[nmlss2006]

Folks,
my X3 started doing something fairly unexpected recently and I do not know enough about the car to even make an educated guess.
It is, first of all, a 04/2014 production 35i with 73kmi on the clock. It's been more-or-less OK in the past, save for necessitating a rather premature - I thought - belt tensioner replacement around 55kmi. And don't get me started on the procedure to get that done.

Be that as it may, as the title mentions the car has started smoking - fairly copiously, white smoke - AFTER the engine is stopped.
You can start it - it will be fine. You can leave it idling, hood open or closed, no issue. You can drive it around, in any manner you choose.. no issue.
You can then leave the car running after driving it around ... still no smoke.
The moment that you turn OFF the engine after driving it at least 15 minutes, within two minutes the car will start smoking white. The smoke will typically start coming out of the REAR wheelwells and within a minute or so will progress to the front and start coming out of the bottom of the windshield. It will last three or four minutes and then disappear.

If you open the hood, it will rapidly dissipate. It's white smoke, without a particularly strong smell, though there is a certain 'burning oil/overheated engine' smell to it - slight, however.

The coolant level is fine and stable. The engine does not overheat. There is no visible sign of oil in the water or water in the oil. The trans seems to behave, plus trans fluid and gear oil are pretty distinctive and the smell doesn't quite match.

Has anyone encountered this? Any ideas?

[Wgosma]

When driving are you using the A/C?

White smoke might be condensation from the evaporator is not dripping straight downward and water spotting the driveway, perhaps the underbelly pan's drain holes are clogged and the moisture is getting routed elsewhere…. It's one thought.

[Opie55]

That is odd. Does it do it the same every single time you shut the car off or only sometimes? I think we can eliminate oil. Oil burning to produce that much smoke would smell like burning rubber. Coolant would smell kinda sickly sweet. Plus I can't think of why coolant would only leak after you shut off the car.

That would seem to leave water as the source. I'm guessing your washer fluid level is not decreasing, and I can't see how that would get on a hot engine part unless you are using it every time before shutdown.

That would seem to indicate the AC. If your car reliably smokes each time, then you have an easy test. Start it from cold with the AC fully off and drive until it will normally smoke and report back.

[nmlss2006]

Thank you guys. I was thinking A/C too - by process of elimination as you very correctly described - and that's a quick way to figure out if it's a factor. I will test later today and report back.
It is true that for some reason unknown to me several of the plastic belly pan bolts went... missing. I wonder if that is a factor (pan flaps while driving -> catches more dirt -> holes get clogged -> pan gets flooded -> white smoke).
The only thing is, in theory (?) it should happen when the engine is running as well, I'd would think. In any case: I'll try and report back.

[nmlss2006]

So this is going slowly but it's going - it's my MIL's car, so diagnosis is... less immediate and exact.

Anyways - it seems like the A/C is not the ONLY issue, though it's certainly one of them.

Now there is an amount of semi-regular white/blue smoke coming out of the passenger side in the engine comparment, in the rear (i.e. by the firewall).

I am pretty persuaded that this is the valve cover gasket, I'll take a look and report back.

[sibhusz06]

Check your valve covers or oil filter housing to make sure your not leaking oil, as the oil will run down the passenger side of the engine onto the exhaust enroute to the transmission and cause odd white smoke.