Silverado running hot/ no heat?

I have a 2005 Chevy Silverado, and recently it has been running hotter than usual (I dont know the exact numbers, but it usually runs at the halfway mark, and is now at the 3/4 mark) and the heater doesnt work… as long as I’m going UNDER about 60 mph. When I get on the freeway, the water temp goes back down to the 1/2 mark, and the heater comes on! Occasionally when I’m going around 30 or even stopped the water temp gets even hotter, and it tells me that the engine coolant is hot. I’ve tried adding more coolant… maybe I have a leak? I also know that I need new AC/Serpentine belts, could it have something to do w that? Thanks!

Unfortunately, it sounds like the symptoms of head gasket failure.

If you have recently had work done on the cooling system, it could be something simple such as air trapped in the coolant.

Low coolant level may make the car run hot, while the heater, which is located higher up, does not get any hot fluid.

Pressure test the clloning system and if no leaks, fill it and get the air bubbles out.

@HaddlesRB, was the coolant level lower than the full hot/cold mark when you added coolant? If so, then that can definitely be the issue.

Check your waterpump. They tend to leak after several years of service.

It would seem to be time for a new thermostat, due to the age of the vehicle. As well as a cap.

Are your fans coming on?
I believe you have electric fans, correct?

@db4690 the fans do come on, they are very loud from outside the car. @98caddy and @Docnick, I have tried adding coolant (the level is always low when I put more in), which fixes it temporarily, but the issue comes back and it seems to be sucking down coolant.

@HaddlesRB okay, so your fans are working. Good.

Since you’re losing coolant, somebody needs to perform a coolant pressure test.

In the meantime, why don’t you crawl under the truck and look for signs of leaking coolant.

Particularly, look at the radiator and the water pump. If anything looks crusty, it might be the leak

If you wind up needing a water pump, do the thermostat and pressure cap also.

If you are losing coolant without any visible leaks, that is one sign of head gasket failure. There is a test that can be run to determine the presence of exhaust byproducts in the coolant.

I hope that’s not it, but I would hate to see you waste money replacing other things if you are going to need a head gasket replacement.