Dear Tom and Ray:
Recently I was driving my 2001 Ford Focus in cold weather on I-70 in the mountains west of Denver. There are two long, steep grades. After completing about half of the downhill run on the west side, I noticed that the engine temperature had gone completely cold and I was getting no heat in the passenger area. When I opened the throttle slightly, the engine temperature began to come up some (so did the speed — not what I needed). When I stepped on the gas to go up the other side of the hill, using the engine for propulsion instead of braking, the engine temperature returned to normal and I got heat again. The same thing happened on the return trip. What’s causing this? — Jim
Ray: I think you have a classic case of low coolant, Jim.
Tom: The way you get heat is that the coolant circulates through the engine and sucks up the heat created by the combustion process. It then releases that heat in one of two ways: either by going through the radiator, or by entering the heater core and releasing heat into the passenger cabin.
Ray: So the heater core is like another small radiator that’s only active when you crank up your heater control. And on most cars, the heater core is the last thing to get coolant — it’s the farthest away from the water pump, and often the highest thing in the system, too.
Tom: So if you were low on coolant, the first way you’d notice it in the winter is from a lack of heat in the cabin.
Ray: If the coolant were low enough to fall below the coolant temperature sensor and leave the sensor exposed, the engine temperature would read “cold.” So it all makes sense.
Tom: The reason it came back a bit when you went uphill is because when you’re revving the engine, you’re making the water pump turn faster, and that’s getting a little bit of warm coolant into your heater core. At least temporarily.
Ray: So you need to do two things, Jim: You need to see if you’re low on coolant. And when you find out that you are, you need to find out why. It could be anything from a loose $2 hose clamp to a $2,000 cracked cylinder head.
Tom: You want to catch it and get it fixed while it’s still a hose clamp — before you overheat the engine and make things much worse. Good luck, Jim.