I believe he's talking about the line between the head (connects next to the upper hose) and the expansion tank.

I have had a problem with it running hot for as long as I've had the car. I was having trouble testing if I had it fixed after my new water pump because there was air trapped. Apparently my 603 doesn't bleed air as well as everybody else's, so I had to drain and refill very very slowly.

After getting all the air out I am still having the same problem. It feels like there's very little water flow when I squeeze the upper hose at any temperature. I'm in the process of replacing my radiator with a Nissen unit now.

For anyone curious, the current Nissens (build date 2015) seem to not have metal reinforced necks, and there is a plastic tab on each side at the top that needs to be trimmed off/bent for the radiator to fit. The Behr unit I pulled out doesn't seem to have either of those issues.

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