Overheating engine above 8,500 feet.

Dear Car Talk

Dear Car Talk | Dec 01, 2003

Dear Tom and Ray:

I have a 1987 Dodge D50 4x4 truck. The truck's cooling system has a problem of overheating when I get to altitudes over 8,500 feet. The radiator fill cap is rated at 9.5 pounds. It acts like a university student after a frat party. It starts to gurgle and heave from the radiator when the temperature gauge hits the high mark. Do you have any suggestions on how to remedy this problem? And don't tell me to get a Ford or Chevy! -- Scott

TOM: It has trouble once it gets over 8,500 feet? Most vehicles this age have trouble even getting TO 8,500 feet, Scott.

RAY: My brother has trouble getting to 2 feet in the morning.

TOM: Understand that any problem in the cooling system will be magnified at 8,500 feet. Just the work involved in climbing that high is going to make the engine run hot, and that's going to tax all of your cooling components. But the component I'd focus on is the radiator cap.

RAY: You have the wrong radiator cap for this truck, Scott. It's supposed to have a radiator cap rated at 13 psi. Yours is only 9.5. What you're doing is allowing your cooling system to operate at a lower pressure. If you remember your high-school chemistry, you know that when you lower the pressure, you also lower the boiling point.

TOM: So, the lower-pressure cap allows the coolant to boil sooner. And that's exactly what it's doing.

RAY: There could be other things wrong, too, Scott. I assume you've got a hundred bazillion miles on this thing, and at this age the entire radiator could be held together by nothing but the cap. But the wrong radiator cap will definitely cause overheating, so get that changed. I think that'll fix it.

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