Fuel Pump Buzzing - Good Or Bad?

CARRERIC · 04-09-2002, 07:09 PM

1975 911 2.7L CIS

My fuel pump buzzes loudly with ignition on and while motor is running. I changed the fuel filter and accummulator one week ago. Fuel filter was dirty! When ignition is on the pump buzzes steadily and constantly also while motor is running. I can feel a strong vibration when I place my hand on the fuel filter and accumulator and the CDI has a high pitch.

The car runs great. Only have a slight momentary drop in idle when I pull up to a stoplight.

I did a search on the fuel pump buzz and read most of the threads. All the cars were from different years so it was hard to diagnose.

Do I need to replace the fuel pump to be safe? Do I have a fuel pump relay on a 1975?

Any comments would be appreciated!

Thanks
Eric

john walker's workshop · 04-09-2002, 07:12 PM

if it's still loud after a new fuel filter, it's probably on it's way out. just a matter of when. might be 27 years old. the relay is above the fuse panel. the one closest to the windshield.

CARRERIC · 04-09-2002, 07:15 PM

Jhon, thanks for the reply. I pulled all three round (black) relay's out while ignition was on. The buzzing did not stop!

Early_S_Man · 04-09-2002, 07:16 PM

Eric,

You may want to check the current draw of the pump ... it isn't supposed to be greater than 8 Amps.

If it is more than 8 Amps ... time to replace!

CARRERIC · 04-09-2002, 07:20 PM

Thanks early S Man. I don't mean to sound like the village idiot, but how do I check the draw. Can I find this in a tech note.

Eric

emcon5 · emcon5's Garage

Quote:

Originally posted by ERIC_75
My fuel pump buzzes loudly with ignition on and while motor is running

Is it running any time the ignition is on? It is only supposed to run when the engine is running, causing the sensor plate to lift, or when the starter is engaged.

Tom

Early_S_Man · 08:29 PM

Eric,

First ... you need an analog VOM or digital multimeter with a 10 Amp or 20 Amp Direct Current range ... not that common, though the better 'traditional' VOMs such as Weston 660, Simpson 260, and Triplett 630 all had them. Few of the inexpensive DMMs have that range these day! You have to spend a bit more than average ($180+) and get a 'better' quality Fluke DMM to have that capability.

With a meter that has the capability in hand ... you need a set of high-quality test leads, either with sturdy, secure alligator clips, or individual banana plugs on the leads to be connected to the car's electrical system.

With alligator clip test leads ... remove the fuel pump fuse and clip the alligator clips Start the engine and note the current drain of the pump.

With banana-plug leads ... remove the fuel pump relay, and look on the underside for the terminal locations. Put a banana plug into the relay socket terminals #30 and #87a. Start engine and note current drain.

After turning engine off, remove test leads and replace fuse or relay.

If you don't have the necessary meter, you can probably get the draw tested at a major auto service chain store for free. Print this thread and take the instructions along!

Just as a suggestion ... used VOM's of the models I mentioned above are available on eBay quite frequently in the $15 - $30 range. I bought a used Weston 660 (unbreakable Lexan case, very rugged taut-band meter movement) for around $15 a couple of years ago ... beat-up looking case, but works fine.

sleepdoc · 04-09-2002, 09:28 PM

Eric,

Recently my engine started to stutter, and the fuel pump started to buzz and howl, and I thought the pump was toast. When I took off the inlet line to the fuel pump from the fuel tank I found it to be essentially occluded. Once I had fixed the occlusion proximal to the pump ( the fuel line was kinked where it passed over a hot oil line) the pump immediately quieted down to a nice purr and the fuel delivery problem dissapeared. The noise was evident way before the running problem.

Just a thought.

best Mark

3.2L

john walker's workshop · 04-10-2002, 07:17 AM

oops, sorry, there's no relay on a 75. that started in 76. power comes off the top of fuse 12, which gets power direct from the ignition switch.

Superman · 04-10-2002, 08:04 AM

Sometimes fuel pumps will make noise when crud in the gas tank is plugging the inlet line to the FP.

CARRERIC · 04-10-2002, 08:31 AM

Superman, how would you clear the line without spilling gas all over??

Eric

john walker's workshop · 04-10-2002, 10:21 AM

pull out the fuel sender and have a look at the screen with a strong flashlight first. easier to see with a low tank.

CARRERIC · 04-10-2002, 10:45 AM

Thanks John. Unfortunately I filled the tank last Sunday. Looks like I'll take Friday off and drive down to Key West for an overnight. This should empty the tank so I can have a look!!

CARRERIC · 04-10-2002, 10:48 AM

I know this may be a stupid question! Does the fuel go from the tank to the accumulator first or the fuel filter first?