Odd fan sounding noise from front

[NotsosmartCDI]

I have a Smart ForTwo CDI, and intermittently getting what sounds like a fan coming on quite loud behind the dashboard. It is not the heater fan or air con. No warning lights or loss of power, although that issue we have as well, like a lot of other CDI owners apparently. Anyway, my girlfriend drove 10 miles and each time she stopped at lights the sound went off. Started again when she set off. What on earth is this? I didnt think there was anything to do with the engine at the front. Is it some kind of cooling fan? Its only happened a couple of times since we got the car 2 yrs ago. Engine revs dont increase while its doing it, and its not the dpf being cleaned, as we know when it does that. Can anybody help? Dont want a silly priced Merc assessment

[ColSmart]

Must be the engine cooling fan as thats the only fan up front that you have not mentioned, as it cools the radiator

it should switch on only when the engine is really hot, then off again when the temperature elect switch tells it the engine temp is normal again. (usually this happens in a long queues on a really hot day, ie, the engine is not getting cooled because car is not on the move, so no natural flow of air. The electric fan creates air flow through the radiator instead.)

THIS IS A NORMAL FUNCTION to cool the engine.

Because of the hotter temperatures in the UK at the moment, this is bound to operate 'on and off' if stationary for a while.

When it goes faulty.....
If it is doing this all the time on colder days then your engine is overheating for some reason, faulty thermostat, etc.

The usual way to check for overheating is via a temperature gauge which we do not have on the Fortwo, or at worse smoke coming out the engine, as coolant is too hot. When that happens its time to stop at the side of the road.

[NotsosmartCDI]

Thank you so much for that. Very helpful. Much appreciated.

[Thrumbleux]

So the sound only occurs when the car is in motion - implies something at the wheels to me. I'd check for a broken spring rubbing on the tyre.....

[tolsen]

You can read out coolant temperature with a cheap diagnostics kit. Worth while checking coolant level and top up if required. Mine is a 450 Cdi and radiator fan only runs if I run flat out up steep hills. Keep monitoring coolant temperature for a while to confirm if normal operation.

[10:45PM]

It's common on the 451 for the cabin circulation fan motor bearings to go dry and sometimes make a right racket...

BUT simply turning off the fan will eliminate that pretty quickly.

I agree after that's been eliminated like so, and if it's only doing it on the move, and not when stationary, it's gotta be delaminated brake pad lining, road spring fragment caught, rusty brake discs catching on the backplate, wheel bearing on way out etc etc... something associated with rotation of wheels.

[lalli_d1]

its not the dpf being cleaned, as we know when it does that.

on a side note, how does one know when a DPF regen is being carried out, because as far as im aware the driver wouldnt know if it has carried out a regen.

[10:45PM]

Don't think there's an indicator/lamp but you'll get an elevated idle speed for one no doubt

[ColSmart]

on a side note, how does one know when a DPF regen is being carried out, because as far as im aware the driver wouldnt know if it has carried out a regen.

This old post answers this question by the fact the revs go up during regen.

http://smartmaniacs.co.uk/forum/the-cars/fortwo-451-sept-2007-present/58570-cdi-dpf-tech-question