Toyota Repair: 93 LE V6 Camry blue/black smoke out tailpipe when driving over 50 MPH, oil leaks, toyota camry

Question
Model: Toyota Camry
Year: 1993
Model LE V6
Milage: 150,000
Tran: Auto

My car drives great around town with no problems, however, if I take it on the Highway (over 50 MPH) plumes of black/blue smoke pour out of my tailpipe.  11/2 years ago I had this same problem.  I took the car to my mechanic and he found oil leaks and replaced the headgasket and oil filter gasket.  Its been working great until now.  What could be the issue.  My mechanic says he cannot find any problem (oil leaks), but something has to be causing the smoke.  He seem to think the smoke is comming from the manifold.  Please advise.  Thanks, Jennifer

Answer
Are you sure the smoke is coming out of the tailpipe?
If that is the case there is an internal engine problem, possibly worn piston rings.
External oil leaks can also cause smoke behind the car if the oil is leaking on hot exhaust or engine parts.
If the mechanic you have been dealing with can't find the reason for this smoke I would get another opinion and diagnosis somewhere else.