The third-generation Eclipse was a bit of a change-up over previous models -- more boulevard cruiser than road-and-track bruiser. But, in the process, the Eclipse became a more versatile and highway friendly creature, with all of the amenities and equipment required to go the distance. Radiator leaks aren't especially common with this chassis, but it may fall victim to leaks in other areas.
Radiator Damage from Debris
Radiator leaks aren't especially common on newer cars -- but not because he radiators themselves are so much better. It's because they're fairly well protected on the front side of the vehicle by the air conditioning condenser in front and by the plastic electric cooling fans behind. Even with the Eclipse's radiator opening being as low as it is, stones and debris kicked up into the opening are more likely to damage the AC condenser than the radiator. So, unless the radiator fan motor is somehow shoved forward into the radiator, you don't have much to worry about in terms of damage from debris.
If the Eclipse's radiator has one Achilles heel, it's the plastic radiator reservoir tanks that many came with. Plastic reservoir tanks are nothing new, and they don't usually cause problems -- but it is one more thing that can go wrong. Manufacturers create these tanks by "welding" the plastic together, then join the plastic tanks to the aluminum core. These tanks can crack and leak at the welds, but are just as likely to leak where they meet the aluminum radiator core. Aluminum and plastic expand and contract at different rates, so, some kind of failure isn't unlikely as time goes on.
Most radiator leaks don't happen at the radiator at all, even if that's where you find the water; the vast majority of leaks happen at the hose ends and at the radiator cap. Rubber hoses, after constant subjection to heat, stress, vibration, oxygen and exhaust fumes, will harden over time. The hose conforms to the shape of the radiator outlet and clamp, and shrinks slightly away from the fittings. Mitsubishi coats the nozzles with a layer of sealant before they leave the factory, but subsequent repairs and removals will disturb the seal and make the hoses more prone to leakage.
If the Eclipse's cooling system does have one design flaw -- which the tanks aren't, really -- then it has to do with the upper radiator hose nozzle. The upper cooling system nozzle is made of aluminum, which tends to corrode and pit differently than steel. The aluminum nozzle is part of the reason that Mitsubishi used hose sealant from the factory. So, If you or a previous owner have ever removed the upper hose without sanding the nozzle to rid it of corrosion and old sealant, then applied sealant afterward, you will eventually have problems with it. If you do remove the hose for whatever reason, you know now what to do to prevent problems in the future.