Have you checked the coolant level? It’s sounding like you’re low, and your heat’s depending on the extra pressure when the engine revs faster than idle. (The water pump’s speed and flow is directly related to engine RPM.)
I agree, it sounds like the coolant level is significantly low, causing a big air pocket in the cooling system. (Note that coolant refers to the liquid in the system, which should be a 50/50 mix of antifreeze and water.) Top up the coolant as soon as you can (cold engine only!) and have it fixed promptly. The next symptom will be severe engine overheating, which can cause expensive damage.
An important detail - check the coolant level in the radiator, not in the overflow jar. The level in the jar is not a reliable indicator of the level in the rest of the system. (This is why the engine needs to be cold to check it - the radiator pressure cap must be removed, and it’s not safe to remove it when the coolant is hot.) Coolant should be up to the top of the radiator, and the overflow jar should be about half full.
GaryT you were correct. There was an air pocket in the cooling system. This was discovered when the car over heated today on my way to work. I immediately pulled over, stopped, and turned the engine off but had the heat blow on Hi (cold air only came out).
AAA towed it to my friend’s house and he diagnosed a cracked intake manifold gasket.
He says the engine should be fine and I am having him repair it
Slight hijack, but I figured its ok now that the OP’s problem has been resolved. Do you always have to have a 50/50 mix of anti-freeze and water in your coolant tank? I’ve always used just antifreeze/coolant that they sell at safeway or something like that. Am I damaging my car?
From my understanding the downsides to doing it this way include (there may be others):
Propylene glycol is not as efficient as carrying heat from your engine block to the radiator - it has a lower specific heat than water - so your cooling system is not as efficient if you run straight antifreeze rather than an antifreeze/water mix.
Antifreeze is more expensive than water, so you are paying more to fill your cooling system than if you ran antifreeze/water mix.
The other reason is that a mixture of antifreeze and water has a lower freezing point than either straight antifreeze or straight water. The phenomenon is called “freezing point depression” and it’s common in mixtures.
What Schuyler and Rocketeer said. It’s not necessarily damaging the car (although it could), but it could certainly lead to problems. Presumably you’ve been topping up, rather than filling a totally empty system, so there should be some water in the mix you have now. I would recommend fixing the leak that necessitates topping up (it shouldn’t be losing coolant at any appreciable rate) and restoring a 50/50 mix. In the future, premix the antifreeze with water, or at least add equal amounts (pint of antifreeze, pint of water, another pint of antifreeze, another pint of water, etc.). It doesn’t have to be exactly 50/50, but the closer the better, except for extremely cold climates, where up to 70% antifreeze can be used.