Volvo 240s are good about coking-up, Lots of oil vapor works its way into the engine despite a large oil separator. We would routinely take off a vacuum line off the intake and allow it to siphon Seafoam at higher RPMs; this did the same thing... GM dealers also have a product for this purpose alone, does good work at getting rid of carbon in the bores/heads.

__________________
---------------------------------------------------------------

1998 VW Cabrio
2001 Audi SomeRoad A/T (no air suspension)
2003 Audi DeadRoad M/T (no vroom, for later)
2002 Audi NoRoad A/T (nothing under the hood, being rebuilt)