Summer. Dry weather, air temperature 26oC, the sun is shining. I’m on a little trip thru Europe on my F32 440i (B58 engine). I draw for around half an hour on the excellent German highway. I stopped in the parking place for a short moment to answer the emails. I started the car. I waited for some 10 seconds, then began to drive without any hurry. At that moment, the lady and her 7-year-old son standing nearby were looking at my car with wide eyes. What happened?

It turned out that a massive cloud of white smoke was coming out of my car’s exhaust! Truly impressive! At that moment, I thought only of technical nuances, so – sorry, there was no picture of the actual event. The image with a similar situation is “stolen” from the internet, but in my case, the actual cloud of smoke was much more impressive!

My first thought – summer, warm weather. The engine has been warmed up, and I have already driven for half an hour. Water condensate was not assumed as an option. 

Other possibilities – unburned fuel (for example, the defect of the injector) or the injection damage – the cooling agent gets in the burning chamber(s). 

A logical conclusion – the trip is over. The engine should be stopped immediately, and the car evacuator should be called out. I decided to find out though what had happened to my car. I got out and approached the exhaust. It turned out that the smoke didn’t have the stench of the fuel. The smoke doesn’t have the soppy stench of the cooling agent!

The steam comes out of the exhaust! In the summer! After driving 50 km with a warmed-up engine! How is that possible?

Explanation, in my opinion, is interesting,

A comment is in order here: this applies only to 440i with Performance and Sound kit exhaust. If your car does not have this exhaust – don’t worry; such an “effect” is not possible. 

Note: Performance kit exhaust has two valves, each on the other side of the vehicle (info here). The flaps are closed the first minute after starting the engine (regardless of the driving mode chosen). DME warms up the CO catalytic converters; the exhaust VANOS is activated to speed up this process. 

In the morning, DME warmed up the CO catalytic converters when starting to drive. Considering that the weather outside is warm, the engine has a load (not large – I don’t torture unwarmed engines), and CO catalytic converters warmed up very quickly – it happened during the first minute. The exhaust flaps were closed, and the exhaust gases flowed via the “bundle” of the exhaust system. As everybody knows, the side product of the catalytic converters is water. This water is accumulated in the bundle of the exhaust system. 

Note: Typically, I drive in the Sport+ mode. In this mode, flaps are always open, which means – the exhaust gases are not flowing via the bundle but “directly” to the exhaust exits.

After 60 seconds, during which the DME warmed up the CO catalytic converters, the exhaust flaps opened (because the Sport+ mode was chosen), and I, not expecting anything bad, continued to drive. The water was “sleeping” in the exhaust bundle. 

When stopped in the parking lot, I allowed the DME to “go to sleep”. After a moment, when “waking up”, DME again started to warm up the CO catalytic converters and closed the exhaust flaps. Now the exhaust gases were flowing via bundle. As the engine was still warm, the CO catalytic converter had not cooled down either; a flow of hot exhaust gases got in the bundle. Hot exhaust gases heated up the bundle swiftly. The water accumulated in the bundle, started to boil and evaporate. 

This time this is good news – there is no doubt that the CO catalytic converters performed their duty perfectly. But I assume that many car users (and I understand them – the symptoms are critical) would call the evacuator and tow the car to the service. And I’m sure most of the mechanics would immediately disassemble the engine to perform its capital repair.