A bad EGR valve, plugged passageways or a bad vacuum
or electrical signal to open the EGR.
A lean condition created by a vacuum leak, a
weak fuel pump, a bad fuel pressure regulator, plugged injectors or a sensor that can
control fuel delivery that is lying about what it sees and creating the lean condition,
like a bad 02 sensor or a bad coolant sensor.
Carbon build up created by oil consumption
or low speed, short trips, granny style drivin’.
Timing not adjusted properly, too advanced.
Poor quality gas. This is an easy one for
the driver to determine.
The engine is running on the edge of overheating. This
one is also easy for the driver to see, because it will take a long time for the ping to
develop.
The knock sensor is bad or disconnected. This will
normally turn on the check engine light as well.
So I can't tell you which one is causing your problem, someone has
to go in and check each one of the above listed areas.