right when the real cold of winter flares up, my truck decides it doesn't need to keep me warm anymore. Basically it started the other day with ...stays cold after about 20 minutes of run time, then gets sorta warm...well tonight I ran it 50 minutes on the interstate and I could still see my breath in front of me as i got to the house..my toes are numb.

I noticed that the factory temp gauge would go from Cold...slowly crawl to Hot, then instantly drop down to a mid range level...seems to do it nearly everytime.

I checked the heater hoses after shutting it off..they are only warm from the engine heat...the top radiator hose is fairly warm..the bottom one is warm from the engine heat..nothing feels toasty hot.

Any input? 150,000 miles, i replaces pump about a year and a half ago.

replace the t-stat

Check the water level and antifreeze mix. I had a weak af mix in one of my cars and it actually froze when I drove without letting the engine warm up. Darn near cost me a motor before I caught it.

If anyone has ever run stop-leak through the system there is a good chance the heater core is plugged. If so, you will have to remove both heater hoses and flush core both ways.

sounds like no coolant or air in the system

I've got the same problem in my 98 V8 dak. However, I noticed a tiny drip when I'm running the heater coming from a weep tube about middle of the firewall near the bottom. My diagnosis seems clear, someone put stop leak in it, it completely plugged it with the exception of the leak out of the condensate hose. I'll be putting in a new core and flushing.

I have a 92 with the same problem, haven't been worried about it but now its getting cold out.

The heater core gets easily clogged. If you don't flush the system regularly, just flush your heater core by disconnecting the hoses and running water thru the core. Chances are if you disconnect them at the firewall, you will see the blockage. Run the outlet hose into a catch can. I bet it will look like mud coming out. Do that before you change T stats or spend any money onany expensive service.

thanks for the advice guys, I'll try and mess with it tomorrow as I have another day off.

My 93 V6 would do the gauge thing, go from cold to almost pegged, than back down. It would fry your feet if kept at highest hot.

Sounds like a blockage or air. Have you put the hoses on backwards maybe?

Sounds like your blower works, but no heat?

My '90 3.9 Dakota would not product heat brand new off the dealer lot. Defective thermostat was stuck open. New thermostat got the heat back. On a higher mileage truck though, I'd agree an air bubble or heater core blockage.

I did have a friend that had a F250 diesel with no heat. His problem was a big air bubble in the heater core, system had to be burped, then he had heat.