1993 Ford Aerostar coolant temp guage swings

[quote:44357386ee="ray333"]Other Category problem
1993 Ford Aerostar 6 cyl Two Wheel Drive Manual 170000 miles
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Temp guage has always stayed the same. Then one day it started to make big swings - from the lower "O" to upper end of the "M" on Ford (NORMAL) guage scale.
Replaced thermostat; replaced coolant sending unit; replaced radiator. No change.
Nothing in oil. No evidence of water in exhaust. Never overheats, just big swings

Appreciated,[/quote:44357386ee]

There is a slosh module in the back of the instrument cluster that controls guage movement and that may be failing on the temp sender side, usually when completly bad guage needle will swing to either extream of the scale. I once had an arostar temp guage issue that was resolved after adding a ground wire at the temp sender to a ggod ground off of the engine, aparently the intake manifold was loosing electrical continuity with the engine block. I had a few other Fords that the base problem had to do with poor electrical grounding of engine. Like I'm sure at this point you or someone has had to service the battery terminals, and if not start with the negative one, The terminal end will look normal but they corrode from inside the wire out.

Yeah. I have seen this problem. I had a 96 awd aerostar. I replaced the thermostat. Aerostar thermostats are installed vertically. This tends to trap air in the engine. When that happens you get highly variable temperature readings. The thermostat I used in the 96 aerostar had a air bleed port in the thermostat housing. When installed correctly the port is at the top of the thermostat so it lets air out of the engine as you refill the coolant. I have a 95 aerostar that had similar simptoms. We replaced the thermostat in it too. The thermostat that was removed had no air bleed port. The replacement had none either. The wild gauge swings did not go away. I pulled the new thermostat and put a small hole in the thermostat housing and reinstalled it with the hole in the top side. All is now well "like new" behavior. Heater works. Engine runs at "N" in NORMAL. Apparently the FORD/MotorCraft thermostats come with the air bleed port.