The Radiator

The radiator is a heat exchanger — a large flat panel made of thin aluminum tubes and hundreds of tiny cooling fins. Hot coolant flows through the tubes. Outside air passes over the fins. The fins are attached to the tubes and transfer heat from the coolant into the air stream. Think of the fins like a stack of playing cards spread apart — they create maximum surface area for the air to contact. A radiator's cooling capacity depends entirely on airflow through the fins and coolant flow through the tubes. Block either one and the engine overheats.

Most modern radiators have aluminum cores with plastic end tanks crimped onto them with a rubber gasket. The tanks distribute coolant across all the tubes. The core is the flat section with the tubes and fins. Cross-flow radiators have tanks on the left and right with coolant flowing horizontally through the core. Down-flow radiators have tanks on top and bottom with coolant flowing vertically. Cross-flow is more common on modern vehicles because it fits the lower hood lines. The transmission cooler is often built into one of the radiator tanks — a small internal heat exchanger where transmission fluid passes through and dumps heat into the coolant.

External leaks — from cracked plastic tanks, damaged tubes, or corroded core seams. The plastic tanks become brittle with age and heat cycles. A small crack at the tank-to-core crimp joint is one of the most common radiator failures. Even a small coolant leak eventually leads to overheating if not addressed. Internal restriction — sediment, corrosion, and old degraded coolant build up inside the tubes and restrict flow. A radiator that is partially clogged runs hot even though coolant level looks fine. You can identify this with a thermal camera or infrared thermometer — cold spots in the radiator indicate blocked tubes where coolant is not flowing. External blockage — bugs, leaves, road debris, and dirt clog the outside of the fins and reduce airflow. A garden hose from the engine side of the radiator washes debris out — spray from back to front, not front to back.

Pressure test the cooling system with the engine off. Pump it to the cap's rated pressure and watch the gauge. If it drops, look for the leak — wet spots, drips, or seepage at the tank seams and hose connections. With the engine at operating temperature, use an infrared thermometer to scan across the radiator face. The temperature should be relatively even with a gradual drop from inlet to outlet. A cold band across part of the core means those tubes are blocked internally. If the radiator is restricted, flush it or replace it — a partially clogged radiator never gets better on its own.