How do you know when the impeller has to be replaced. We have a 2005 4 stroke 25 hp Nissan that we took apart today. The impeller looks good. But how do you know? The issue only a trickle to drops of steam of water comes out of the motor when is running.Almost a sputter to a very week stream. We dropped the lower unit and looked at the impeller. Blew out all lines nothing seems to be caught anywhere. No water is getting to the Thermostat. We took that housing off And ran the motor in water. Not even a drop of water was getting to the thermostat. Now would this be the impeller or something else that would be causing this?

Just by looking at it, it shouldn't have nicks or cuts in it and it should look really bent. Is the key still in it? If not, the shaft could be spinning, but the impeller could be just sitting there doing nothing or barely spinning.

Your intake screens may be plugged.

Is there noticable wear in the impellers backing plate? If so water could be bypassing the impeller.

The vanes on the impeller need to be straight, not curved. What does the pump housing insert look like?

I've never worked on a nissan. If they need to be straight and not curved it's my bad for giving that chunk of bad info. However, I find that interesting. I've never seen a centrifugal pump with straight impeller vanes, and I've worked on an awful lot of them.

Learn something new every day.

Look at a brand new impeller, it has straight vanes. Over time they will take on a curved shaped. Especially from sitting the winter. The impeller itself does not sit exactly in the center of the housing. If it is too curved it will not pump efficiently.

If the impeller were exactly in the center of the housing, it would turn with the drive shaft, spinning water within the housing, but it would not pump water.

However, it is designed with an offset impeller. When the impeller turns, it has a lot of room on one side of the pump, and not much room on the other. As the drive shaft and impeller spin, water enters on the side that has a lot of room, and is pressured out on the side that doesn’t have much room. I rambled a bit there didn't I

When you reassembled the motor did you pay attention to line up correctly the water tube ? Sometimes it gets line dup crooked and breaks or doesn't allow any flow up to the motor.

My bet is on the key, it's been lost, shaft spins but not the impeller, very common issue.