How can I tell if my radiator is bad?

Coolant coming out clean is a good sign.

How to spot a bad radiator

A radiator is a device consisting of tubes through which coolant flows and fins connecting the tubes. The fins increase the surface area of the radiator (dramatically), increasing cooling efficiency.

The surface of the radiator should look uniform. If there is an area where the fins are rusty, different color, or missing, that area is not cooling effectively. Here is an example of a radiator in poor condition:

If you touch the fins with your finger, they should not crumble. If they crumble the radiator should be replaced.

The air must be able to go through the radiator. If the fins are bent/smashed so that they are vertical rather than horizontal, they are preventing air from flowing through. If the fins are in good shape otherwise they can be carefully straightened.

Overheating diagnostic

Besides what you already replaced, consider the following:

• Are you sure the car is overheating? Many if not most cars do not indicate actual temperature on their temperature gauges. Simply having the needle on the right does not necessarily mean the car is operating out of specifications.

• Is the radiator opening unobstructed, such that air can get to the radiator in the first place?

• Some cars have a plastic undertray that serves two functions: it protects the engine bay from water splashes and it guides the air through the radiator. If your car doesn't have such an undertray/air guides, find out if they are supposed to be there and are missing. Here is a picture of an undertray:

• Is the car overheating when it is moving or standing still? A moving car is cooled by ambient air going through the radiator due to vehicle movement. A stationary car is cooled by the cooling fan behind the radiator drawing the air through the radiator. If the car is overheating while stationary but not when moving, inspect the fan and make sure it is operating.