engine overheats when speed is constant 130kph

[splink48]

The technical way is to take a pressure reading off the system while the car is running and full of coolant. If it reads 16 lbs, the cap is good.

The redneck way is to take off the cap, examine the seal to see how good it looks and make sure its not dryrotted or cracked, then push the metal tab at the bottom to ensure the spring isn't locked up. If you're applying more than 16 lbs of pressure, there's a problem, if its way less than 16 lbs of pressure, theres also a problem.

I doubt the water is leaking out of the cap...you never know, but I sincerely doubt it. It could be evaporating in the passages and seeping out, but you would probably have smoke either out the tailpipe or under the hood while driving. Keep in mind that the water goes somewhere, it doesn't just disappear, so as long as you're losing it, you've got some kind of problem.

[8695Beaters]

Sounds like an air bubble in the system. Refill the radiator and while the car is running, knead the coolant hoses. Leave your radiator cap off while you do this. You should see some bubbles (big ones) come out. Your coolant level will drop down while you are doing this, so make sure you keep it topped up.

Absolutely do not swap out to a plastic radiator. I've never heard of any manufacturer using a plastic radiator, so I hope you're not getting mixed up somewhere.

You must have a leak somewhere in the system. Possibly just a hose with a pinhole. That won't make any smoke, it will just slowly pee the system dry. Start up the car and then check over everything, you might find where the hole is. It probably only happens when the system is under pressure (ie running).

[splink48]

If the radiator has plastic end caps, thats ok...but as Beaters mentioned, not fully plastic, by any way shape or form. I'd second the pin hole theory too. It wouldn't leak when the hoses are cold or don't have any pressure....good idea Beaters!