...on my 525 EFI which resulted in a tow. I carry towing insurance through BoatU.S. so the tow cost nothing. I first checked the intake hose which is clear and has no leaks between the drive and the sea strainer. I took the pump apart which is a Hardin marine SS pump, second generation and 2 years old. The impeller looks fine. I suppose that next I'll start looking for an obstruction and backflush the various coolers but I am running a sea strainer. Any other ideas where to look? After letting the engine cool, on startup it would pump 5 psi momentarily and then back to zero with very little water visibly flowing through the sea strainer. Thanks.

strainer lid allowing air in ? how does it seal

Large O-ring and the wing nuts were tight. I will look further into any possible air leaks on the intake side though. Also it will be getting a new impeller while it is apart. (that pressure relief valve and hardware are gone now)

are you picking up water from the nose of the drive ? might check for blockage

No blockage. I stuck a hose tight into the inlet in the strainer and could freely blow through it. I then had a friend block the intake on the drive with his palm and I could not blow through it. I plan to back flush water through it and look for any water leaks in the hose but this doesn't appear to be the problem. I also blew compressed air through the hose. I also suspected an intake side problem but haven't found one.

What about your circulating pump?

Did you check the water hose that connects the bell housing to the gimble housing? They can kink.

I did check the hose visually and by blowing air and then water through it. I don't think suction could pinch it as the drive running through the water should provide plenty of pressure to keep the hose open. I think so anyway. What I did do is use a hose to pressurize the inlet section going through the sea strainer to rule out an O-ring failure or a hole in the hose. Everything seems good. The hose from the strainer to the pump is wide open. I got in there and was able to blow through the pump outlet hose without too much effort and that is blowing through 4 coolers so there is no obstruction. The impeller is two years old and I have heard of instances where a good looking impeller doesn't pump so all I can do now is wait for the new impeller to arrive and put it back together and try it, unless I'm overlooking something. I'll also seal the strainer lid O-ring with grease.

make sure you buy Merc or Q silver impellers not the knockoffs

https://www.ebay.com/itm/33271501529...cAAOSwCR9cnVV6

If I'm following the progression of events correctly, you were on plane when you lost water press. Shutdown and got towed in. After it cooled, you restarted and got 5 psi at idle for a little bit then lost water press again. If that is correct, then maybe you ran over a plastic bag and it wrapped itself around the drive shutting off the water. Bag falls off while being towed in. When bag blocked water flow, it created a very high vacuum within the pump inlet hoses and caused the inner and outer plys to separate from each other within a short section of hose. Now when you restart at idle. The normal vacuum inside the hoses causes the delaminated inner hose ply to collapse and shut off water flow. Outside of hose looks fine. Just a WAG.. Not even sure if the hoses are 2 ply.

I've seen this happen with a vacuum hose once. Drove me nuts for hours.