The intercooler over time gets contaminated with oilresidue. The intercooler is basically a radiator, and it's an air-to-air-system. The ICI Genklene cleaner, is difficult to obtain, but any radiator cleaner can be used - though it's a problem that most cleaners are supposed to be used i conjunction with water and engine running... So I used white spirit, but soon found I needed so much of the liquid to fill the intercooler, so I turned to paraffin from the house tank... First filled half a liter of white spirit, and started shaking and turning upside down, downside-up - really letting the stuff run through the small holes in the intercooler. It turned dark black immediately. Then I filled it fully with paraffin, and just left it there for half an hour. Then poured out some of it, and started shaking again. There was a lot of oil built up. Then flushed both ways with a hose - long. Neither paraffin nor white spirit is good for engine air, so making it clean is important. Drying took a day and a half in hot sun. Some roverists say they put the intercooler back in, letting the turbo push air into the intercooler, but not connecting it to the engine. This allows the turboair to dry out the intercooler.

To remove intercooler, you:

Remove all hoses to intercooler (up and down).

Remove any brackets

Lift out intercooler element.

Drying out the intercooler takes time, and suspect the engine to stay in your garage for two days. This is good time to clean cyclone engine breather, as you use paraffin and because this one also has to dry out before refitting.