Stone cold Mazda truck

I inherited a 1994 Mazda B2300 Pick Up 5 speed with just over 100K miles. Problem is the temp. gauge will just barely move off “C” at full tilt on a summer day. Confirmed the temp gauge was right. Replaced the thermostat and flushed out the system and heater core. Hoses are “warm”. Cardboard over the radiator will push the temp. up to the middle after 20 minutes on freeway. Next checked the fan clutch. No dice. The fan spins free and can be stopped by hand.
So, what else is “open” allowing too much coolant flow? All help appreciated.

Mr 2

So, how much heat do you get from the heater?

There’s been a small run of “running too cool” posts lately. The bottom line is that, while there are lots of reasons one might run too hot, a thermostat problem is pretty much it when it comes to running too cool. Lots of aftermarket thermostats are not so good. So before I did much else I’d put an OEM thermostat in it & then see where things stand.

If you are absolutely sure the temperature sensor and gauge are working correctly then the only thing I can think that would cause this trouble is a air pocket at the sensor. Topping off the coolant through the top radiator hose may fix that if that is the case.

If the radiator hoses get hot before the engine gets to normal temperature then the thermostat isn’t doing its job.

My guess is you got a cheap thermostat from an auto parts joint.
Go to the Mazda dealer (or a place that sells OEM parts online) and get the proper thermostat.