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View Full Version : Possible reasons for fuel pump failing to prime?



dr_always
07-12-2012, 04:04 PM
Are there any other reasons other than a bad fuel pump? I just unplugged/replugged my entire motor wiring harness, does anyone know which plug(s) relate to the fuel pump? Could this green plug have anything to do with the fact that my fuel pump isn't priming even though I just bought one in Feb. to replace my old pump which was doing the same thing?

http://img.tapatalk.com/d036568f-0d4f-0359.jpg

dr_always
07-12-2012, 05:57 PM
Anyone have one for sale?

turbo_bootin
07-12-2012, 06:49 PM
Is your ignition switch good?

dr_always
07-12-2012, 07:29 PM
I'm pretty sure, how would I check? I took out the fuel pump and hooked it up to a 12v source and it was unresponsive, though. Is this method of checking the fuel pump failproof?

walky_talky20
07-12-2012, 07:29 PM
The pump prime is controlled by the ECU. Sometimes mine will prime the pump indefinitely each time I turn the key off and back on. Other times it does it once and subsequent off/on cycles doesn't produce another prime. I believe it has something to do with coolant temperature, but I'm not positive. Either way, the fuel pump will always run during engine cranking, and it continues to pump for a second or 2 after you let off the starter if the engine start was not successful.

Anyway, are you getting a Check Engine Light with the key ON? Nothing in the engine bay really relates to the pump priming other than the ECU being plugged in, really. And perhaps the coolant temp sensor as I mentioned earlier. Is the car not starting? Is this a no-start thread? Or just a no-prime thread? I is confused.

walky_talky20
07-12-2012, 07:34 PM
Applying 12V/Ground at the pump should definitely make it run. Perhaps it is stuck from sitting. You can try to tap the tank near the pump with a hammer while power is applied to perhaps get it to move. You can also try applying power backwards to see if some reverse action will break it loose. Of course you must make sure the pump is unplugged from the car's harness before you do that.

dr_always
07-12-2012, 08:07 PM
I tried applying the power backwards with not a single movement or any indication of life in the pumps motor. You would think if it were just stuck and still internally operable that it would make some minute noise or movement of some sort but, no. I tried cleaning the contacts and nothing. A friend of mine who's been mechanic-ing for several decades on all makes/models says that because I have some very nasty and outdated gas still in my tank, this is causing my fuel pumps to fail (just bought my 2nd replacement in 6 months tonight). According to him, once I get my car back on the road and a few tanks of gas through the lines this problem should relieve itself. Does this sound like an expert opinion to you, walky?

walky_talky20
07-12-2012, 08:11 PM
The original probably failed just due to miles. This is not uncommon for Audi's. The second one could have failed for some other reason. I'm leaning toward it just being a lousy pump to begin with. What brand is it/where did it come from?

dr_always
07-12-2012, 08:20 PM
The second one is what looks to me like an OEM audi part that I bought from a junkyard for pretty cheap. You really think that my car had nothing to do with either failure? Here's a pic if the second one:

http://img.tapatalk.com/d036568f-936b-70a0.jpg

http://img.tapatalk.com/d036568f-93aa-ddb3.jpg

The pump I just bought for my 2.8 12v motor came from a S4, does that mean my gas mileage is going to get worse?

walky_talky20
07-12-2012, 09:02 PM
Yeah, looks like the replacement pump was OEM. So your first 15 year old pump failed. Then you replaced it with another 15 year old pump, which failed after another 6 months. I don't see anything terribly shocking there, or any cause to believe there is something specific that is killing them. Maybe if the 3rd one dies in short order I'd start asking questions. You can certainly inspect and/or empty the fuel tank while you are changing the pump this time if you wish. Save the gas to use in your lawn mower or something. That crap is expensive.

S4 pump will not change your fuel mileage at all. Line pressure is determined by the fuel pressure regulator. And the S4 pump may be identical anyway. Not sure.

dr_always
07-12-2012, 09:09 PM
How would I go about emptying the tank while the pump is out, syphoning (yuck) ?