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Repeated loss of coolant issue

I have a 2005 Ford focus. I haven’t even had it 8,000 miles. THREE times it has overheated, and I’ve had to replace over a gallon of coolant. The first two times, the cause was a leaky ‘vacuum’ cap. The third time it was a different cap that was leaking. This radiator has two (that I know of) unused ports on it. A 3/8" one on the top left, and a 5/8" one on the bottom left. They are capped off with little rubber vacuum caps, which are quite simple to replace. But, these things don’t last long before they start to weather and crack and leak. I just replaced them after 2,000 miles (before they started to leak), and they’re already cracked. But oddly, only on the OUTSIDE. Inside they’re still smooth and unbroken.

I’ve asked for advice at auto parts stores, mechanics, and even at a radiator shop. They all say they’ve never heard anything like this before?!

I just got some silicone vacuum caps which are specifically rated for high temps, but they’re so thin I’m not sure I wanna install them.

Anyone ever hear of a problem like this?

Does your Focus have a manual or automatic transmission?

Tester

It’s automatic. Also, the radiator is plastic, so not possible to crimp or weld shut.

My Toyota has a cap on an unused coolant port that was designed to be used on left-hand drive cars. Toyota uses a cap that is approximately 5/8" ID. These are not vacuum caps, but made similar to radiator hoses, complete with reinforcing and high-temp rubber compounds. When I had to replace mine, I special ordered mine from the Toyota parts counter. I suggest you go to a Ford Dealership’s parts counter for the factory Ford caps. Vacuum caps will not do the job, as you have determined.