Repeated impeller failures, help

[mikereno1]

We have had 3 impeller failures over the last few seasons. This seemed to start after we had the gimbal ring replaced. We did the impeller twice with new housings and after maybe an hour of use it started overheating again and acting as if the impeller is out again.

We hooked up a clear tube between the inlet and impeller and noticed bubbles coming in from the sea water intake. Would some air getting into the system cause impeller failures? What could cause air getting in?

We have a mercruiser 350 mag with bravo 3 outdrive. 2000 Cobalt 206

 

[tank1949]

20 year old boat? Is it salt water used? Water may be getting past internal seals around tube due to corrosion of metal. Or, you have incorrectly installed it. I'd lean more on corrosion causing issues. I am not real sure on B3s plumbing, but I had an 86 MC that had a similar tube issue. If this is the case, you may be able to JB weld the seal in place (be careful)? However, If my memory serves me correctly, I replaced upper. Lots of new or used fresh water Merc parts out there for sale. B3 availability may be a problem.

 

[tpenfield]

If the intake tube that goes between the bell housing and the transom assembly has a crack in it, then the intake will suck air. So check the intake hose . . . it is on the opposite side as to what is shown in the parts diagram.



Are you sure the impeller actually failed (i.e. disintegrated) or was it just blamed for the overheating?

 

[mikereno1]

I havent pulled the impeller yet this time but the first time it was missing vanes, etc and the second time the housing was worn so it was replaced along with the impeller again.

Can you see the intake tube with boat on water? Not sure where to look

Boat is fresh water only with 500 hours on it.

 

[tank1949]

If the intake tube that goes between the bell housing and the transom assembly has a crack in it, then the intake will suck air. So check the intake hose . . . it is on the opposite side as to what is shown in the parts diagram.

View attachment 344173

Are you sure the impeller actually failed (i.e. disintegrated) or was it just blamed for the overheating?

do B3s use the same water pickup and distribution as B1s? I purchased so-called special kits to eliminate "Brovo-itus" during my rebuild. But, mine also had crank water pumps. The above looks (to me ) identical to B1s.

 

[kenny nunez]

The best thing that people n the Donzi site say to do with a Bravo is to plumb a sea suction through the bottom and with a “Y” connection from the drive joining with the through hull before the pump your problems should be over.
Greco STH-1000. 1” NPT scoop strainer
IBV- 1000. 1” NPT ball valve
You will have to get the various other pieces of pipe fittings and hose to finalize the set up.
The only other thing to try is by back flushing from the pump suction side to see if there is a partial blockage in the drive.

 

[tank1949]

The best thing that people n the Donzi site say to do with a Bravo is to plumb a sea suction through the bottom and with a “Y” connection from the drive joining with the through hull before the pump your problems should be over.
Greco STH-1000. 1” NPT scoop strainer
IBV- 1000. 1” NPT ball valve
You will have to get the various other pieces of pipe fittings and hose to finalize the set up.
The only other thing to try is by back flushing from the pump suction side to see if there is a partial blockage in the drive.

I didn't know that any Bravos had impellers in the ODs. But, I am getting too old to remember....

 

[Scott06]

I didn't know that any Bravos had impellers in the ODs. But, I am getting too old to remember....

No impeller but that is where the water is picked up by the sides of the drive , if there is debris the engine mounted water pump wont be able to suck up the water. people bypass any of these issues plus barvoits by using a thru hull mounted water pickup

 

[tank1949]

No impeller but that is where the water is picked up by the sides of the drive , if there is debris the engine mounted water pump wont be able to suck up the water. people bypass any of these issues plus barvoits by using a thru hull mounted water pickup

Thanks. Original post indicated that he had replaced 3 impellers, then mentioned tube? So where was impeller? There are only 3 places, that I am aware of, that may have an impeller: OD foot, Motor pully or crank pully, but I am no expert. Perhaps he has an Alpha??? Very curious now....

 

[Scott06]

Thanks. Original post indicated that he had replaced 3 impellers, then mentioned tube? So where was impeller? There are only 3 places, that I am aware of, that may have an impeller: OD foot, Motor pully or crank pully, but I am no expert. Perhaps he has an Alpha??? Very curious now....

OP stated Bravo 3 drive in first post. B3 is going to use a belt driven pump unless someone has changed it out. To my knowledge Merc never used a crank driven pump like Volvo, but either design works

 

[tpenfield]

I haven't pulled the impeller yet this time but the first time it was missing vanes, etc and the second time the housing was worn so it was replaced along with the impeller again.

Can you see the intake tube with boat on water? Not sure where to look

Boat is fresh water only with 500 hours on it.

You will probably not see much of the intake tube with the boat in the water, as it is fairly well tucked in along side the main bellows.

The best way to check it . . . or test it . . . IMO . . . would be to run the engine with the boat at rest in the water with the clear intake hose and see if/that there are no bubbles in the water leading to the sea water pump. If not get the boat slightly on plane and see if there are bubbles in the intake hose.

If you see bubbles regardless, then the culprit is probably not the intake hose that connects the bell housing to the transom assembly.

 

[mikereno1]

Impeller is belt driven on underside at front of engine, not in the outdrive.

If there is bubbles only on plane then the intake tube?

We're planning to take it out this week and to a shop next week so hopefully I can use it the following week when I'm back at the lake.

 

[CayugaCobalt]

You will probably not see much of the intake tube with the boat in the water, as it is fairly well tucked in along side the main bellows.

The best way to check it . . . or test it . . . IMO . . . would be to run the engine with the boat at rest in the water with the clear intake hose and see if/that there are no bubbles in the water leading to the sea water pump. If not get the boat slightly on plane and see if there are bubbles in the intake hose.

If you see bubbles regardless, then the culprit is probably not the intake hose that connects the bell housing to the transom assembly.

Would have a diagram of the intake hose connecting to the bell housing? I too am overheating a 7,4L and cannot find or see where it connects at transom.
Mine is gen VI 1997 7.4LX MPI