Just changed the theromstat and sensor in my truck and now the truck will overheat and not blow hot air.

If the fan is turned off and then you turn it on you get warm air for about 5 seconds and then it gets cold.

Before the temperature gauge would rise to max hot and then randomly fall back down to normal temperature. Now it just climbs and stays hot.

Could it be a bad water pump?

EDIT: Fixed. Had a pretty large leak near the overflow reservoir.

 

Maybe waterpump, when engine is hot sqeeze upper rad hose and release you should notice pressure on it if pump is working. If pump is really old well a new one wont hurt either way, But try this park truck with front end higher than rear (just enough so you can tell) turn heater to full heat remove cap from rad, top off fluid let it run getting up to temp and watch for air bubbles from filler neck, and make sure to keep topping off fluid level.Run till no more air can be seen.This is just a possibility. Or your thermostat could be bad or installed wrong another possibility.g/l

 

you might also have an air pocket. Like 413 said check your upper & lower hoses and see if your thermostat is working.

 

I suggest this

Remove the thermostat and sensor in your truck redo it sometimes the thermostat is in too far and kind of blocks flow.

Just redo what you did and check things out maybe the gasket is blocking flow the wrong gasket.
Best is to redo it all