The water usually doesn't hit that high up, so the plaster covers and intake manifold could be completely dry, but the bottom end is wet. When it's raining heavily, water gets pass the radiator and hits the block, the oil sump or the exhaust (especially the exhaust manifold and cats) and instantly turns into steam. If you have driven aggressively enough, and the radiator is trying to dissipate that heat, the radiator itself could vaporize outside water droplets directly as well. When you slow down, the airflow that was pushing it out underneath the car is gone, so it goes up. But the highest point of most car's hood, which is where most BMW's cabin air filter box and tray sits, is sealed with a rubber gasket. So the steam will escape out the kidney grills which is the next biggest opening.
This phenomenon only appears when it's cold and wet outside because 1.) moisture, 2.) it's cold enough that airflow to the radiator is all the cooling it needs. and 3.) it's cold so you're not using A/C, which won't command the auxiliary fan to run.
So let's say your car is sitting there in traffic at a red light. You've been driving the last 20 minutes in stop and go traffic with more stops then go. Your engine thinks someone has stolen the throttle because it has barely rev'ed beyond idle for most of the trip. The radiator outflow pipe temperature sensor is telling the DME that the coolant/water temp isn't hot enough to take a bath in, and if it dares to turn on the radiator fan it will tell the firewall drain plug to clog up and drown the DME. Your A/C isn't running because you've either turned it off manually as most sane people do in winter, or the condensation sensor inside the cabin says there's no foggy windows to clear, so the A/C won't run even if you're insane like me who leave the A/C on auto because the LEDs light up the on the buttons in a nice straight line. Damn my OCD... Now your car is sitting there looking like a cool kid blowing cigarette smoke out it's nose.
Easiest way to test if this is the case... When you see the smoke/steam, stop the car but leave the engine running and pop the hood. Let whatever was in there vent out then close the hood. It shouldn't steam up again until you are moving again unless it's absolutely pouring outside.
My E60 was steaming these few days as well, but mine was directly on top of the hood cause the insulating blanket is old and not that good anymore. Plus E60 hoods are aluminum so it sucks up the heat and radiates it even faster then steel.
Of course, you may bring it to BMW for them to check, especially if you're still under warranty. But remember to check over your car like it was a rental before you hand it over to them... My bumper still bares the scratch marks they left on it pulling my lights out to change 2 simple D2S bulbs, and somehow during a routine firmware update, they managed to kill the CAS, then the LM. So what should've taken 15 minutes ended up taking 2 months...
thank you so much for your detailed sharing. much appreciated.
I opened the hood yesterday but it was when the car was sitting cold. I saw some ? water at the bottom, beyond reach of my hand, beneath all the machinery. I wonder if that could be rain that went in through the radiator like you described. I checked again just now and it wasn't there anymore. if it was coolant leak coolant probably won't evaporate?
I also checked the coolant level and it was just above minimum level. So I added 200ml Watson's distilled water.. Drove 35km afterwards but all the time below 80km/h and it wasn't raining. Didn't see any smoke.
my car is at 5.5 years and 30k km and no warranty. I'm still thinking should I take it back to BMW cuz the previous owner has been taking it back to chai wan yearly.
Coolant leaks are usually a constant stream and won't stop until the seal totally fails. I suppose what you saw was liquid collected on the underside aero covering. If you see liquid there and it dries quickly, it usually isn't a big concern. Coolant will indeed evaporate. Next time, you can even shine a flashlight on it and look for a rainbow reflection. If you see that, you have a oil leak somewhere.
Depending on how long it took for the coolant level to drop to that point, it might or might not be an actual leak. If it took a year to get from max to min, then I wouldn't worry about it. If it took a week, then you have a problem. BMWs have a plastic coolant expansion tank, and that may crack or leak after some time. The expansion tank cap may also fail as it actually holds quite a bit of pressure and is again, plastic. To look for a coolant leak, look for a white chalky trail. The corrosion inhibitor sticks to things and gets left behind when the water in the coolant evaporates.
If you are not under warranty, start looking for trustworthy mechanic and shop. Going back to the dealer for everything will get less and less economical as your car ages. BMWs have a lot of plastic components and they do wear out with time, even if you are way low on mileage. Even if the previous owner took it back to the dealer annually, dive into the records and see what was actually done. Sometimes, owners will defer on service items and since the car is out of warranty, the dealer won't force them to do it. I would look for things like expansion tank replacement, water pump replacement, and coolant pipe replacement. The most common failure items are the coolant pipes. They are rubber and the heat cycles will harden them until they fail. I replace the entire set every 4 years. It's under $2.5k with labor for my M54 using genuine BMW parts. I forgot if the N13 uses an engine driven mechanical water pump or an electric one. If it's engine driven, the pump seal and pump bearing may leak coolant slowly and you see a white trail down the timing chain cover.
Also, the mechanics at the dealers are getting less and less experienced. With modern cars, they only need to hook up the ISTA and look at that the codes tell them. They don't have the troubleshooting skills that old school mechanics have for actually looking at the oily bits and figure out what's wrong.