On modern diesels with EGR, the intercooler internals slowly fill with an oily soot sludge — a mix of EGR carbon and crankcase breather oil mist. It can cost you 15–20bhp before you notice.
Why it happens
EGR routes hot exhaust gas back into the intake. Crankcase ventilation routes oil vapour. The two meet, condense on cold intercooler walls and bake into a thick paste.
Symptoms
Slow boost build, reduced top-end power, occasional smoke and gradually worsening MPG.
Cleaning process
Intercooler removed, flushed with proper degreaser, agitated, rinsed and dried before refitting. £150–£250 mobile depending on access.
When to combine with EGR work
If your EGR is clogged, the intercooler will be too. Do both at once and you save hours of labour.