Now for my own details and some notes on my experience so far. My car is a 2006 A4 Quattro Sedan, 2.0T, 6 Speed Manual, 71,000 miles. Purchased used 18 months ago with 50,000 miles. My daily commute for the last four months has been 150 miles round-trip per day. I was getting 28-29 mpg at 75-80 mph, but when mileage dropped to 26-27 at 70-75 mph, I knew there was a problem. Also during this time, engine temp was hovering low, and would drop significantly on downshift. With NO CODES. Some general internet searching brought me to Audizine and a discussion of B7 frequent problems. After reading through those discussions, I ordered new cam follower, revised diverter valve, PCV valve and hoses, thermostat and fuel filter, snub mount, N80 Evap Purge Valve and spark plugs. Since the problem wasn't so severe, I installed each part individually to measure the difference and hopefully capture the exact root cause.
1) PCV/crankcase breather - Ordered "Emissions Service Kit, for VIN from 165001", which includes breather valve, gaskets, metal breather tube, and replacement screws. I also ordered the plastic tube from the breather valve to the intake manifold. I also did an oil change at this time. The car ran better at first, but the next day developed a surge at idle. I tried to pull the codes but only got soft codes p0171 and p0507. I cleared them and the car ran great. Probably should have left it alone at this point, but MPG was still low, temps were still low, and more service work is due, so...
2) Cam Follower - bought an OEM cam follower from the dealer, The old cam follower looked normal for the age, installed new part anyways. Car was much, MUCH quieter for about one minute, then the "normal" 2.0T ticking that I have always had returned. I had also been watching my High Pressure Fuel Pressures and I seemed to gain about 100 psi across the board. Also watching the Long Term Fuel Trim, and noticed that it was right around 7%. NO CODES
3) Diverter Valve - ordered the latest piston type diverter valve from ECS. Also coincided with an oil change. The old diverter valve looked fine, installed new part anyways. I disassembled the old diaphragm valve to see if I could confirm an issue, and only noticed that it didn't slide smoothly or seem to hold pressure, even though there were no tears. The next day, boost seemed stronger again, but other issues remained. And soft codes p0171 and p0507 returned. Cleared and ran as well as baseline.
4) N80 Evap Purge Valve - don't remember where I ordered it from, install should have been very easy, but it has those damn cinch clamps and the hoses are just barely big enough to fit over the barbs. No change in how the car ran. None. At all. But, NO CODES.
5) Thermostat - I think I ordered this from Europaparts.com, again install should have been easy, but I tried to take shortcuts and it took forever (three hours?). The moral to that story is to completely remove the alternator, also remove the intercooler to throttle body intake tube, don't drop any important nuts or tools. Once complete the engine warmed up much faster, and gauge stays perfectly centered (torque pro shows coolant temp ~195, consistent. Previous was 160 to 170). No other changes or improvements. NO CODES
7) Snub Mount - Ordered from ECS, installed at the same time as Thermostat, super-easy, especially after driving myself nuts with the thermostat. The difference in the way the whole car feels is extremely difficult to articulate, it is really subtle... but really significant? Does that make sense? It really does make the whole car feel more connected and direct.
6) Air Filter - Fram filter from Amazon... I know, I know. It was cheap, and I would like to reconfigure the intake soon, but I wanted to stay OEM until I get this all sorted. I also cleaned the snow filter... completely full of sand and road salt. Then... WHOA! I noticed that the MAF to turbo inlet tube wasn't clamped on either end. So I re-installed the clamps, confident that must've been it, and wow, the car ran FANTASTIC, for one day. Now I finally kicked hard codes and CEL on the same p0171 and p0507. Thinking that all this new air was freaking out the learned calibrations, So I cleared the codes and watched the LTFT climb back to the baseline of 7% over the course of several days. I then went through and performed Throttle Body Adap, fuel pump trim, and Idle Air Adap. Everything changed in all the ways you would expect after resetting everything... then about two days later, hard codes p0171 and p0507.
I have in hand the Fuel Filter and Spark Plugs, but I have not installed yet as fuel PSI is perfect, misfire count is 0, and I really believe this is a vacuum related issue. Today I disconnected and corked the PCV inlet on the intake manifold, no change in idle RPM's or AFR (high and lean), but confusingly ignition advance went from -3 deg steady to swinging from +3 to +9 deg.
With the intake inlet corked, I pinched the throttle body inlet tube as closed as much as I could with my hands, and there was no change. None.
That's where I am so far. Where else could the Intake being drawing air in from, if the PCV inlet and the trottle body itself are blocked?
Read here for a possible solution to the P0171 and P0507 CELs
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Originally Posted by CharlesP2009
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