Superb II 2 ltr 170bhp. Why is it every now and again the revs go to 950 RPM and I get blue smoke out of the exhaust with no loss of performance, after about 15 to 20 mile of driving it goes back to the normal 650 idle speed? 

4 hours ago, Plip said:

Why is it every now and again the revs go to 950 RPM and I get blue smoke out of the exhaust with no loss of performance, after about 15 to 20 mile of driving it goes back to the normal 650 idle speed? 

\how often is "every now and then"?

There's no set time or mileage, I get to a junction and notice the revs up and blue smoke when I pull away, it's done it since I bought the car 2 year's ago

It's regenerating the DPF. It shouldn't be doing it so often though, so you have a problem in there somewhere. You'll need diagnostics to figure out what. Differential pressure sensor is a good possibility but don't go throwing parts at it blindly.

That's what I thought but I've been told the smoke during regeneration is white, Been on diagnostics and they can't find nothing wrong. No fault codes. 

If a sensor fails with a plausible value it's never going to set a code. You'll have to monitor things over a period of time I think. The VAG-DPF app with a Bluetooth obd adapter could be helpful here.

 

Another possibility is that you have an engine problem that is resulting in excessive soot generation and leading to frequent regens. A check over fuel and turbo systems might be no harm.