Fuel pump issues

[Cobra351]

Hey guys.

So I believe one of my fuel pumps might be on its way out. I need help diagnosing which one- I know that there is the HPFP, LPFP under the passenger rear seat(mine is Quattro), and a transfer pump with a sending unit under the drivers rear seat. I do have the newest version of VCDS- but won't have access to my laptop til tomorrow.

1) So it started about 6 weeks ago. Parked my car for an hour or so, got back in, tried to start it. Engine cranked, but ran rough, and died after about moving about 5 feet. Car would not start afterwards. I had about 100 miles to empty. Friend bought me a gas canister with 2 gallons of gas, we put it in, and it fired up right away. To test, we turned off and turned on the car 5 times, and all 5 times it started up right away.

2) Saturday, I had a full tank of gas. Drove about 25 miles, parked my car for 10 minutes. Tried to start it, started for 2 seconds, ran rough and died. I attempted 4 more starts after that- the 4th time left the car in the "On position" for a few minutes, and it finally started. Since then, the car has been starting ok- sort of.

Notes: Car takes 2-3 seconds(of holding the key to the engine start position) to crank on a totally cold start. When warm, it starts up noticeably faster. My fuel level reading is wrong- the needle never moves to a full tank(stays at 3\4, then goes to like 90% full sometimes, but never actually full) - even when I know the tank is full- I filled to overflow once. Only check engine light that I've had was for a small\tiny evap leak.

So what tests can I run in VCDS to further troubleshoot which fuel pump is dying, and why the fuel level reading is wrong? Any help would be appreciated, thanks in advance!

[Charles.waite]

Could be an issue with the EVAP system. Might want to try replacing your N80, though I wouldn't expect miracles. Intermittent issues like this are hard to diagnose, but try to make a list of conditions when this is happening. aka: before/after long drive, engine hot/cold, outside temp hot/cold, etc. Its sort of old school but it might help narrow down the conditions its happening.

My first thought its that the level being off and the starting issues are separate issues, but i'm not sure.

[Denio24]

The n80 evap system can prevent the car from starting on a full gas tank. If the N80 valve is leaking then all the fuel vapors from the fuelup on a full tank will richen the A/F ratio and prevent you from starting.

[o1turbo30v]

Start small, replace your fuel filter first. Secondly the fuel gauge sending unit is built into the LP, in-tank pump assembly. You can check pretty much everything with your vag-com, the first thing I would check is the LP pump duty cycle at start up, if it's above 60%, replace it (assuming you changed the filter already).

Good luck

[Cobra351]

Could be an issue with the EVAP system. Might want to try replacing your N80, though I wouldn't expect miracles. Intermittent issues like this are hard to diagnose, but try to make a list of conditions when this is happening. aka: before/after long drive, engine hot/cold, outside temp hot/cold, etc. Its sort of old school but it might help narrow down the conditions its happening.

My first thought its that the level being off and the starting issues are separate issues, but i'm not sure.

Will do. Have the N80 valve already on hand. Thanks!

The n80 evap system can prevent the car from starting on a full gas tank. If the N80 valve is leaking then all the fuel vapors from the fuelup on a full tank will richen the A/F ratio and prevent you from starting.

Now that you mention it- often, the check engine light comes on after filling up, or driving while the engine is cold(warmed only 30 seconds until the idle is normal). Once every few days to a week after resetting it. Thanks!

Start small, replace your fuel filter first. Secondly the fuel gauge sending unit is built into the LP, in-tank pump assembly. You can check pretty much everything with your vag-com, the first thing I would check is the LP pump duty cycle at start up, if it's above 60%, replace it (assuming you changed the filter already).

Good luck

Also have a new Mann fuel filter on hand that I'll replace this weekend. I assumed the low pressure pump was on the passenger side, and the level sensor\sending unit was on the driver side, as shown on Phils DIY. Will look at the LPFP duty cycle at startup as well after replacing the fuel filter. Thanks!

