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Help with diagnosis?

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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).


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.


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!


Edited by pkhoffman
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Thanks for the reply, Loren! Excellent point about the AOS. Where would you recommend I start looking with the P0446 and P0455's?

No P1126?

How could the AOS only affect one engine bank?

I would start with the P0446 and P0455 - you've got a leak that needs to be addressed.

Fix that problem - clear the codes and see what comes back. I thing at a time as they say...

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P0446 EVAP Canister Shutoff Valve (Function) - Below Lower Limit

Possible causes:

- Purge air line blocked.

- Flow resistance of EVAP canister too high.

Most of the time this problem is the purge valve.

P0455 Fuel Tank Ventilation System (Major Leak) - Below Lower Limit

Possible causes:

- Tank cap missing.

- Ventilation lines detached from EVAP canister.

- Ventilation line detached from tank.

- Ventilation lines detached from operating vent valve.

- Ventilation line detached from fuel tank vent valve.

- Break in ventilation lines.

Start by inspecting the cap for cracks and the o-ring on the cap.

If all looks well there then remove the right front wheel and start infecting the canister and the lines for road damage.

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