My car has never had a serious mechanical problem, but it does have a rather frustrating problem.
When I am on side roads, the temperature goes up like normal(stopping at a normal level). I can use the heat normally and everything is fine. However, when I go fast on the express way, the temperature drops down and the heat blows quite cool instead.
I can also hear a whistling noise when I go at fast speeds, sounding like it is coming through the vents.
I have no problems with the air conditioner in the summer.
Your thermostat is probably stuck open. At highway speeds the radiator is cooled by wind. At slower speeds there is less of a cooling effect, and the stuck thermostat has less of an effect. It’s the same reason why traffic jams cause cars to overheat.
No insult intended, Mahaloth, but to elaborate on Bill Door’s post, the thermostat is typically located under the radiator cap near the front of the engine. It sounded as if you might not be as familiar with such things as some, so you should also know that the engine should be quite cool before you attempt to remove the radiator cap to service / replace the thermostat. If you knew all of this already, I apologize.
The radiator is the only thing under the radiator cap. The thermostat is generally located under a housing at the engine side of the upper radiator hose. To help the OP, it would help to have the following info:
Year, make, model, and engine size of the vehicle.
Does the vehicle come up to operating temperature in what would appear to be a normal amount of time (5 to 10 minutes) or does it take longer?
Does the engine coolant temp rise when you come to a stop of more than a few minutes or if installed, does the electric cooling fan kick on every time you are stopped for more than a few minutes?
At this time I am thinking it might be a sticky thermostat. It will operate normally when the coolant is coming up to operating temperature then stick open. When the car is shut off and the coolant cools, it will cool to a point that will allow the thermostat to fully close. The above can be tested by removing the thermostat and placing in in a pan of water on the stove. Generally when a thermostat sticks open, the vehicle takes a much longer time to get up to operating temperature and they tend to overheat in stop and go driving and run cool on the highway.
The radiator may indeed be the only thing under your radiator cap, Racer72, but that’s not the case in all vehicles, particularly those of older North American origin. I may have generalized, but at least I did so politely.
Well, Racer72, it seems that not only did I generalize, but I hallucinated. My mechanic tells me they haven’t made cars with thermostats below the radiator cap since Christ was a corporal. That’s a quote. I humbly apologize.
Just on the whistling noise, which I don’t think has been addressed yet…
Are you sure it’s coming through the vents? Noises can be hard to pinpoint in a car at speed, but I’ve usually found this whistling at high speed to originate in the door or window seals. Back in the 70s, when cars were assembled by drunken bonobos, it was an utterly normal noise. In modern cars, you shouldn’t hear it unless there is a faulty rubber seal somewhere.