Engine Cooling problem
1997 Ford Aerostar 6 cyl Two Wheel Drive Automatic 134534 miles
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My van has been over heating, I've changed the thermostat (the fail-safe model), I've even ran the vehicle with out it but it still over heats. I've looked to see if the radiator had circulation, and it does, I've even did a radiator flush (Preston super cleaner). In order to stabilize the high temp. I run the Defroster on high to keep it from going in the RED( the temp gauge goes to the L in NORMAL). Also I did pressure check I pumped 16psi into the radiator and timed it for 3 minutes before the gauge lost pressure and even then it was not at all fast in loosing the pressure. What is the problem.

Try block testing it for combustion gas leaking into the cooling system

Things to try before blocking testing.
The Radiator cap should hold 15-16lbs of pressure for a period of five minutes before any leak down.
I would replace the cap first.
Another thing often over looked is the fan clutch. With the miles you have on your vehicle I would check for proper operation of the fan clutch.
Also because of the miles you show I would not rule out a water pump. Aerostar water pumps have very thin cheap impellers and if the cooling system has been neglected the impeller could be bad.
If all these suggestions fail to cure your problem then proceed with the block test.