it on the possibly offending part, then put your ear against the broomstick. It will transmit the sound. Try a few places, then lift the car and try the pump unless it is pretty obvious when you get anywhere near the area of the trunk above the fuel pump that it is indeed the pump.

angela

sean m · sean m's Garage

Before my fuel pump went out it was making a "buzzing" noise.....not a "hissing" noise.

richard003@wind · 05-14-2011, 10:54 AM

Try pressing the brake pedal and see if the "hiss" changes ...

j911brick · 05-14-2011, 11:54 AM

I had a fuel pump that only made noise after I washed the car. Same car different pump: worked perfectly unless wide open down a long straight.

porschenut · 05-15-2011, 02:57 PM

Thanks for all the suggestions, gents. The hissing noise is definitely coming from the fuel pump area. No noise of any kind coming from the brake booster, and it doesn't change when I step on the brake. The closer I get to the fuel pump, the more it is apparent that the "hiss" is actually a subtle mechanical noise, kind of like chain lash only it's rather quiet so from a few feet away it sounds like a hiss. Come to think of it, it sounds a lot like the hissy bearing noise I got from my 928's water pump when the fan belt was a little too tight.

The fuel pump has been in service since 1995 so I may go ahead and replace it before it leaves me stranded somewhere.

E Sully · E Sully's Garage

Just an additional note, when the fuel tank is low, mine makes a little noise as the return fuel flow enters the tank.

Bob Kontak · Bob Kontak's Garage

Quote:

Originally Posted by sean m

Before my fuel pump went out it was making a "buzzing" noise.....not a "hissing" noise.

Mine (81SC) always makes the buzzing noise when almost empty but recently makes the buzzing noise when tank is full. Buzzing goes away after warm up. I think a new one is in my future.

Dogbreath · Dogbreath's Garage

Kind of a newb question.
On an 82 911SC should the fuel pump be running when the key is in any On position? As soon as I put the key in any position other than off, I can hear the fuel pump running. I am working on some wiring issues and it is a little bit on a concern to have the fuel pump running for long periods while I troubleshoot other issues.

JP911 · JP911's Garage

Quote:

Originally Posted by Dogbreath

Kind of a newb question.
On an 82 911SC should the fuel pump be running when the key is in any On position? As soon as I put the key in any position other than off, I can hear the fuel pump running. I am working on some wiring issues and it is a little bit on a concern to have the fuel pump running for long periods while I troubleshoot other issues.

No, it should not run with the key in the on position on an '82. I think '75 was the last year the fuel pump would run in this manner. Possible that your fuel pump relay has been bypassed or the pump wiring was messed with by a previous owner (some people splice the fuel pump wiring into alarm systems). I would start with a Haynes manual wiring diagram and trace the wiring for the pump. Good luck!

ossiblue · 01-02-2012, 07:45 AM

Short follow up to JP911's post. Your running pump could be as simple as someone having disconnected the wire from the air flow sensor to the pump relay. Take a quick look near the 14-pin connector in the engine compartment where the engine harness plugs into the electrical panel--there may be a single wire connector that exits the harness that has been disconnected.

Bob Kontak · Bob Kontak's Garage

Quote:

Originally Posted by E Sully

Just an additional note, when the fuel tank is low, mine makes a little noise as the return fuel flow enters the tank.

Ditto. Not hissing but a louder buzzing.

Edit - Sorry - responding to stuff posted months ago.

Walt Fricke · 01-02-2012, 08:54 PM

The air flow sensor is a switch which opens when the air flow measuring plate raises up off its stop. Even at idle speed the plate raises up enough to open this switch. When the switch is grounded, it prevents the fuel pump relay from turning on. The start position on the ignition bypasses this and actuates the fuel pump relay (so you can get fuel flowing and start).

A common way to test a fuel pump is to pull the air filter and, with the key on, reach in and push the air measuring plate up a little. Pump should run.

There is one of those nifty Bosch plastic two wire connectors which attaches to the switch. You can just barely reach it if you put your hand into the engine by the throttle plate, and reach around forward and to the right until your hand is over by the right side of the air box, where the measuring plate is. Right side of that rubber boot which connects the two sides of the intake up top.

If you pull the connector off, you defeat this interlock system, and the fuel pump will run every time the key is turned to "on."

The other way to defeat the interlock is to bypass pins under the relay, but that calls for remembering which pins to connect, and making up a bit of nicely bent copper wire to go between the selected pins.

My experience with fuel pumps is the sudden death one. Though if one which has been silent starts making noise, prudence suggests replacing it.

I took a sudden death one apart once. It wasn't all that old, either. And inside I found absolutely nothing which looked worn or melted or siezed or whatnot. Another poltergeist item to confound us.