Hook up an OBDII reader and see what the air fuel ratio is. It should read 14.6-14.7 to be in the correct range, when the engine has warmed up.
Those engines have a habit of burning the rear spark plug wires but that is usually noticed by a misfiring.
The Tech should have been able to tell you what the problem was or at least an area of concern other than to just say the engine is running rich. In the USA most large auto parts stores will check your OBDII port to see if there are any codes.
Are you burning an excessive amount of oil?
