I'm scratching my head over this one and would appreciate some advice. My 04 Ram with the heat on in any fan position blows alternating hot and cold air.

When this first started I replaced the radiator as it was leaking. Still had the problem, so last Sunday I replaced the thermostat. It pegs the temp gauge hot while blowing cold air. Then, after a few seconds on hot, it starts blowing real hot and the temp gauge drops back to normal. Once at normal, it starts blowing cold again until it heats back up pegging the temp gauge and blowing cold. it repeats this process. I suppose the new thermostat could be bad, but is there anything else I'm missing? Because the hot/cold air coming out of the vents seems tied to the temp gauge, I'm thinking it's a thermostat issue and not a relay. Thoughts?

I replaced the blower motor and resistor back in the summer when the A/C was acting up.

Thx!
Randy

Thats not good. I ASSume ume you have properly bleed the cooling system? If so, It sounds like you may have a head gasket issue. This is common on an engine thats been overheated, especially a 4.7.

Have you checked the blend door? Common problem o the rams.


Maje sure that all the air is out system. My 5.7 has a bleed screw on the top of the thermostat.

Another vote to bleed system

My company 02 Ram has similar issues. Heating all over the place as well. We just got it back from the shop. They tested the cooling system & found exhaust gasses in there. We believe it's head gaskets & I'm about to start tearing into it. My concern is the heads themselves.

Thought I've read that these magnum heads are very prone to cracks. Anyone got any more info on that before we tear into this truck.

Sure does sound like air in the system. And it could be from a head gasket as was said. When I fill them up I fill them and leave the cap off. Then I run the eng with the heat on until it either starts blowing warm air or starts to get to hot then I shut the eng off and wait a bit until the eng cools. While doing this it will usually spit coolant out of the rad but it is pushing the air out. After I do this few times it will get all the air out as you will know when the heat stays warm and the eng does not overheat. If you cant get it to fill at all then I would test for exh in the rad as they make testers for that as it may be a head gasket. I also like to fill some at the end of the day and let them sit overnight with the cap off as some of the air will sometimes work its way out just sitting for a good bit like that. You can try to loosen the the heater return hose to bleed some air as sometimes I do that but it does not always work to good. Ron

Which engine are we dealing with?

Yes which engine? 4.7 or 5.7? Magnum engines were done after 2002.

The bleed on my 06 5.7 is not that obvious. had to look it up in the book. It is an inset allen head screw