Wearing my coat, hood, and winter gloves last week, I was cruising in my sweet Lexus LS 400, and freezing. The heater was working on and off – mostly off. Going up hill for a short time it kicked in for a few minutes, then again when I reached highway speeds. It seemed that there was something to my wife’s reports of problems about it after all.

Several years earlier I had a similar problem with my 1999 Chevy Suburban. It was cold in the passenger compartment in the winter and was almost impossible to warm up. But, as turned out to be the case also with the Lexus, adding the proper mixture of water/anti-freeze did the trick. Fill the radiator, and your heater should work fine as long as your heater core and your blower motor are in good shape.

Technically speaking, one might intuitively think that low coolant would mean that the engine would run hotter and thus so would the heater. Well, the first part is true, but is not necessarily good, even in winter. However, the second part of that assumption is incorrect because of how a typical heater works…

It works like this: Coolant or antifreeze (or whatever you decide to call it) circulates through your engine block and heads to collect the heat from your engine. Your water pump circulates the heated liquid from the engine through the radiator where the heat is transferred (“radiated”) to the air that passes through the radiator fins (same principal as radiator-type heaters used in older homes). However, there is another mini-radiator inside your car called the heater core. Similar to the situation of the main radiator, the warmth of the engine-heated liquid is extracted via the heater core by way of the heater blower motor. That is why your heater works only when your engine is warmed up.

Now, the reason that a vehicle with low cooling fluid does not heat properly is because there simply is not enough liquid to be pushed by the water pump. So, your engine could theoretically overheat while at the same time you are chilled inside the car. And, your temperature gauge might not even indicate a problem.

Check your coolant level regularly! It can disappear inexplicably over time. In the case of the Suburbans of the late 1990’s, there was a defect in the heater hose nipple that comes out of the intake manifold. The metal would corrode very slowly and would cause micro leaks that were not detectable. The fluid that escaped would evaporate before it could cause a puddle. The antifreeze would be lost and the engine would overheat and the heater would underheet, and the driver would see no evidence.

With no help from the dealership, I finally discovered what was happening and replaced the faulty heater hose nipple. From then on, the truck’s heater has worked much better. Unfortunately, the overheating from early on caused warping of the head and/or exhaust manifold. So now there is a slight but noticeable exhaust leak sound that is anything but cool. However, you can keep your custom rides cool – and warm – by checking your coolant regularly.

All the best!

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