8Pack

Can anyone explain to me how the fuel transfer pump works in the '04 A8.

Dealer says I am getting lean bank1 and bank2 MIL codes and this is the cause. As explained to me, this pump is in the fuel tank and provides additional fuel under engine load moving fuel between chambers. Does that sound right? If so, if it fails, can you still use up a full tank of gas since it is no longer pumping between chambers?

Does it do more than that?

Any advice would be appreciated!

PaulW

At first look, I would say no. Lean bank 1 and 2 would normally be an air leak on the intake side.

Short term fuel trim is when you press the gas down, the computer adds fuel immediately (throttle angle has a lot to do with it also).

Long term fuel trim lean (which is what I would guess you have) means it gives the engine enough gas for the air being metered in through the mass air flow sensor.

If you have an air leak downstream of the mass air flow sensor before it reaches the cylinder, the ECU is not measuring this air and is only providing fuel for the air it is measuring.

Therefore, the 02 sensors on the exhaust side see that the engine is running lean and it throws a long term lean fuel code.

Air leaks on the intake side can be:

1. Rubber seals and clamps on the intake hoses.

2. Intake manifold gaskets.

????

If the fuel pump wasn't pumping enough fuel, you would loose HP, you are not loosing HP are you? And if it's a long term lean problem, it's only when the car is consuming the least amount of fuel. I would bet a ham sandwich it's not the fuel pump. And replacement of a fuel pump at your expense would be over $1,000 at the stealer. Don't let them change it, plus it creates a real potential for them not reinstalling it correctly, it's tricky.

You need to post the actual code.

8Pack

Paul, thanks for the speedy reply. The only codes I have are off my CANOBDII reader that I took. P0171 and P0174. I don't have the VAG data. I bought the car used (from an independent dealer) on September 22, it has been at the dealer all but 3 days since then and the check engine light has come on within 300 miles each time it has been returned "repaired"

The car apparently has a 2 year history of the MIL coming on. The intake has been taken off and remounted, the fuel pressure regulator has been replaced and all 8 fuel injectors replaced since I have brought it in.

The dealer said that the car is "going lean under load", that they reviewed the history and called Audi Engineering and they said to look at the transfer pump. The tech says that when he pulled the tank, the transfer pump was disconnected and the hose was laying on the bottom of the tank and appeared to have never been connected properly from the factory. He has reconnected the hose and reinstalled the tank.

It seems that if that pump was not working, the car would have poor fuel range (haven't driven it enough to know) and that it would never empty a tank of gas....(if the fuel wasn't being pumped over would the gauge still read empty if the other chamber was full? or are there multiple fuel level senders so that the car would run out of gas at 1/4 or 1/2 tank? I did get it down to near empty at least once when the fuel light came on...)...

The dealer said that when under load, the transfer pump acted as an auxuliary pump to add more volume...doesn't seem to make sense to me, but it could be hooked into the main fuel pump I suppose....

By the codes that I could read, it looked like a MAF sensor or maybe the 02 snesors, but odds that both of those are bad are very long...but they say that the MAF was replaced already (not since I owned the car though)....maybe the replacement is bad....

Good news is it is under factory warranty.

Thanks for the ongoing support......

For all those wondering.....ser# 12140......

tozoM8

It is possible, that you have an air leak at the intake since it was removed.

ih8emos

It might be labled a transfer pump but it does way more then just move fuel fron tank to tank. both pumps will turn on to prime the system for start up and run for about 30 sec then the transfer pump will shut off, and come on again under load conditons as needed. it is a 2 stage pump one stage for transfering fron tank to tank and the other for assesting the main pump. it the transfer punp is faulty you mignt not even know it because it only helps under load. with a scan tool you should be able to do a final contol output test to check both pumps and see the pressures they put out.

hope it helps
-Justin