renierk wrote:Hi Thank you all for the replies. I am not a boffin on engines. What is the EGR system?

EGR = Exhaust Gas Recirculation system
- it bleeds a portion of the hot exhaust gases back through to the inlet manifold and is designed to reduce the emmissions. Based on revs, engine temperature, exhaust gas temperature, throttle opening etc. the ECU determines the amount of opening of the EGR valve - i.e. how much, if any, exhaust gases are recirculated.
One of the issues with EGR is that the hot exhaust gases can cause the oil in oily inlet air (oily because the sump ventilates to the inlet manifold and also can be because of some degree of oil leakage through the turbo seals) to partially burn in the inlet manifold leaving carbon deposits which choke the inlet.