badco87

So I have a 1995 90 quattro sport with the 2.8L 12v V6. It recently stopped working on me it started to chug on me, like it wasn't getting fuel, before it died, i got it home replaced the fuel filter and in the process of testing found my pump was intermittently working, it normally pre-runs when you turn the key on but not cranking it. Now it needs a mallet to the fuel tank to get the fuel pump to run about 50% of the time. I checked my fuel lines/rail for any blockages, none found. While doing that I tried to check fuel pressure, now I didn't have the correct fitting to get a tight seal with my pressure gauge but when i would turn the key ON and the pump ran it spiked my gauge right past 100 PSI before the pressure would leak out, I am in the process of getting the right fittings to do a proper fuel test. I also opened up my gas tank to check any connections and they looked much better than expected however i only read 10.3 volts going to the fuel pump and i am unsure whether this is correct or not. I also checked my pump relay and it seems to work fine however when testing the connections in the fuse panel something came up below I have attached a picture of my relay and my labeling of the pins. Now pins 4 and 5 are the 12v power to activate the relay and they appear to work fine. Pins 1 and 3 are the questionable ones when the key is ON i get a +12 on pin 3 but a -12v on pin 1, i.e. if i read across pins 1 and 3 i get 12 volts which doesn't seem right to me. I am unsure of that pin 2 does. I also came upon this link talking about over fuel pressure on the Bosch CIS system.



So in summary my questions would be.

1. Can you get an over pressure from the pump that would cause this system to fail.

2. What is the proper voltage the fuel pump should be getting and is it getting it(10.3 Volts)?

3. Do I have an electrical issue in my fuse panel causing the relay to work improperly?

If you have any ideas or thoughts please let me know I am at a bit of a loss here, also if more tests are needed please let me know and ill get my results posted asap. Thanks for reading and/or help.

socalclimber

My answers:

1. Doubtful. System pressure is controlled by pressure regulator which "vents" excess pressure back to return fuel line to gas tank. You can't test pressure by ONLY plugging a pressure gauge onto output of fuel pump w/o a return loop. I once put a pressure gauge on a CIS pump w/o a return loop and pressure went to 160lbs+...nearly broke my gauge! You must test pressure by "T-ing" into main fuel line (rail) with pressure gauge.

2. 10.3v seems low, maybe you have a weak fuel pump relay, or a wiring issue, or fuel pump is drawing circuit down?

3. Maybe, probably not.

Pin 1 = 30 Battery voltage (should be 12v approx)
Pin 3 = 87 goes to fuel pump

As a test, remove fuel pump relay, and put a jumper with large male spades on pins 1 and 3. This will energize fuel pump directly. Could measure voltage while fuel pump running, if low, maybe fuel pump drawing too much.

Measure voltage with neg. on ground, + on circuit you want to test. My guess is fuel pump is going, maybe drawing too much power, and the fact that you have to bang on fuel tank to get it going.

badco87

So further testing reveals that Slot 87 on my fuse panel is being shorted by a ground, gonna have to try and repair or replace. Is this uncommon?

socalclimber

Terminal 87 feeds power to fuel pump and fuel injector (+) power (on a 1990 20v MPI engine, don't have diagram for 1995, but I assume it's similar), so unless you unplugged fuel pump connector AND all injector connectors (or ECU connector), you may see continuity to ground through pump and fuel injector windings?

Injectors get triggered by a ground pulse from ECU. What happens when you jump 30 and 87? You should hear fuel pump running. If not, power pump with external power source and see if it's working.

Fuel pump is probably on its way out, or you have a loose/bad fuel pump ground circuit, follow brown ground wire and make sure it's good.

badco87

It turns out a bad power line caused the fuel pump to fail. I believe this was one of those rare cases of multiple things failing at the same time.

Thanks for all the help