0.9 or 1.1 bar cap?
 

ive had my engine rebiult a month ago because when it was getting hot the coolant buzzer was going off and when i was stopping somewere the expantion tank was overfilling and leaking.now im still breaking it in its doing the same again.i took it to my mechanic and he told me its my a.s.t cap.i only put a new one 2 weeks ago.my a.s.t had a 0.9bar cap on it he told me it was releasing coolant from the radiator too soon so he put a 1.1bar cap on.please if anyone knows is the 1.1 cap o.k or will it do any damage? one more question ,how many miles do i have to do to break the engine in well?
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New engine break-in period should be around 800 miles (no hard acceleration, no high rpm runs, proper warmup and cool downs)
As for the coolant, I had a similar problem and it turns out to be a kinked return hose from the overflow tank. Check that first, there are quite a few reason for the low coolant warning, worst of which is a bad water seal (but shouldn't be since the engine was just rebuilt). So start from the simplest.