2003 350 Magnum MPI Bravo III SeaWater Pump Photos and question

Some background. I just completed major (for me) work - exhaust manifolds, plugs, wires, fluid, filters, etc. Included was a new seawater pump impeller. Pump worked fine before performing any of this work. I read on the forum that the seawater pump was self priming so instead of struggling to get muffs on (pain in butt to get too when boat is on my lift), I lowered it into the water and fired it up. Started right away and idled smoothly with no odd sounds. I started checking for leaks and found none, however, when I peeked at the temp gauge it was not moving. Sitting about 80 deg which was ambient temp. I felt the manifolds and exhaust elbows and temp was rising but not uncomfortable to touch. I then touched the seawater pump and it was hot. So I scrambled out of the engine compartment and shut her down. Not sure of exact time but it was running for over a minute.

I raised the boat out of the water. I now suspected I had destroyed the impeller. Popped open the seawater pump and was pleasantly surprise to see that it appeared OK. (Maybe all that lube I used to get it into the housing gave me a larger margin for error.) Photo of new (left) and one I had replaced (right). Also shot from the side to show they are same height which apparently was/is an issue related to this problem.


Unfortunately, with the boat in the water I could not see if water had actually coming out the drive ports (lesson learned for next time). Remounted the pump and decided to get the muffs on for one more test to see if water emerged. Only ran for 30 seconds. No water. Shut it down. Started process of elimination.


1. Removed pump and inspected impeller and o-ring. All seated properly and in accordance with manual.
2. Ran water through muffs and got good flow out of the pump inlet hose into the bilge; (Ran water back down through the inlet hose to the outdrive - redundant but wanted to get a good flush in both directions). Seems to eliminate the possible "constriction of the plastic fitting inside the outdrive snake neck hose" problem I have read about.
3. Rigged up a garden hose to the pump outlet hose and ran water through the engine. it emerged from the outdrive ports in a few seconds. Good flow so doesn't appear any blockage there.

So seems the pump with new impeller is the problem. (I know I could have installed the old impeller and tried again or connected a clear hose to the pump output and measured flow, if any, but I was beat so decided to try to get more info before taking those steps.)

I took the pump to my parts shop and they mentioned a problem with worn face plates such that when a new impeller is installed, there is sufficient clearance within the wear grooves to prevent the pump from priming and moving water. He showed me two such pumps and said mine appears to have this problem. I asked why it worked with old impeller and not new and he said Mercruiser Techs told them the old impellers were swelling to fill the void but the new one would not.

Here are a couple photos of my pump:


So my questions (finally!)

1. Is what the parts guy said true regarding old vs new impellers and this pump wear issue.
2. Is this pump bad (it's a 2003)
3. Are pump rebuild kits such as this one water_pump_kit worth it or should I just get a new pump.

Small apology for lengthy post. I know I could have just put the pump photos up and asked if it was bad but thought the background might help another amateur such as myself who encounters this issue. Thanks in advance for your help.


Location - St Petersburg, FL
Boat - 2003 Sea Ray 260 Sundancer
Engine - Mercruiser 350 Magnum MPI Bravo III
Engine Serial Number - OM632029

Re: 2003 350 Magnum MPI Bravo III SeaWater Pump Photos and question

Yes the deep grooves will create an issues just as what you went through with only installing a new impeller and not at the very least smoothing down the back cap with water hose connections.

By smoothing all the grooves the best you can should help avoid the need for a new pump this time around.

Yes the rebuild kits do work well and would help save you some $$$ as well.

Yes heavy lube will help save the new impeller. Also running the engine RPM up to 1300 RPM at first start up is recommended and helpful for priming the cooling system. Also submerging the drive in the lake is much more effective then using the muffs.

One other tip for your bravo 3 drive that i have found works for me over the years, Always trim the drive up & down one full time to help purge out any trapped air betwen the drive and impeller housing. I've had many that would not pump water at first, so i would remove the muffs trim the drive back up then down put the muffs on and it immediately would start pumping water for me.

Re: 2003 350 Magnum MPI Bravo III SeaWater Pump Photos and question

Fun Times said:

One other tip for your bravo 3 drive that i have found works...to help purge out any trapped air betwen the drive and impeller housing....it immediately would start pumping water.....

Click to expand...

Just to throw this option out there also to check into if you want, but of course is not as cheap as what Fun Times uses for priming his Bravo water pump:
The PO had this Merc kit installed on my 270 so what I do if my boat hasn't been run in awhile is hook up the garden hose to the fitting and turn on the hose low flow and just watch for water to come out the idle relief ports. Then, remove the garden hose from the fitting, hook the garden hose up to the muffs/flusher and start the engine as normal and water will begin flowing within a couple seconds since water was introduced through the fitting.

Here's a link with a picture of the kit (898235A01) and is inserted into the input hose from the transom to the water pump with the fitting being secured to a bolt from the exhaust riser.

Photo(s) - Mercury Quicksilver 898235A01 FLUSH KIT-MCM 898235A01 - MarineEngine.com

Mark