I have a 1997 ford f150 and the motor is only heating up to 99 degrees at the thermostat. The heater inside only blows luke warm air. I have replaced thermostat twice and have flushed radiator and heating coils. What could be wrong.

 

How do you know it's only getting to 99deg? Are both heater core lines hot?

 

Pointed laser thermometer to thermostat to get reading and across motor. It was 99 degrees. Both heater core lines are hot

 

Does the dash temp gauge get up into the normal range or is it reading low, or peaking really high and then staying low? The usual causes are thermostat related.. either it's stuck open or it's installed backwords, theremostat should be installed with the spring towards the motor. Are you installing failsafe thermostats? Are they stuck open when removed? If so that indicates an engine overheating problem which could be mean a blown head gasket but could also just be trapped air in the motor. What motor is this and did this just suddenly start happening or were you trying to fix a problem?

P.S. You're technically in the wrong forum for that truck but for this problem the solution is the same regardless what year or motor you're dealing with so it's not a big deal IMO.. if a MOD wants to move this thread so be it.