Here is my most educated from the cracker-jack box responce...
My 94 Jeep Cherokee started to run hotter as it got older, and here is my most educated guess's as why this is happening.
1. As antifreeze gets older its chemical ability to dissapate heat is reduced along with its freezing ability.(coming from Canada trust me on this one)
2. As the rad hoses get older they may become encrusted on the insides with gunk.
3. As the thermostat gets older, the wax and spring do not respond as good as they did when new, meaning it will (may) not open as well as it did when it was new.
4. The impellar on the waterpump slowly wears out, and the coolant mixture will not get forced through as much. ( like lower the amperage in electrical paths).
5. Look at the rad from the inside, is it full of junk and debris?. As the passages get full of rust and other junk, so goes its ability to shed heat.
6. How about the cooling fins?. Is there bugs and junk covering them up?, if so clean the rad from the inside-out, meaning get your hose and spray the water from the inside of the engine compartment to the rad, forcing the junk back to the front bumper.
Some of these are more important and a higher percentage than others but it is usually a slow combination of all of them acting together that does it.
The best analogy is to draw a parallel to your blood vessels clogging up with bad cholestrohol(sp), it happens slowly over a period of time, until your blood pressure is to high. A maybe somewhat crude explanation, but I am sure you get the point.
And thats my 2 cents worth....
which being 2 cents Canadian means very little...
98 300 DT