1992 Audi 100 Fuel Pump

Tiny
LUCKYPUNCH
  • MEMBER
  • 1992 AUDI 100
  • 6 CYL
  • FWD
  • MANUAL
  • 210,000 MILES
Hi guys �here�s the problem with my son�s 1992 Audi 100 CS 2.8L Front-wheel-drive, 5 speed: It won�t start and run.

I replaced the fuel pump because no fuel was getting to the filter or fuel rails. (The spark was OK and the engine would fire if fuel was introduced directly into cylinders.)

But the new fuel pump wouldn�t charge the filter or rails either. So I tried the following diagnosticsâ� ¦

Fuse is OK. The fuel pump relay clicks when key is turned on. Relay should be OK too, then?

When ignition switch is turned on, without starting engine, test light will light (brightly!) For a second and go out when I hooked up leads to pump. This is correct, I think.

Another on-line mechanic told me it was OK to hook up pump directly to power to check if pump is running. (The pump is protected by 20 amp fuse so direct hook-up should be OK?) I did this and I can hear the pump humming and it did charge the filter once and the car ran a second after I started it.

However, now the pump just hums when connected directly to power and nothing comes out, even at the tank.

Any suggestions for how I can get the fuel system working again?

Thanks a bunch!

Steve
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Sunday, March 28th, 2010 AT 6:38 PM

5 Replies

Tiny
DAVE H
  • EXPERT
Hello .. thanks for the donation .. much appreciated

From reading your post, everything seems to be working correctly ! I know this may sound like a stupid suggestion, but have you checked you have fuel in the tank still ? If the fuel pump is running (humming) but not pumping anything out from tank .. then either the tank is empty or the pump is not sat in the fuel correctly .. especially if you direct feed the pump from a battery (12v) supply ! Generally a pump will not hum if it is not working, it will just stop altogether ... you are getting power to the pump at start position of key (you checked with test light) 1 or 2 seconds is good ... now make sure the testlight glows brightly when cranking, if it does you are getting power correctly, so there is either no fuel in the tank, the pump is not sat in the fuel, or the pump is bad still !!

Hope this helps


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Monday, March 29th, 2010 AT 3:22 AM
Tiny
LUCKYPUNCH
  • MEMBER
Thanks, I tend to agree that this new pump is itself bad. Yup -there is fuel in the tank! About 6" over the top of the pump. It scared hell out of me, taking it out because I thought it might explode like "Hurt Locker!") Also no cap either so I know there no vacuum stopping fuel from flowing.

I will remove the pump and check again.

But before I decide to return the pump I want to confirm that I didn't damage it by connecting directly to the battery. What do you think? (It must take a lot of current since the fuse is larger (20 amp).) The pump charged the system once when connected directly and then nothing.

Thanks for your help!

Steve
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Monday, March 29th, 2010 AT 7:13 AM
Tiny
DAVE H
  • EXPERT
It should not have damaged the pump by giving it a direct 12v feed from the battery ... i have done this hundreds of times .. it may have been defective from the outset ..

Hope this helps


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Monday, March 29th, 2010 AT 6:49 PM
Tiny
LUCKYPUNCH
  • MEMBER
Thanks for your response on my last question about the direct hookup.

I pulled the pump out and noticed that I had the leads wired backward! The old pump and new pump have the terminals switched for some weird reason. Looking at the pumps with the outlets in the same position the positive and negative terminals are switched.

Although I took great care with masking tape and Sharpy to make sure the wires didn't get switched, I didn't notice the little "+" and "-" marked on the old pump until the new one was in.

Lesson learned! Thanks for your quick responses and help.

Steve Rumley - Helena, MT
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Monday, March 29th, 2010 AT 8:07 PM
Tiny
DAVE H
  • EXPERT
Thanks for letting us know the outcome of the problem .. please use our services again in the future and spread the word of our forum to friends and family etc. ..

Happy motoring.


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Tuesday, March 30th, 2010 AT 3:30 AM

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