I've only had one T-stat that was marginal out of the box, it was opening early. I ran it for a couple years and then changed it with a coolant flush. Now my engine runs about 5-10 degrees warmer, and mileage has improved by about 10%.

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC