Hi ManaAdaSystem
Yes it is your right to refuse an aircraft you are not happy with, but when an engineer sign it you have to accept the delay code 65.
Some examples out of AMM(737NG):
...In very cold weather (outside air temperature less than -4 degrees F (-20 degrees C)), leaks from the drain can occur on an engine that does not operate or during an engine start. These leaks are not unusual, and should stop after the engine operates at minimum idle for 5 minutes...
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or:
Aft Sump (Overboard Drain)
NOTE:
Oil traces at the bottom of exhaust plug, the exhaust sleeve and the turbine rear frame is usual. Oil coking at the flame arrestor and on the rear cover is usual.
NOTE:
An oil puddle in the exhaust plug is usual. After engine dry or wet motoring, a large oil puddle in the exhaust plug is usual.
...
Serviceable limit:
1) After an engine shutdown only, there is not more than 60 cc/hr (20 drops per minute).
( after 2,5 hours you have 0,15 liter of oil which can produce a 30 inch diameter pool of oil)
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or ...
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So if I do a test run and there is no leak I sign it.
Seals at the engine are designed to withheld fluids under pressure. If there is no pressure they could leak.
Therefore every task to identify the leak starts with a idle run or even with a part power run.
But I must say, that I didn´t sign off anything blind according AMM. When the AMM says OK but my experience (or should I say feeling) say NO, I decide for safety. I think that every engineer must have backbend* to say NO!
Safety must be the first choice without splitting hairs. So everyone is doing his job.
Ingo
*(I just translate the German wording "Rückrad" witch means that you stay at your opinion even when it is against the policy)