Well the DTCs (intermittent rich/lean and intermittent minor vacuum leak) took a week off but reappeared Friday, 9 days after installing the new EVAP purge valve. My wife (the usual driver) thinks its related to rainy days. Hmmm.

That pretty well covers all the stuff at the front of this car so I plan to go thru the area around the fuel tank/fill/purge pump. I started with the most obvious/easy - I cleaned up the gas cap gasket.

Interesting note. After washing the car today I pulled it into the garage, shut it off and walked around the rear for something and noticed a slight smell of fuel. Maybe nothing. Maybe a clue...

there is a small valve in the cap that can malfunction
what are the codes???
Also the obvious-are there any cracks in the fuel sender unit or level sender.
then the venting system for the filler is worth a look.
Such a complex system it can be a sod to diagnose.
Good luck man

The codes most frequently thrown and from Friday are 17545 and 17547 plus 16846. I have had 17536 and 17538 as well but not since December before replacing the fuel pressure regulator.

says
V.A.G 1551 print-out
Possible causes of fault
Possible effects
Fault remedy
16515
Bank 1, Probe 1
Voltage too low
- Lambda probe defective
- Exhaust emissions not OK
- Spark plugs sooted
- Lambda control switches to open-loop
- Eliminate short circuit or open circuit in wiring
=> Current Flow Diagrams, Electrical Fault-Finding and Fitting Locations binder


- Signal cable short-circuit to earth or screening
- Increased fuel consumption
- Check lambda control
=>Page 24-42

- Black smoke emission
- Read measured value block, Display Group 11
=>Page 01-123

Then 17547 refers to 17537 which says
V.A.G 1551 print-out
Possible causes of fault
Possible effects
Fault remedy

17537
Bank 2, Mixture adaption (mul)





System too rich
- Fuel system pressure too high
- Vehicle jerking in some conditions
- Refuel vehicle, check fuel system pressure => Page 24-21


- Injectors leaking
- Black smoke emission, spark plugs sooted
- Check injection rate



- Wrong signal from air mass meter
- Increased fuel consumption
- Check air mass meter => Page 24-93



- Leaks in exhaust system up to catalytic converter
- Black smoke emission, spark plugs sooted
- Eliminate leaks



- Fuel in engine oil (may be caused by repeated short trips)
- Poor idling in some conditions
- Drive on motorway for 15 minutes or change oil

Hope this helps

injectors/fuel pressure
Also new plugs if due
My pick is the o2 sensor
Best of luck

The CEL was off when I got the car back Thurs. but came on almost coincidental to the gas light in the dash. I figured it to be the EVAP thang again so ignored it, but then I started it up Fri. night and it had a slight misfire to the idle. So I dumped codes and yes EVAP but then also some code related to misfire on 1/3/4/5 or something like that.

Popped the hood and heard audible hissing from atop the intake manifold in center up by firewall. There's some large round black plastic valve there that I could put my thumb over and stop the hissing but then choke the idle. Never heard this valve do this before.

Cleared the codes a few times, restarted, finally - poof - misfire gone.

WTF?

Mr. Clark, what prompted your FPR replacement? Keep us posted if you would on how you make out with your EVAP hunt, thanks.

This thing can be such an annoying SOB - yet somehow it doesn't seem to matter 8-). Weird.

With the rich and lean codes on both banks, one of the common elements that could cause too much and too little fuel is a unstable FPR. I had already checked the injectors for pattern and leaks and I scoped the front and rear O2 sensors on the VAG-COM and they looked reasonable (based on my somewhat limited experience with good/bad sensors on a couple other vehicles).

I spent a lot of time both at idle and driving with all 4 O2 sensors on VAG-Scope and while the 2 banks didnt behave exactly the same all the time they generally looked the same and close to what I would expect. Front sensors switching either side of .5V generally in a range of .3-.7 with periods of a few seconds going rich or lean on average when first accelerating and coasting. They warmed up at the same rate. The back sensors sort of meander about once warmed up which is about what I would expect. I presumed they are all good and hate to buy $500 worth of sensors for the heck of it. Since I am getting this on both banks I would assume both sides would need to be bad.

I checked (with VAG-COM), cleaned (wasnt really dirty - using that MAF cleaner spray) and rechecked the MAF. No issues came up. My '01 A4 MAF was much dirtier and it had no issues.

I have replaced the FPR and checked the injectors for leaks and spray pattern.

The plugs were replaced by the dealer at 40,000 about 15 months and 10,000 miles ago. I should look at them but they shouldnt be bad.

Sorry to hear of your frustations. Hoping they aren't prevalent amongst others of us here, eeek - no thanks.

Esp. frightening as I do like 20+k / year - yours seems pretty lightly driven!

Good luck, keep us posted.