...all four have heavy dry sooty deposits on them, i cant remember how old they are, possibly 2 years old. its done 20000 miles since new piston rings and stem seals, 10000 miles since new turbo and a new grpa headgasket 1000 miles ago due to manifold studs snapping.it ran well with a puff of smoke on over run but nothing ott. what should i be looking at in general order of likely causes?