Hello, I have a 1999 996 C2 6-speed with 91,000 miles. Only mods are a K&N filter, headers, and an old K-40 radar detector hard-wired into the DME. I bought the car about a month ago for a steal at $9,000.00 because the engine light was on and it had a rough idle. I was hoping for some help with diagnosing its problem(s).

Symptoms:
1. rough idle - rumbles like a muscle car and sometimes idles is at 700rpm and sometimes at 1200rpm.
2. Difficulty starting about 1 in 5 times. Requires excessive turn over and priming with more gas.
3. Poor mileage - 12-13 mpg
4. Sometimes smells like it is running rich
5. Gas tank won't fill to Full; it never shows above 7/8 tank.
6. Check engine light is on most of the time, but occasionally goes off for a day. Usually goes on when I'm in higher rpm's.
7. Very tiny intermittent oil leak coming from passenger side front of engine bay.

Codes:
1. P1539 - Camshaft adjustment bank 2 (4-6) Frequency 166
2. P0446 shutoff Valve Activate Charcoal Filter; Frequency 2
3. P1124 Oxygen sensing range 1 Cyl 1-3
4. P0455 Tank Ventilation (Major Leak)


My thoughts so far:
1. Air oil separator causing vacuum leak, causing it to think it's running lean, and then overcompensating by enriching the mixture, hence the P1124, rough idle, problems starting, and intermittent oil leak.
2. Camshaft problem - I'm not sure there's a problem with the camshaft actuator itself because I great great performance from the car in all rpm ranges; it pulls great above 3k rpm so you'd think performance would be affected if it were unable to adjust. That leaves either a bad sensor, a problem with the DME from the K40 radar detector, or something in between.
3. Fuel tank may be a purge valve issue, blocked vent tube, or the fact that the k40 intentionally causes the gas cap light to turn on when it detects radar and this in turn caused the error. The K40 cause still wouldn't explain the inability to read "Full" on the gas gage.


I'd appreciate any input and/or differing opinions!