The most common cause of a clogged radiator is old radiator fluid. Fluid that has never been changed, or sat for long periods without the engine running, creates rust and sediment that blocks the radiator fins and hoses. Fortunately, you can repair a clogged radiator easily. All it takes is some running water, a can of radiator flush and a few hours.
Park your car on a level surface. Place a bucket under the radiator drain petcock, typically located on the bottom of the radiator toward the driver's side. Wait for your engine to cool down--you should be able to touch the radiator without being burned before proceeding.
Remove the radiator fill cap and pour in an entire bottle of Gunk C2124 Heavy Duty Radiator Flush into the radiator. Replace the cap.
Start your engine and let it run for 15 minutes to move the C2124 Flush completely through your cooling system. Turn your engine off and wait for it to cool again before proceeding.
Remove your radiator fill cap. Reach down and open the drain petcock by turning it counter clockwise and let your radiator fluid drain into the bucket.
Connect your garden hose to a water supply and put one end in the radiator fill cap. Turn the water on and let it run through your cooling system. When the water coming out of the petcock is perfectly clear and draining smoothly (not too slowly or erratically) your clogged radiator is cleared.
