It is frequently observed that fluid is dripping from an engine drain of a parked 737. This leakage occurs mainly when the engine has not operated for some time, for instance after a hangar visit.
In most cases, static fluid leakages come from components such as the Hydro Mechanical Unit (HMU), High Pressure Turbine Air Case Cooling valve (HPTACC) valve or VBV/VSV actuators. Due to the stationary position of an O-ring or a carbon seal on a non rotating engine the fluid can seep through and spills on the ground via one of the drain lines. If one of the valves mentioned above has been tested during maintenance, the chance of static leakage is even greater. In particular Next Generation aircraft are more suspectible to this kind of leakages.
To determine if a leakage is acceptable or not you must consult the tables in the AMM 71-71-00 ‘Engine Vents and Drains Inspection’ task. However, this task also tells to perform a test run for 5 minutes in idle before performing a corrective action, if necessary. If the leakage continues after 5 minutes at idle and the leakage rate is more than the serviceable limit, the source of the leakage must be repaired. But if the leakage stops in less than 5 minutes at idle, no action is required. This is because if the engine starts running the O-rings and carbon seals will settle again, which may stop the leakage.
See these AMM references:
737NG AMM 71-71-00-200-801-F00 2 (A) (2):
Leaks from the drain can occur on an engine that does not operate or during an engine start. These leaks are not unusual, and may stop after the engine operates at minimum idle for five (5) minutes.
And 737NG AMM 71-71-00-200-801-F00 2 (E) (2):
NOTE: Leaks from the drain can occur on an engine that does not operate or during an engine start. These leaks are not abnormal, and may stop after the engine operates at idle power for 5 minutes.