Will water still flow if impeller is “bad”? The dreaded plastic bag aftermath

[the man]

Mercruiser Bravo 3 5.0L with raw water cooling and no thru hull intake

so the engine went into “limp mode”. i was not on the boat when this happened. The low IQ gentleman who was in charge decided to keep using the boat. when i got the report of this event, i went to the boat to check it out. the first thing i noticed was the plastic bag attached to the outdrive. i could not remove it in the water as it was sucked into the intake.

on the trailer now, I am able to remove the bag. When starting with muffs, water does flow out the transom assembly. The engine will overheated at idle and also at about 2000rpm. The risers stay cool. I pulled the thermostat housing as it was the easiest thing to look at- no debris and I put it back together with no thermostat. Now it will overheat at idle but comes right down to 160 or less at 2000rpm at first, but after a while will over heat at the higher rpm also.

Now i know that most likely the impeller is toast, but I like to diagnose as much as possible before starting a repair. It seems to me that if the impeller was bad, I would not be seeing this water flow out of the transom, the risers would not stay cool, and I would not see the temp come down so fast at higher rpm with the thermostat out. So that leads me to think it is a blockage of some kind?

i pulled the lower outdrive and there is nothing obvious that i can see there. Getting to the water pump is a huge pain on this boat- I think i have to pull the manifold at a minimum. I hate to do that if that if it is not the issue!

would i see what looks like decent water flow if the impeller was the only issue? Would the risers stay cool? Other comments?

Thanks in advance

 

[tpenfield]

Often, running on the muffs gives enough water pressure from the hose to get water to flow through the engine . . . particularly if the impeller is bad.

Open up the sea water pump and see what you got.

 

[thumpar]

I would pull the impeller and have a look. Its probably toast. You can back flush it from the thermostat to the impeller house and from the impeller house to the drive once things are apart.

 

[San_Diego_SeaRay]

My 1993 Bravo setup has an oil/power steering cooler underneath the port manifold. It's in the intake water circuit right before the circulating pump. If you have one too, you might want to inspect it as well as that is where a lot of stuff collects if it makes it past the sea water pump.