Having just had my Freelander TD4 pass it's MOT, the tester said that it took two attempts to pass the emissions test and suggested using an additve to clean the engine thereby lowering the smoke levels...

Do they work?

 

Depends on what sort of smoke it's making:

Black = soot from overfueling/restricted air intake - make sure your air filter element isn't clogged. If you're using a modified ECU for extra power (this will cause the extra fueling) put it back to factory spec for the MOT.

Blue = oil smoke - something is wrong, internally, with the engine - usual suspects are piston rings and/or valves.

White = either coolant or unburnt fuel - smell it, if it smells like a cross between week old laundry and an illegal backwoods still, then it's coolant. If it smells like a red arrows fly past then it's unburnt fuel. Coolant in the exhaust is usually caused by a head gasket failing, unburnt fuel usually only happens when the engine is very cold and is highly unlikely on a TD4 anyway.

As far as additives go, fuel system cleaners do sometimes work although diesel in the UK already has detergent additives - pretty much all the others (eg slick 50 or anything that claims extra horsepower or that it has 'teflon' in it) are all a bunch of snake oils and at best they'll do nothing, at worst they'll do some damage.

Hope that helps,

 

Thee are a number of additives designed for diesels to clean up the fuel system, I don't know what is available for you over there, but it would be worth your time and money to find one. Once you put some in, get the vehicle at and on the highway and push her hard to let the chemicals clean her out.
Mike