My volvo runs great. starts great. 238k miles. just intermittantly smoke will come out of the tailpipe, one can kinda tell when it is going to do it because the idle will change to sligtly rough and rpm will drop a bit, kinda like loading up or something. Put the car in neutral, and rev the engine, the smoke goes away for a while, then it will come back if still at the same location is idle. On the freeway or driving at any speed, the car has absolutly no smoke. Please help. (turbo drain has been replaced, intercooler drained, turbo seems to be in good shape --lots of power and no smoke under hard acceleration -- im stumped).