Got this from another site. Hope it helps:
I used a four litre can of parafin, branded Parasene, bought from B&Q and it brought it up like new. The difference in performance is quite amazing, and I hope this will translate to an improvement in MPG.
I took the intercooler out in the morning, laid it on its back and filled it with the parafin (it takes about 3 litres). I then let it soak for about 3 hours, drained it of the now black parafin, and poured in the remaining litre or so. This I sloshed about to rinse out any residue and again drained it. Next, I rinsed it out with a hosepipe and at this stage I was quite surprised to see even more black crud being flushed out. I let the water run through until it was clear. Finally, I laid it in the sun to dry and used a hairdryer to blow out any remaining parafin fumes. Re-fitted it in the afternoon.
NOTE: Make sure your intercooler is completely dry and free from parafin fumes before re-fitting or 'bad' things may happen!