White smoke on overrun

[this is my username]

R55 Mini Cooper D, 2012 with the BMW N47 engine. Has done 120k miles.

Makes white smoke on the over-run. As an example, arrive at a roundabout after a long, fast dual-carriageway run, as a I come to a stop I look in the rear-view mirror and see a significant amount of white smoke. It also produces a bit of white smoke on startup, but they may well just be condensation.

Any ideas of what I should investigate? There are no signs of it using oil or coolant.

Thanks

[anonymous-user]

Sounds like a failing turbo.

[NMNeil]

PCV valve?
On engine overrun the vacuum is at it's highest and the PCV valve should close, but if it's not working it will 'vacuum' out any oil/vapour/mist from the crankcase into the intake manifold and into the engine giving you white smoke.

[Tyre Smoke]

If it's smoke, it's your turbo I would bet. If it's steam, your head gasket is toast, but that would be under load more than over run.

[Krikkit]

Definitely white and not blue/grey?

[Turkish91]

Strong chance it’s turbo. Get it looked at before it properly goes.

[AW10]

Long shot but how’s your brake fluid level? A leaking master cylinder seal can allow brake fluid into the servo and it can then get pulled into the engine. Brake fluid burns white.

[bearman68]

Nobody mentioned EGR valve yet? - what about blanking it off to see if it's that. By far and away the most likely on a diesel.

[Megaflow]

White smoke is normally coolant, but it can also be unburnt fuel. Has your fuel consumption got worse? If so it might be an injector that is failing.

For coolant, have a look at the coolant level to see if it is dropping.

[cuprabob]

You can also get white smoke while the DPF is regenerating.

[this is my username]

Thanks all - the regen option is an interesting one.

I will keep monitoring - maybe I will mount a GoPro on the back of the car to keep an eye on it.

[anonymous-user]

Tyre Smoke said:

If it's smoke, it's your turbo I would bet. If it's steam, your head gasket is toast, but that would be under load more than over run.

when i blew the head gasket on an mx5 it would produce a huge steam cloud on the overrun but nothing when accelerating.

[Megaflow]

JimSuperSix said:

Tyre Smoke said:

If it's smoke, it's your turbo I would bet. If it's steam, your head gasket is toast, but that would be under load more than over run.

when i blew the head gasket on an mx5 it would produce a huge steam cloud on the overrun but nothing when accelerating.

I would expect more steam on the overrun with a head gasket failure. Under acceleration the cylinder pressure is way higher than the cooling system, therefore will try to pressurise the cooling system, but under overrun the cylinder will be in vacuum and therefore try to pull coolant into the cylinder.

[Tyre Smoke]

You may well be correct. I was basing my theory on a Citroën AX GT from decades ago. It never ever did what I expected it to.