Questions and Answers

Car Running Too Cold?? Q. I have a 1990 Lumina 3.1 liter automatic transmission. My son and I rebuilt the engine about 2 months ago. My radiator hose blew yesterday. My thermostat was stuck shut. So, I replaced the hose and removed the thermostat, then turned on the motor.

The bottom hose is still cold and the top hose is hot. I ran water through the radiator and hoses to see if they were plugged. There was good circulation in both. Now I closed everything up and put water in the system to see if it would leak, and I still have the same problem. (Top hose hot, bottom hose barely warm.) I did not put a thermostat in this time while checking. Does that make a difference just to check it out.? I just don't know what to do now. Can you help me? This is the only car I have to get to work with.

Thanks for your time,
Mike

A. I don't think you have a problem here Mike. If you're getting good circulation and the car is not overheating, then you are fine. If you put a thermostat in you'll see the lower hose will get hotter. The thermostat not only regulates engine temperature, it also controls the speed of the coolant through the engine. If the coolant flows too fast, it won't pick up heat as efficiently as if it were moving slower.

There's a basic principle at work in your cooling system. it's that when a fluid is heated, it rises. The "cold" (I'm talking relatively here) coolant is drawn into the engine from the bottom of the radiator. When the engine heats the coolant up, it rises up and "hot" coolant goes back in the top of the radiator. As it cools, it has the tendency to sink to the bottom of the radiator and the cycle starts again. Of course this is not enough to cool an engine without a water pump, but the engineers use this principle to work with nature, not against it.

What is happening is the coolant is not that hot when it enters the radiator and when it is cooled, it cools it down to the point where it is almost actually cold. As I said, pop a thermostat in there and I think you'll be fine.

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