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I have a 1993 Mazda B2200 pick up. The problem I’m having is the truck overheats at highway speeds. It idles fine. It does okay, not great, at 40 to 45 mph. If I drive it between 50 to 75 mph, it overheats. I have replaced the thermostat; coolant; fan belt; radiator cap; and engine oil. I also did a back flush of the radiator. What has me baffled is the truck can sit and idle forever and does not overheat. I can drive it at 30 mph and it doesn’t overheat. What would you suggest I look at next and in what order?

Answer:

It sounds like to me you have most everything covered accept the clutch fan itself. I have seen this several times. As it might not be the most common failure part, it does sometimes do just that.

You would be able to tell if it was a head gasket by doing a compression test. All cylinders should be within 20 psi of each other.

The fan has very little movement at idle and doesn’t do much. During acceleration or continued rpms the fan is needed. You would also see a loss in coolant. If your system is loosing coolant then you need to repair the leak before the cooling system will function properly.

Reference link : Overheating Diagnostics

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