f-150 gas heating problem

[jbradley113093]

my neighbor has a f-150 gas truck. he sorta has a problem with his heat.
he left his truck at idle when he went into the gas stationa dn came back out and it wasnt blowing warm air. when he was driving it blew warm air back out. so if any one has any suggstions on what it could be please let me know.

 

[DEERE3594]

Change the thermostat yet?

 

[jbradley113093]

he just rebuilt the motor about 2 months ago he said it was working fine tuesday but it was acting funny yesterday. but he also pluged it in monday night so the block was already warm. and he didnt plug it in tuesday night so the block wasnt warm then like it was tueday morn.