What is wrong with my Heater?!

It is not blowing hot air. On the highway it get's a little warm, if RPM gets to 5,000 it instantly gets warmer. at 1,500 RPM it get's instantly colder. at idle it is full on cold air like A/C is on full blast! Outside -18c Brrr...!! Is there a quick fix to this? I thought a cardboard cover on the radiator would help, But it is not getting colder at highway speeds.

2004 Dodge Caravan AE

 

How is the coolant level? And when was the last time the thermostat was replaced?

 

coolant was replaced a week ago I guess my sister thought it would fix it. Still the same. I assume my brother replaced the thermostat. Before the coolant was low but still see the coolant, when running with the cap off it bubbled up like soapy water, oil change place said there was a leak somewhere not sure why they still replace the coolant anyway.

 

When was the last time the thermostat was replaced? - did he replace it?

 

If the engine isn't overheating, I replace the t-stat with a Mopar part 1st and see if you have heat. If you still don't have heat. The other problems could be water pump or radiator.

 

Nope, The thermostat wasn't replaced.

2004 Dodge Caravan AE

 

Not that you have to but I always replace the thermostat housing with the thermostat. Saves on a lot of headaches trying to grind out the stakes that hold in the old stat to get the new one to fit back in correctly.

 

The Van just started over heating. thermostat for sure? I hope it is not the water pump.