RE: 1999 Dodge 1500 Van thermostat problems
This sounds serious.[&:]
Yes, change the thermostat to either a 180F or 195F type. Don't go lower than the 180F since the PCM (computer) would get messed up on input values.
When you changed the fluids, did you use "distilled water" with the glycol mixture? If not, then all of the glycol inhibitor protection is now used up due to the use of tap water.
It's strange that the AC has an effect on the cooling. But, both the water pump and AC run off of the same belt. Have you checked the belt tension, the tensioner and the pulleys? If the water pump is shot, it will be either very hard to turn the it's pulley or very sloppy to turn it.
Another odd thing about engine heat is this, when was the last time the engine oil and filter were changed? Whats the level of oil now? Remember to use 10W30 in this 3.9L Magnum engine. If the oil hasn't been changed in a long while, the cooling system can't keep the engine cool enough.
Does the vehilce throw any engine codes when the off/on key trip is done three times?