The large-diameter plumbing lines in a cooling system, typically made of EPDM rubber reinforced with synthetic fabric cords to withstand high heat and pressure. The upper hose carries hot coolant from the thermostat housing to the radiator, while the lower hose draws cooled fluid back into the water pump. They must absorb engine vibration while maintaining a leak-free seal.
Heard on the job
The upper radiator hose feels spongy and is starting to bulge near the clamp, so we should replace it before it blows.
Insider tip
When inspecting hoses, squeeze them near the clamps when the engine is cool. A good hose feels firm but pliable; a failing hose will feel crunchy, overly soft, or swollen.