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Ford aerostar won't warm up

I have a '91 aerostar, 3L, no aux heater, that won’t warm up. Two thermostats, a new temp sensor and a water pump haven’t made a difference. It’s been flushed, cleaned, and everything else i can think of, but rarely get’s above two on the gauge when it should be running at four or five. It also occassionally will warm up some, but then drops suddenly back down. Anybody got any ideas?

Looks like the only thing left is to make sure the gauge is working properly.

Try this very simple little test. Remove the rubber boot from the temp sender, shove a nail in it and ground it to the engine.

Turn the key on & if the gauge is working properly it will indicate full hot.

Gauges are notoriously inaccurate. Measure the ACTUAL temperature of the coolant with a thermometer or infrared heat sensor.

You didn’t mention a new radiator cap, but hopefully you’ve replaced that, too, and made sure there’s no air trapped in the cooling system.

Thank you for your suggestions! Yes, new cap and heater core (old core finally started leaking). Tried the nail trick, gauge shows full hot. Haven’t measured the actual coolant temp, but there’s very little heat coming from the heater, temp probe show’s 100 degree air with ambient at 60. Felt the heater and radiator hoses and they feel like plenty of water is moving, but for the 192 degree speced it doesn’t feel that hot. Flushed the core seperately and checked it with engine running, plenty of water going through. If there’s a trick to releasing trapped air i don’t know it, followed the Chilton manual directions for the drain and flush. Blocked off 80% of my radiator and it finally heated up, but that’s not a solution. Most of the time the guage sit’s right on the second mark, and that’s about what the air from the heater feel’s like. I’m stumped.

This might sound funny but make sure you have you’re oil topped off . Low oil will cause an engine not to warm up fully too.

Could be an aftermarket thermostat that doesn’t fit properly.
If you are using aftermarket 'stats take one to the dealer and see if the parts counter person will let you compare it to one from Ford.

"Blocked off 80% of my radiator and it finally heated up, . . ". I think your 1991 Aerostar has a mechanical fan clutch. If this clutch is engaged all the time, you may be pulling too much air through the radiator and this won’t allow the van to warm up. I had the opposite problem on a 1990 Aerostar. The clutch slipped and wouldn’t pull enough air through the air conditioning condenser. The engine didn’t overheat, but the van wouldn’t cool down. A good mechanic finally ran down the problem–directed air from his shop fan through the radiator and condenser. The refrigeration pressures dropped to normal and the air conditioning started cooling. A new fan clutch solved my problem. You could have the opposite problem. When the car is cold, try spinning the fan blades. If you can’t spin them the fan clutch is locked and the fan will run all the time. What makes this a possibility is that you do get heat if you block off 80% of the radiator.