The brief
The check engine light on the 116i had been on for weeks. The owner could live with the light, but the rough idle at the lights and the longer-than-usual cold crank in the morning had started to bother him, so he brought it in for a proper diagnosis rather than guessing at parts.
The EVAP purge valve is part of the fuel-vapour system: it controls when fuel vapour stored in the charcoal canister gets drawn into the engine to be burned. When the valve stops sealing, it leaks vapour, and unmetered air, into the intake whether the engine wants it or not, which upsets the mixture, makes the idle rough, and can make a cold start drag while the engine sorts itself out.
The diagnosis
On the BMW handset the stored codes told a consistent story, a lean fuel mixture and a catalytic-converter efficiency complaint, both of which point at extra, unmetered air or vapour getting into the intake. A smoke test confirmed it: vapour was leaking through the purge valve when it should have been sealed shut. Bench-testing the valve out of the car backed the diagnosis up.
There is no adjusting a purge valve that no longer seals. The fix is a new one, which is a small, cheap part for a fault that had been throwing a check engine light and roughing up the idle.
The work
The purge valve was located in the engine bay, the harness clip and the two vacuum lines freed, and a new BMW-spec EVAP valve fitted in its place, the lines reseated. The stored fault codes were cleared on the handset.
The engine was run and checked for a clean idle before the car went back to the owner.
The outcome
Check engine light off after a drive cycle and staying off, the idle smoothed out at the lights, and the cold crank back to a normal half-second start.
The 116i went home with the dashboard clean and the fuel-vapour system sealing the way it should. For the owner, a small part for a fault that had been nagging at the dash and the idle for weeks, and one that, left alone, keeps the engine running a touch lean, which over time is harder on the catalytic converter than it is worth.