RE: carbon buildup
Exactly what does it do at idle? Does it run ok at higher engine speeds? I think that Your mechanic is a bit off base. Check the idle control solenoid and the EGR valve. These are the kind of things that will cause an idle problem. If it's running ok at above idle conditions, carbon buildup is not the problem. The carbon buildup occurs in the EGR valve which does exactly what it says ( Recirculates Exhaust Gases ) which is where the carbon would come from. Thanks for listening.