924Board.org

chickengod · Posted: Thu Jan 29, 2004 10:26 am Post subject: Fuel pump missing

Hi,

Now I know what the fuel pump sounds like I am wondering if it is causing a problem I have been trying to cure.

When the car is idleing it misses about every two or three seconds. This can been seen as gaps in the exhaust vapour when it's really cold. Every time the engine misses the sound on the fuel pump misses at the same time.

Now..

Is this the fuel pump reacting to the engine or the other way around? If the engine was missing due to something else rather than the fuel pump, would not the pressure of the fuel be the same. The fuel pump would be making a constant sound.

Going to have to check the fuel pump out anyway but does anybody know?

Iain

Paul · Posted: Thu Jan 29, 2004 10:36 am Post subject:

Is the tach dropping to zero at the same time?

If so, its probably your ignition switch or ignition wiring.

Lizard · Posted: Thu Jan 29, 2004 10:52 am Post subject:

if the tach doesnt drop then bridge the fuel pump relay terminals #30 and #87, dont bridge #15 and #87 #15 is the relay switch cable, it is not of thick enough guage to run all the componants for long, I have a few wires I had to replace due to this,

if this does cure the problem and the tach doesnt drop then you may consider just bridging the connection and installing a switch under the dash, good luck with it, also might be a good time to see how many omps your fuel pump is pulling to estimate how much wear it has
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3 928s,

chickengod · Posted: Thu Jan 29, 2004 10:54 am Post subject:

Thanks for replies

Tach doesn't drop. When it misses it is for a fraction of a second. I will try your suggestions soon as I have time.

Iain

Paul · Posted: Thu Jan 29, 2004 11:01 am Post subject:

As Lizard said the next step is to bypass the fuel pump relay. If that fixes it, I recommend you buy a new relay. If it doesn't, perform the fuel pump checks that are listed in the tech section of this board.

D Hook · Joined: 02 Nov 2002 Posts: 3158 Location: Omaha, NE

I had a similar problem and it turned out to be vaccuum leaks causing the missing. Replaced the large boots between the fuel distributor and the intake and it runs smoothly now.

Have you put your ear right next to the pump to be sure it's cycling like you say? It'll help when you jump the relay so you don't have exhaust noise competing with the sound of the pump. Good Luck and let us know how it turns out.

chickengod · Posted: Fri Jan 30, 2004 7:41 am Post subject:

Ran a bypass from 30 t0 87. The fuel pump is making a load whiring whining sound that misses in the same way the engine was. I have not checked out the fuel pump with a multimeter yet but it sounds like it's on it's last legs

. At least that seems to have pinpointed the problem.

The Boshe fuel pump seems to cost around £70. I heard there is one by piermont (think that's the name) that fits and is quieter. I have not seen this alternative pump advertised anywhere. Anybody know a supplier and price in the UK?

Thanks for help

Iain.

Lizard · Posted: Fri Jan 30, 2004 8:21 am Post subject:

well at least you didnt have to go through everything else, I gave you enought to keep you busy for a week

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3 928s,

Smoothie · Posted: Fri Jan 30, 2004 9:36 am Post subject:

John Brown · Joined: 07 Nov 2002 Posts: 903 Location: Leesburg VA

Information on the Pierburg pump is here: http://users.adelphia.net/~johnbrown3/

May also be elsewhere in this web.
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John
80 931 - #931 44Cup
99 Escalade - tows track cars
gone but not forgotten: original 924.org car - 82

wdb · Joined: 02 Nov 2002 Posts: 2024

I dont think the fuel pump is effecting the engine , with probably 20 feet of lines it would take a second or 2 for there to be a pressure drop to effect air fuel ratio . sounds more like the electricity is cutting in and out ,shutting off the pump and ignotion, momentarily .

Lizard · Posted: Fri Jan 30, 2004 2:33 pm Post subject:

well if he bridged the connections wbd then it is not the ign switch and the problem lies either on the wire to term #30 which runs right to the battery, or terminal #87 which runs to the fuel pump WUR and the Thermo time switch, I would recommend hooking up a volt meter right to the pump and seeing how much voltage is there? with it running, now you wont be able to use a digital meter they dont react fast enough use an analog 1, although if the pump is stuttering it can cause the system not to run properly I know, but the other thing that could be happening is the check valve in the fuel pump could be fried not allowing all the pressure to get through and the pump could be hitting max pressure and then cutting itself off, highly improbable but is possible
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3 928s,

Joes924Racer · Posted: Sun Feb 01, 2004 4:30 pm Post subject:

I found loose a connection at the pump when I had one doing the cutout
caue after I jupmed the terminals it still didnt work. To test
it I ran wires from the bettery to the pump and the idled car it was fixed
I then checked connections..found a loose one on the pump.
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1979 porsche 924 Na
1980 porsche Turbo 931GT Replica
Have u ever driven a turbo.

mick · Joined: 13 Jun 2003 Posts: 183 Location: ohio

im experiencing almost the same thing with my 80 931 but i keep frying relays every 7 months. were is the fuel pump at on these cars under the rear carpet i assume?
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80 931 intercooled,1 bar, and kinda runs right

chickengod · Posted: Tue Feb 03, 2004 8:11 am Post subject:

I checked the fuel pump out with a multimeter today.

When running it is using 8.35 amps and 10.5 volts.

Amps are ok (under 8.5) but the volts should be at least 11.5.

I checked the in tank fuel pump that branches off the in-line fuel pump circuit at the connectors and it is getting 11.5 volts.

When the fuel pump is running it is making a grating rattling whing sound that is varying in pitch, vibrating a bit too.

When I use a multimeter to check the amps (inserting the meter into the circuit) , the pump runs quieter!!!! I don't know why.