What issue would you suspect if a pump operates but does not deliver water?

When a pump operates but does not deliver water, one of the primary concerns is the presence of an air leak in the suction line. This is because an air leak can disrupt the pump's ability to create the necessary vacuum or suction to draw water into the pump. Even if the pump is functioning (i.e., the motor is running), if there is air entering the system, it can prevent water from being effectively pulled into the pump, leading to a situation where the pump appears to be running, but no water is being delivered.

In contrast, while a worn impeller may lead to decreased efficiency or capacity, it typically would still allow some water to be delivered. An incorrect voltage can affect pump operation, potentially causing it to stall or not start rather than simply running without delivering water. A low oil level could lead to pump damage or failure, but it would not directly cause a working pump to just not deliver water while still operating. Thus, a leak in the suction line is the most direct reason for the pump operation without water delivery.