Ford Aerostar :: 1997 AWD - Heating Up To L In Normal

1997 AWD 4.0L with 112k

Background : Last year van suddenly overheated (almost to red). Turned on front and rear heat full blast and drove home - about 5 miles. Tested thermostat in boiling water and found that it would not open.

Fix : Pressure-washed radiator fins. Flushed system with Prestone kit and cleaner. Replaced thermostat and temp sending sensors (gauge and ECU) with Autozone parts. Added 50/50 coolant and correct amount of Bar's leak. Temps returned to normal.

Problem : As outside temps warm, the van now consistently runs between "A" and "L" in "NORMAL" on the gauge. AC on/off makes no difference nor does sitting or highway speeds.

Initial Fix : Suspecting the non-OEM Autozone thermostat to be the cause, I replaced the thermostat with a 197 degree Motorcraft unit from the dealer. The van still runs at the same temp.

My Thoughts :

Fan Clutch - Unlikely. Resistance when turning by hand. No fluctuation between sitting and highway speeds implies airflow is adequate

Thermostat - Unlikely. New OEM unit.

Temp sending sensor for gauge - False high reading a possibility. May try to measure temp of radiator supply hose directly to see if it's accurate. May also try to measure sensor resistance to see if it's within specs.

Radiator - Likely? Older van, so scale may be causing blockage. Temps have increased with time. No fluctuation between sitting and highway speeds supports blockage. No issue when outside temps are colder also seem to support blockage theory.

Conclusion : Radiator is okay with winter temps, but reaches it's cooling capacity with summer temps.