No heat when running, only when climbing a hill or stationary - KW T2000

I'd say the thermostat too, plus if the engine runs too cool when it's cold out, it'll hurt your fuel milage.

 

change the thermostats, takes longer to drain and refill the antifreeze than it does to change them.

 

i had a bad rad cap do the same thing this week had no presure put new cap on went back to normal but this was a detroit

 

From cold the top rad hose/pipe should stay cold untill the thermostat opns at 195 degrees. if it does not it could be the thermostat seals, many people will change the stats and use the old seals, similar to sex with your socks on. You could also have a leak between the top surge tank and the radiator core if it is a KW or Pete. The fill line should be at engine temp, the bleed line goes into that tank.
You also want to have the preasure tested of the coolant flow Cat has a lot of water pumps that the impeller will slip.
I take off the top heater hose and insert a preasure guage, when running the guage should show positive preasure , up to from memory 27 lbs at 1800, when it is shut down you deduct the radiator cap preasure.
Just a thought!

 

Wanted to revisit this thread to say a big THANK YOU to everyone's suggestions and especially those that recommended I check the thermostat. Long story short, I'm staying in trucking (was thinking of getting out) after changing the two thermostats and gaining 1.3 or so miles per gallon! I'm at 6.4 now from 5.1.
The darn things were stuck open and have been for the last two years that I've had the truck. I'm angry at the mechanics whom I've asked about my low mpg and cold cabin and didn't ever think about this. I guess my inexperience is to blame too. Anyhow, each thermostat costs ~$50, plus $200 labor, so this $300 is hands down the best dollar I've spent on this truck.
I won't say go out and blindly change your thermostat, but if the cabin temp is remotely not hot enough, just replace those thermostats. It will be well worth it.

 

I have a similar problem. on a 3406E 1997. this first started when i was running thru some very cold weather in Mt-
Wy and Sd prior to christmas. it happens when i have been running for a while and stop at idle. I had also noticed that the truck wasnt running at 180 degrees going down the road, more like 150 but temp outside was minus 20 deg . so at first, at idle would start to blow cold. if i rev'd up the truck the air would warm up. that was a few months ago. now i am in some cold weather again. and as before after running for several hours and stopping, it started to blow cold at idle, but when i revved it up, it didnt heat up the air, i had to rev up to 1500 to get warm air. outside temp now is about zero. fortunately i have an espar heater that didnt let me down. so this morning i fire up the truck, let it idle, and go in for a few minutes. come back out and the heater is blowing real warm. so i let it just idle for another hour and it blows hot all the time. my water temp was stayed at about 150 deg.. which is telling i suppose after reading the other comments.
but i dont understand why when the engine water temps were about the same last night and this morning, would it blow cold last night and nice and hot this morning...???

 

Back to the basics.
Check the coolant level. first and foremost. very good idea to do that.