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If the Gas Cap fixed it temporarily.
Then you most likely have a evap leak.
This is a test our cars do on a drive cycle.
Like every 5th start after been driven x miles and stopped x time longer than 4 hours.
Hence why it comes and goes.
The only way to find out what the leak is is to do a smoke test of your fuel system.
Minimal charge to figure it out.
They hook up a smoke machine to gas lid, at the engine after the charcoal filter and possibly before the charcoal filter in the back of the car.
Our charcoal filters at least on a 335i is right behind the left rear wheel under the rear bumper.
Get it smoked, professionally.
DN
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