I have a 2000 Ford Expedition, Triton V8 approx 143000 miles. The weather is starting to heat up and it is starting to overheat. I have added coolant, and there does seem to be a really small leak. If the weather stays under 80 degrees outside there are no issues, not even when driving for extended periods of time (1 1/2 hours straight) , the gauge never shows its getting close to the red, just stays in the middle. If it gets over 80 degrees outside then after about 25 min, pretty exact, the gauge starts rising quickly...so I turn on the heater full blast and it comes right back down to normal. Any ideas on what might be wrong so I can see if it's something my fiance can fix or have an idea before I take it in? Thanks