Aaaahhh... I love delving into an entirely new part of my car that I have never had any experience with or problems with before...

Well, I jsut replaced my fuel pump a few weeks ago and I thought that by installing a new pump, with higher pressure and better flow it would increase the performance of my engine slightly as my old pump had been slowly dying and performing worse and worse by the month. However, when I replaced the pump, I fould the my engine was even less peppier than before...
Tell me what you guys all think... since then i have redone cap and rotor, and am thinking about fuel system cleaner. But I suspected, that since the pump was slowly dying it was sending an increasingly rich mixture to my engine to suffice for the loss of pressure. and hence, 1) that messed up my mixture and 2) maybe dirtied my O2 sensor and is now not easily adjusting to the leaner mixture. (but that was proposed by a non-audi driver) so... what do you guys think? When I initially start my engine I idle around 900, normal, but then after it warms up a bit, I settle back down to 650-700, pretty low. And when I take it out of gear and the RPMs fall, it dips even lower somtimes... what do you guys think?
Scott,

You think the pump was failing and affecting the mixture? I'm not so sure..
(What was it that made you think the pump was having problems? Noise?)


Here's why I'm not so sure:
Remember, there's a pressure regulator at the fuel distributor, that sends the excess fuel back to the tank (fuel is always circulating).

So until the pump was pumping a lot less, the fuel pressure regulator would keep things 'normal' as far as the CIS was concerned.

If the pump DID start to mess with the mixture, the o2 sensor would detect the change and correct the mixture BACK TO NORMAL as best as the ecu/dpr could. So as long as still withing the control range, as far as the motor (and o2 sensor) is concerned, nothing out of the ordinary in closed loop running. Shouldn't have dirtied the 02 sensor. Open loop would be a different story though..

For fun, try tieing the full throttle switch (the one the throttle works, on top of the intake manifold) closed, and take a test drive.
(that takes the O2 sensor off line, and makes the ECU run open loop).

I'm suspecting that you *may* have had nothing wrong with the pump, but may have been having a gradual failure of the 02 sensor, or perhaps a developing vaccum leak somewhere. Maybe .. :-)

Like going to the doctor, hard to tell without a visit!

Thanks Doc, I love any diagnosis I can get!
Ya, my pump was making quite a loud buzzing noise, so I replaced it. Do you think replacing the 02 sensor will help things out? or is there another way to test that as well?

Well, the 'proper' way to see if the o2 sensor is to measure the DPR (Differential Pressure Regulator) current, and watch for it changing up and down, as the ecu adjuts the mixture.

Using somthing like this adapter:
<img src="http://www.humanspeakers.com/audi/images/timing10.jpg">

Adapter connects to this connector on the DPR, and lets you measure the current going through it:
<img src="http://www.humanspeakers.com/audi/images/cise02.jpg">

Pictures are form the writeup here onhow to set the mixture:
http://www.humanspeakers.com/audi/timing2.htm

If the above is not possible, try tie wrapping the full throttle switch closed all the time, and take a test drive. Forcing the switch closed all the time makes the ECU ignore the o2 sensor.

If you notice an improvemnt, then *good chance* that your o2 sensor is tired/dead. Not an absolute test, but will give you a good idea.