I have a 1998 chevy silverado z71 5.7 that currently has a thermostat and heater problem. I have a week old thermostat rated at 180 and my truck won't hit anything over 140s. I have serviced the fluid level. My truck just won't heat up. I know the gauge works because when I off road it does exceed 200s. Now for the heater im just getting lightly warm air. I replaced the heater hoses. I installed a new heater core and re filled the fluid level. I am still getting cool warmish air. I also noticed no pressure in the system as I can drive for 30 minutes and remove the radiator cap with no pressure or fluid bursting out. The heater core is getting hot I checked the heater hoses and both are equally hot. Please give me your opinions on what the problems might be thanks.

 

Thermostat may be stuck open? Best guess I have. There could be something in the heat level adjustment switch too that would be causing it not to allow full heat, but if you can pull the radiator cap, I would guess anti-freeze is constantly circulating.