The van is always reading engine heat at 140 degrees. Shouldn’t it be closer to 210 degrees to run more efficiently? And would relate to how the airconditioning is exceptional and heater is not good. We’re really writing about the heater, as the '99 van runs really good. (Already changed thermostat, and everything is the same.)
One question you need to ask is if the van is really running cool or is it the temp gauge. Does the van have two temp sensors, one for the indicator and one for the PCM? I suspect it does. If you van does not get to operating temp, you should get a CEL. OTOH, you feel that the heat is not hot enough which is a real tangible indication that the coolant is not getting hot enough. It is time to get some quantitative information to replace the SOTP evaluation. Find out what the real temp of the coolant is.
140 degree thermostats are common. There are 160 and 180 thermostats too. Find out what the spec degree thermostat is supposed to be. If the spec calls for 140 you are there already. You need to stick to the spec temperature thermostat, it is what the engine is designed to use. Increasing the temp above the spec will not improve efficiency. New motors do use higher temps but they are designed to take adavantage of the higher temps.
Using a higher temp thermostat than called for can result in pre-ignition knocking and or other problems that can result in damage to the motor over time.