overheat only over 3000 rpm

Inertia said

The interesting thing about the lower hose sucking shut, - is that as soon as you let the rpms drops from the revs nec to suck it closed, it will immediately open , and stay open till next time revved, then shut.
Cruise at 60 mph, it gets hot, pull over, idle, open hood, inspect for problem, hose is open .
Ask me how I know.

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Hi
thanks for the reply.
No, it wasn't sucking shut.
A long time ago I hadn't the spare to put on and I adapted (on the road) a Ford Transit coolant hose, which has he same bends. Only a little longer.
AS it came out, when is new and helastic it holds the rear bend without problem. Then, after some time ( heating/cooling cycles) it collapses in that bend and reduces the flow.
At least, that's what happened to me....