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Heat not working at low speeds

I have a question that I hope someone can help me with. I have a 2001 Infiniti I30. Last August, after a long drive from Florida to Virginia, my radiator burst and I had to have it replaced. Since it was summer, I didn’t notice a problem with my heating, but since the weather has gotten cold I now notice that I have no heat at lower speeds, even though the fan is blowing. If I drive at high speeds (40 MPH and over) the heater works fine, but once I slow down, the fan continues to blow but the heat goes away. I can also cause the heat to come on by revving the engine while still in Park. My mechanic insists this is not related to the radiator being replaced. Any thoughts on this? Possible diagnosis? I can give more details if needed. Thanks to anyone out there who might be able to help!

Two of the things that can this problem is either a low coolant level or air entrained in the cooling system. Have you checked the coolant level?

Tester

Since the rad was replaced I would say the tech left some air in the system and it needs to be “burped” to remove the air. Some vehicles have a bleeder for this.

Does the mechanic refuse to diagnosis the cause of the concern or are you refusing to pay for him to look because you think the heat problem is related to the radiator work?

Tell him you will condider the heat problem unrelated to the radiator work unless diagnosis proves otherwise,you trust him don’t you?

Thanks for the replies. When I asked my mechanic to check the problem, he did notice that the coolant level was low, so we added coolant, but this did not fix the problem. This was when we tried letting the car run for a while, to demonstrate that the heat did not work when the car was not moving. Then he had me “gun” the accelerator while in park, and after a couple of minutes the heat started blowing, but as soon as I stopped accelerating, the air kept blowing but not with heat. Re the “burp” issue, I really don’t know–I’ve been hoping that if I keep driving the car on the interstate, etc., it will work its way out, but this doesn’t seem to be happening.

I originally had the radiator replaced by a AAA-owned garage, which I don’t have a lot of confidence in. But since I had assumed that the problem was related to the radiator having been replaced, I had hoped that the repair would be free or inexpensive if I took it to the same shop. However, since they’re telling me they don’t think the problem is related, I wonder if I should take the car to a locally-owned shop which is known to be very good, although pricey. Perhaps, all things considered, that would be my best option. I guess that if they do fix it and IF their assessment is that it was related to the radiator replacement, the AAA-owned shop would have to acknowledge that and reimburse me. (I would, of course, discuss this with them beforehand.) Thoughts?

Take it back to your mechanic, and get the engine up to operating temperature. With the engine idling, loosen up the upper radiator hose clamp slightly. Take a flat bladed srewdriver and slip it between the upper radiator hose and radiator hose neck until all the air is purged out. Retighten the clamp.

Tester

Thank you so much for this advice. Since I posted the original query, I had the misfortune to have my alternator go out, and when I took the car to the shop to have that replaced, I also asked them to look at the heating problem. I read your response to them verbatim and they got the problem fixed–they said this was exactly the problem. So. . . thank you very much!