The temperature gauge shows overheating on the freeway when going over 70 MPH. When I slow down and turn on the heater it cools back down. Replaced the temperature sensor. Still doing it. Not using any coolant. When I pull over and turn the engine off and check the fan is running, radiator feels cool. The upper hose is hot but the lower one is much cooler. Could it be the thermostat if the heater is working?

 

In my opinion it sounds like a coolant flow issue?

First two things to check.

1. blockage in the radiator

2. water pump flow rate.

From reading some past threads on here..your engine was prone to water pump impeller fins failing.

How many miles are on the vehicle and have you ever flushed the coolant?

 

+1 get to the source of your over heating condition. Do not drive it until you do and resolve

 

It could be if the heater feed is from head side of the engine as I would expect to make sure heating starts early. It is not an expensive thing to swap out. Is the engine/engine bay area hotter than expected?

 

If the radiator fan is running when the ignition is off, and the lower hose to the thermostat housing is cool, the thermostat is sticking or has stuck. Replace the thermostat. If the water outlet pipe hasn't been recently changed, this is a good time to consider doing that as well since it's prone to failure and can leave you stranded.

Do not rely on the instrument cluster temperature gauge to determine the coolant temperature. Use a smart phone with an app such as Torque and a Bluetooth interface connected to the vehicle's OBD port to read the current coolant temperature.