I do not think the purge line makes much of a difference at this point.
You are getting a fixed reading of 7% when the engine is warm. A fixed reading is a fault code. The manual does not have a fault at 7%, but the closest one is 10% (air flow position sensor / potentiometer).
It appears the potentiometer is worn at the idle position.
See if you can rig long wires into the cabin with the meter connected, monitor the duty cycle while driving and see if it sticks to any fixed value.
If it just keeps oscillating you do not need to adjust your mixture, mixture is OK. If it sticks to any value, then you have a fault.
The pots are known to wear right at the idle position, hence rough idle and fixed duty cycles at 10% (warm and idling engine).
10% is also a code for faulty throttle valve switch.

My 2 cents.

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1999 Porsche 996 Carrera Convertible
1994 420E - SOLD
1986 300E - SOLD, what a car
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