A bad radiator cap.
Dear Tom and Ray:
I have a 1982 diesel Chevy Malibu wagon. The problem is that the water from the radiator is drawn into, and overflows the over flow reservoir. Then it's not drawn back into the radiator, making the temperature light come on. I've had a new radiator put in, as well as new head gaskets and bolts. Still, the problem continues and my mechanic is getting as frustrated as I am. What could it be?
TOM: Your mechanic may be getting just as frustrated as you, Joanne, but remember, at least he's building up his IRA in the process.
RAY: The coolant is supposed to flow into the overflow container. That's normal. Your problem is that the coolant isn't being sucked back into the radiator. What that leaves you with is low coolant in the radiator, which makes the car overheat. In fact, this may have been responsible for your blown head gaskets.
TOM: Here's what's supposed to happen. When the radiator cools off, the pressure inside the cooling system drops, and suction is created. That suction is supposed to draw the coolant out of the overflow bottle and back into the system. There are two things we can think of that can prevent the coolant from being sucked back in.
RAY: One is a very small hole somewhere in the cooling system. It could be a pin-hole in the radiator, a crack in a hose, or even a loose clamp. That would cause air to be sucked in through that hole, instead of the coolant from the reservoir.
RAY: The other--and more common--cause of this problem, is a bad radiator cap. The radiator cap controls the flow of coolant in and out of the over-flow container, and a faulty cap could definitely make this happen. If I were you, I'd start by trying a new radiator cap before I spend any more big money on this heap.
TOM: And here's a prediction. If it turns out that a ten dollar radiator cap was all you needed to fix this thing in the first place, your car's cooling system isn't going to be the only thing that's boiling over!