Here are the codes VCDS pulled from some weeks ago:

Mode 03: Emission related fault codes

Address 7E8: 2 Faults Found:
P0442 - EVAP Emission Control Sys: Small Leak
P0456 - EVAP System: Very Small Leak Detected

[Cobra351]

Ok so I replaced the N80 valve yesterday. The old one, I was not able to blow air through it, still swapped it anyhow since I had the new replacement part.

This morning, I put about 4 gallons of gas(had 75 miles to empty) in only, as I wanted to pull out the fuel sender unit this weekend. Drove ~25 miles to work. Parked the car for 10 minutes, tried to move it again, and the car started and stalled after 10 feet. Of the next 5 tries, it started 2 times before running rough and stalling out in 3 seconds. 6th time, I left the key in the on position, held the gas all the way down and the car started up. Since then, turned off and started the car 5 times successfully.

Since the 2 of 3 times this happened approximately after 20 miles of putting gas in, is it possible it has to do with the charcoal canister being flooded or something? Remember reading something about it.

[Charles.waite]

That was what I was going to suggest. The charcoal canister. There's more info in the b6 section on this as the cars are older and more prevalent but they use basically the same EVAP setup and the info should be relevant.

[Cobra351]

That was what I was going to suggest. The charcoal canister. There's more info in the b6 section on this as the cars are older and more prevalent but they use basically the same EVAP setup and the info should be relevant.

There's a DIY on the B7 as well, looks pretty easy:

http://www.audizine.com/forum/showth...ng-Gas-Problem

I hope this is the problem. My LPFP was cycling at 60% at cold idle and then dropped to 50%. And this only happens when the car is off- never died while actually driving.

[audifahrer]

Hey guys.

So I believe one of my fuel pumps might be on its way out. I need help diagnosing which one- I know that there is the HPFP, LPFP under the passenger rear seat(mine is Quattro), and a transfer pump with a sending unit under the drivers rear seat. I do have the newest version of VCDS- but won't have access to my laptop til tomorrow.

1) So it started about 6 weeks ago. Parked my car for an hour or so, got back in, tried to start it. Engine cranked, but ran rough, and died after about moving about 5 feet. Car would not start afterwards. I had about 100 miles to empty. Friend bought me a gas canister with 2 gallons of gas, we put it in, and it fired up right away. To test, we turned off and turned on the car 5 times, and all 5 times it started up right away.

2) Saturday, I had a full tank of gas. Drove about 25 miles, parked my car for 10 minutes. Tried to start it, started for 2 seconds, ran rough and died. I attempted 4 more starts after that- the 4th time left the car in the "On position" for a few minutes, and it finally started. Since then, the car has been starting ok- sort of.

Notes: Car takes 2-3 seconds(of holding the key to the engine start position) to crank on a totally cold start. When warm, it starts up noticeably faster. My fuel level reading is wrong- the needle never moves to a full tank(stays at 3\4, then goes to like 90% full sometimes, but never actually full) - even when I know the tank is full- I filled to overflow once. Only check engine light that I've had was for a small\tiny evap leak.

So what tests can I run in VCDS to further troubleshoot which fuel pump is dying, and why the fuel level reading is wrong? Any help would be appreciated, thanks in advance!

I am in what appears to be very similar situation as yours. 2008 165k km. I own VCDS, logged 000135 P0087 Fuel rail/system pressure to low. Channels 103 and 106 log running pressure at 4.5b, duty cycle at 53%, which are in acceptable limits. That rules out LPFPump. Car runs very well, acceleration is superb, restarts after 15+ minutes have become a headache (starts then cuts out after few feet). No issues after several hours. N80 gas tank vapour purge valve was replaced recently. Fuel filter preventative 2 1/2 years ago. My thoughts are towards LP Fuel pump control module and low pressure fuel sensor (at side of HP mechanical fuel pump).