I just bought a 1991 Aerostar 3.0 with 108k miles.

-The temp gauge on the dash barely goes up when driving, staying well below the "n" in normal. (I can see it moving though)
-I'm getting very poor gas mileage - 12.5 mpg with full time city driving
-I tried the heater today, and it produces very little heat

What do y'all think? Thermostat stuck open? Coolant temperature sensor non-op (if so, can someone tell me the location?)?

Something else? DIYer here, no expert....

 

That sounds most likely to be a thermostat stuck open. It's a cheap part to get, a bit of a hassle to replace, but an easy test to do.

 

Pull the top radiator hose off and you can see the thermostat, if there's one in there. My 87 didn't have one when I got it and I just left it that way. Not much heat but I'm in Florida. If you decide to go any further, be very careful, it's real easy to break those bolts.
Don't expect any more than 12 to 14 MPG in town.

 

 

Thank you for the advice!

I'm lucky that this is a Mississippi (non-coastal) vehicle with almost no rust present.

I'm glad to hear that the vehicle is fine without a operational thermostat. (In fact, it does drive strong)

I will go ahead and change out the thermostat tomorrow, as I wouldn't mind it operating more efficiently. And as a new owner, I would like to know that the coolant is fresh and clean.

I will be very careful with those very thin bolts on the casing.

I don't really need a heater (I live in New Orleans), but I would like to see the engine working optimally.

I will post an update indicating if the thermostat makes a difference in the coolant temp or mpg.

Thank you for your advice.

 

There are replacement parts that you can skimp on. Don't do it with the thermostat. There are some reputable aftermarket brands out there, but be safe and get the Motorcraft/Ford replacement.
Your gas mileage will improve and your engine will wear less when the coolant is at the proper operating temp.