No heat... Please help

On Detroit engines...there are 2 valves you can close for cold weather.

Follow the lines from the radiator and close the gate valves.

 

Have you checked the gate valves for the heater hoses going to the cab if the temp gauge dosent move then I'd say you need to check the thermastat it sounds like its stuck open if the gate valves are open then the hoses running to the cab should be hot if they are then the problem is in the cab could be the rheastat on the control panel on the dash or could be the actuator for blend

 

Just as Otherhalf said. I had this issue, one of my valves were closed. Follow the heater hoses in the engine compartment to find them both and make sure they are all the way open

 

I have two sets of valves on the heater hoses. One set off the water pump and the return above the oil cooler. Mine look like hose spigot on my Western Star, but other trucks I've owned have had ball valves. I imagine if you don't have temp registering on your guage you have other problems.

 

Or the blend door in the heater box isn't working and stuck on cold.

 

Yah beat me...

 

Do you have a water filter?
When they changed the thermostats did they change the seals?
Do you have a winter front?
If you are in an area that is cold you may need a belly tarp, from the front bumper to the middle of the transmission.
You want to also check the rad cap, if it is not holding preasure the system does not work properly.
With the Cap tested and good, the system full of coolant, top rad pipe bleed, and the engine cold, oil level good, fan belts, and fan all good. Run the engine for 1 minute at Idle, you should buy an infered temp gun they are very handy, run the engine up to 1200 rpm and monitor the coolant temperature at the oil cooler housing, and the top rad pipe, it should stay cold, turn on you lights, pump the brake pedal, no cab heat, when the temp gets to 120 degrees, run at maximum RPM, and monitor the top rad pipe, it should stay cold untill the thermostat opening temperature, 190 degrees.
If that tests out OK then turn the heater temperature control to full, lower the RPM to 1200, and check the shut off valve, works, and opens fully. It has a little lever and a cable, full out is on, full back is off. Measure the temperature out of the engine and into the engine threw the heater core, without the fan on they should be very close, put the fan on full, and measure the temp then.
You may have a filter, in the air inlet for the heater that needs changing every year. the heater fans may be dirty and not work properly. remove them and wash if needed.
Trucking is so much fun!

 

So the fan is on 100% of the time after start up?