2000 W900L Weak Heat

What temp is the engine running? If it running cold the thermostats are not closing and need replaced.

 

What does the eng temp gauge read when you are saying its not blowing hot?

 

Do you have a filter installed? If you do its located on passenger side behind the engine air cleaner. Its not a factory option but a lot of people get it installed aftermarket.

 

Try this: when the truck is warmed up, open the hood and locate the valve for the heater. It'll be under the evap box on passenger side firewall. There's a cable going to it. That cable may be stretched or out of adjustment and not opening the valve all the way. See if you can push the valve open a little more. Give it a minute and see if the air gets hotter coming out the front vents.

Also, I'm assuming you've made sure your water valves where the hoses come out of the water pump and engine are all the way open?

 

A quick test to see if it's a coolant flow problem is to trace your lines which go to the cab heater and the bunk heater. Usually black rubber hoses. Feel them with your bare hand, with the engine running at operating temp. They should be hot. Very hot. The one going in to the heater will be slightly hotter than the one coming back out. If this test passes, then you know you are getting proper coolant flowing. If one is hot but not the other, you likely have a problem with the temp control valves. On my 04 WS, the one for the cab is manual, controlled by a cable. You should be able to find it in front of the firewall. My valve for the sleeper is a bit harder to check, in my case the heater is under the fridge. That valve in my truck, is an electronic valve, so far haven't had an issue with it. Your sleeper temp valve may be as mine, electric, or could be a cable. In any case you need to check them for proper opening. Good luck with it.

 

Someone posted a picture not long ago about mixing diffrent types of coolant. He cut the heater hose open and it was clogged shut. You can't mix coolant. Should be able to tell by feeling the in and out hosses and they should both be hot. If one is cold you have a clogged system.

If they are both cold and you have a summer shut off valve it's probly a gate valve (like a garden hose bib) they suck. They can feel like they are working but are stuck shut. Replace with 1/4 turn ball valve.