89 Ranger will not blow heat

I've got a 89 ranger xlt the temp gauge does not work to tell me when the truck is warm. When Idling will blow heat, but when driving down the road does not. The heat travels through the heater core and is warm on both hoses. not losing any water. think it might be thermostate. Need help any other ideas that it might be?

Sounds like it (thermostat) could be stuck open. Does it take a long time to warm up?

Pete

It doesn't take long to warm up. When sitting still it will blow luke warm air, when i start to drive down the road it blows air that is barely warm.

Probably not the thermostat, check the fan clutch.