Changed out the impeller and top housing on the Water pump. The steel impeller housing was spinning a little inside the housing is the reason for changing the top housing.

I am getting an alarm now and I know it is directly related to the water pressure as it goes off when my water pressure drops. To keep the water pressure in the correct range I have to trim the motor down. This is a new occurrence as it previously did not occur. No motor height changes have occurred. Also changed the water lines coming out of the T-Stats.

The only constant is at WOT fully tilted it loses pressure dropping to around 8 psi and the alarm sounds, trim down and the pressure comes back up and the alarm stops. At lower speeds it is fairly random at when the pressure drops and I wont always get an alarm.

Questions
1. Can you over tighten the top housing?
2. This unit requires a dam of sealant. I had a time getting the lower unit back in, is it possible that the dam failed?

Could either of the above cause this?

I haven't checked either of these yet, but I can certainly see either 1 or 2 happening. I plan on checking them this weekend, but just wondering if I should be looking at something else, maybe T-Stats or a partially blocked line somewhere?

you need to check the water pump base underneath the stainless steel wear plate it may be warped or partially melted allowing air to get into the water pump housing

Make sure you put it back together with the fins on the impeller going the right direction.

when ya develop any kinda "unknown" over heat problem, first thing is a complete OEM kit.. base, stainless ride-plate, impeller, housing and all new gaskets then go from there.. tha sealant "dam" won't have anything ta do with it...jmo

It's all new OEM kit except the bottom housing. Must have put something on backwards.

Very easy to get wrong. There is a You Tube video showing a how to do waterpumps on Mercs and he puts the waterpump plate gaskets on wrong. Sounds like a housing not sealing right because of having to trim all down to get enough water pressure.

did you put the key back inn

If the water tube seal is damaged or missing it will not pump properly once the pump get above the water line