I installed a fail proof thermostat a month or two ago on my car. The car does not overheat on trips less than 25 miles, but on longer trips it overheats. The radiator is full and the cooling fan works as well. I noticed that the coolant tank may be leaking; could that be the problem?
I have never heard of a fail proof thermostat. If your coolant tank has a leak then it may be getting low on coolant. A leaking coolant tank that is unable to hold pressure can also cause the temperature to get higher than normal. The coolant needs pressure to help with the anti-boil properties of the cooling system and its fluid.
Replace the coolant tank and the coolant with a 50/50 coolant/water mix. Check to be sure your fail proof thermostat is opening and at the correct temperature also. Make sure the fans are coming on at the correct temperature with and without the A/C on. If you need assistance with this, contact a professional mechanic who will be able to pinpoint and fix your overheating issue.