I had the intake off about a year ago to replace the glow plugs. At that time I soaked the intake in Simple Green. I did not do much. Should I remove the EGR from the intake and soak it in a strong solvent such as mineral spirits, gasoline, carburator cleaner.....?

How could the EGR be responsible for this?

FYI...In all of this there has not been a Check Engine Light situation.