Changed impeller - now overheats 350MAG Bravo III Closed cooling

I did preventive maintenance to change impeller myself - used non-OEM Sierra to replace previous Sierra impeller that lasted over 100 hours with no problems and no previous overheating. Boat alarmed (high temp) on 1st outing after changing impeller.

Pulled belt and turned pump by hand and impeller sounded free and moved fairly easily. Thought about priming and put boat in water and used my flushing attachment to flood the imput hose and started boat that way. Took long test run including full speed and everything was normal!

Put boat up on lift after the successful test ride and today (a few days later) on 2nd test run boat alarmed once again (happens very quickly), this time saying low water flow but also of course had high temp. I am thinking something to do with fit of impeller leading to priming issue once on lift? Other possibility is some debris plugging downstream however this seems less likely to me because it seems like that problem would have appeared while running on the old impeller.

Any ideas? Am thinking to order OEM impeller and change it out to see if there's a slight difference before I try disconnecting multiple hoses to backflush.

There are a number of possibilities. I think the most likely is a worn housing. This usually manifests itself with a brand new impeller that has not conformed to the sides of the pump housing. It pulls air and therefore loses suction leading to reduced water flow.

Another possibility is your inlet hose clamp is loose, or the inlet line cracked during the flex of installation.

And did your old impeller come out completely intact? If nor go look in the first cooler downstream for the pieces obstructing water flow at that unit.

Old impeller came out intact but there were a few small chips missing but I am assuming that shouldn't be the problem as it ran beautifully up until I changed the impeller. The old impeller also still seemed quite pliable and had about 200 hours on it (just about the interval they suggest). The housing itself is the brass housing it did show a little wear but the engine itself does not have a lot of hours (around 350) which I would think it should last longer than that. The fit seemed to be tight but on the other hand I feel like it was almost too easy to turn by hand when I took the belt off. Maybe I should swap out the impeller 1st in case there are any slight dimensional differences and then if still a problem look at replacing the housing (plastic now??). The inlet hose I think is quite tight as the bilge is bone dry. I didn't flex the hose too much because I took out the pump the 'hard way" by sacrificing the plastic "Y fitting" between the pump and the junction of the inlet hose and the flushing attachment hose, i.e. I cut through the fitting with a recip saw and replaced it ($7 part). Now I learned that splitting the pump (rather than removing the entire pump) might have been a hell of a lot easier.

Another longshot - there are 3 air hoses connected to sea water pump. 2 are vacuum hoses and there was another hose that was larger and not a vacuum hose and went straight up and was pinched off (in other words, did nothing?). The fitting for that hose got dinged during my pump removal so I taped over the fitting since the hose apparently went nowhere. Is there a possibility for air to leech backwards in the pump itself through that fitting or is it sealed off from the rest of the pump?

Following

The BIII pumps of that vintage were known to wear out quickly... like 100 hours. So, 350 is actually a lot. I highly suspect the pump housing is toast. After my boat went through a couple sets under warranty (before 200 hours), I finally had Inconel wear plates custom made. That was about 15 years ago, and they still work perfectly. There are now a number of companies that offer these wear plates for sale. Many years ago, I believe Hardin Marine offered a stainless kit. They cost less than a new pump and will outlast one as well.

It should take a fair amount of effort to turn a well pre-lubed impeller in the housing. Also, when you reassemble the pump, you should have to slightly compress the impeller to get it to close. Oh, and make sure you have sealed it up tight. Air leaks at the pump will cause this problem, too.

keokie said:

The BIII pumps of that vintage were known to wear out quickly... like 100 hours. So, 350 is actually a lot. I highly suspect the pump housing is toast. After my boat went through a couple sets under warranty (before 200 hours), I finally had Inconel wear plates custom made. That was about 15 years ago, and they still work perfectly. There are now a number of companies that offer these wear plates for sale. Many years ago, I believe Hardin Marine offered a stainless kit. They cost less than a new pump and will outlast one as well.

It should take a fair amount of effort to turn a well pre-lubed impeller in the housing. Also, when you reassemble the pump, you should have to slightly compress the impeller to get it to close. Oh, and make sure you have sealed it up tight. Air leaks at the pump will cause this problem, too.

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The impeller is probably too easy to turn and I think I didn't have to compress quite enough to close when re-assembling. I may just go ahead and order a new housing and I am thinking of the Hardin SS housing that people have been recommending -

If you do a google search, you will find mixed reviews on the Hardin Marine pumps. When I had a similar problem on my searay, I contacted Mercury marine for advice. My pumps were brass and slightly scored. Mercury gave me the part numbers based on the engine serial number, a new "smaller impeller", 2 wear plates(Inner and outer), (original factory pump came with just the outer) . Problem immediately went away after the install.
IMO, Its worth a 5 min call to Merc and see what they have for repair kits for your pump

hpk1012 said:

The impeller is probably too easy to turn and I think I didn't have to compress quite enough to close when re-assembling. I may just go ahead and order a new housing and I am thinking of the Hardin SS housing that people have been recommending -

Click to expand...

So to update I ordered the Hardin SS pump housing with their impeller and wear plates. It is installed now and it seems to be doing the trick - temp has been stable at 160F last 3 outings so I think the problem was cavitation (even though the pump housing really didn't have that many hours as noted above). Thanks for all the help -