If everything is running fine, then the engine isn't having any problems due to carbon build-up.

So it's completely pointless.

If it's otherwise running well, I'd leave well alone. There's always a chance that if all the piston and bore carbon is removed that oil consumption will increase.

There's always a chance that if all the piston and bore carbon is removed that oil consumption will increase

That is exactly what happened to my Anglia in the 70's when a friend of my dads told me to carry out the Redex treatment.

Remove plugs, pour in Redex, turn engine over by hand and leave overnight. Start engine and once warm pour the rest of the Redex into the carb with the engine running.

Before the treatment the engine was fine, after it the oil usage was considerable.

Learned never to use snake oil again.

I would like to buy a Peugeot cabriolet which uses the same engine as some Minis and I believe that there is a carbonisation problem.

Oil from the crankcase breather is fed into the airflow and carbonises on the back of the inlet valves; it is not washed off by the fuel as it is gasoline direct injection.

I have seen this hydrogen cleaning advertised but can’t think how it would work and it does say hydrogen as opposed to others that say hydrogen peroxide. I can see how that might react and clean but am worried it might cause corrosion.

The company that does carbon cleaning through the fuel (no use here) also has a system that vapourises solvent and it is injected into the air intake.

Any thoughts?

I am getting 45mpg at present and there are no obvious problems. Does anyone think this is a good idea?

I don't. It's a typical example of automotive quackery. Sow seeds of doubt about engine health, flog 'remedial treatment', reap profit. Indeed, there might be a risk of harming machinery that is nicely bedded in and working smoothly. There are many sad stories on this site of expensive problems occurring after unnecessary interference by incompetent mechanics. If it works, don't mend it!

I hardly think that 45mpg from a 1.9D is something t write home about: I get 41mpg from my 1.6 N/A petrol (at 12yo) Mazda3, roughly the same size car. I would've expected over 50 from a diesel. Many aftermarket treatments like fuel injector cleaner, EGR and MAF sensor cleaner have been shown to work well to significantly improve mpg and drivability on older modern cars (as opposed to actually old cars) that are either un predominantly in an urban environment and/or for short trips from cold, or who don't get maintained that well.

The former can be far cheaper than having to use branded super grade fuels, which will do a similar job, but take longer and cost more. I can buy a 4-shot Redex bottle for the price difference between a tank-full of Shell Superunleaded and 95Ron (similar for diesel). Using the higher grade fuels is better over the long term, because it keeps the engine in better health, rather than just helping it recover, but most people can't afford it.

There are many sad stories on this site of expensive problems occurring after unnecessary interference by incompetent mechanics. If it works, don't mend it!

Once again JohnF i am in agreement with you (even if we agree to differ on actual servicing), it aint broke don't fix it.

I'm none too sure what these weird and wonderful concoctions might do to the oil itself, fragile sensors in the intake system, and any aftertreatment or catalysts in the exhaust system, let alone what they might do if the muck seeps down into the piston rings.

The stuff might be entirely harmless, but it might not.