Water pump problems

[howard21]

I'm having problems keeping my engine cool at idle. I have a 21' V-Drive Howard with a 468 BBC. It has a magnaflow water pump. The impeller blew up on Sunday morning, and now it won't stay cool at idle. I made sure I found all the loose pieces from the impeller, and I disconnected the water hoses and watched the water come out. There doesn't appear to be any obstructions in the lines. While we're under way, the engine is at a normal temperature. However, once we slow down, the temperature comes up quickly. When I replaced the impeller, I put in the new o-rings and I'm confident the pump is priming. Does anyone have any ideas??

[Itsahobby]

Do you have a thermostat? Might have some of the
impeller blocking it when open. Or, the thermostat
spring may have gone bad and not allowing the thermostat to open all the way.
Hobby

[howard21]

there is no thermostat. also, the impeller broke off in three pieces, and i recovered all three.

[83nordic]

Are you sure there is no obstruction in the pickup or pickup hose. I am wondering why your impeller blew up in the first place. Maybe an obstruction water starved the pump and is slowing flow at idle but the pressure underway is enough to quench your engine while moving

[howard21]

thanks for the replies guys.
i'm not sure why the impeller went in the first place. we were staying at echo lodge in parker, and making a ski run in the early morning - so there were a lot of weeds near the surface. i'm thinking one got sucked up the intake, but i couldn't find anything once i tore the pump apart.
i separated out all the lines, including the main feed from the hull bottom to the water pump. everything seems to be clear. i checked by disconnecting a line at a time and firing up the engine. once water starts flowing, i shut the engine off and move on to the next line. as i increase the rpm, the amount of water flow also increases.
i ruled out a priming problem because the pump is able to pump water when idling. i ruled out the obstruction because the boat runs well at high speed, and because i can see water flow when i disconnect a line.
Could a partial block explain this??

[Rexone]

Sounds like you may have partial blockage somewhere that is overcome by pump flow as soon as rpm comes up. Are you SURE you found all the impeller pieces? They like to hide in fittings etc. & oil coolers. You may have sucked up some other debris that went through the impeller and is now stuck somewhere partially blocking flow. Do you know if the idle flow now is less than before (out the dump or something)?

[DUCKY]

I'd backflush each line individually with a garden hose..... Also, check your impeller installation. Some brands of water pump will not self right the blades if you install it incorrectly. Make sure the blades are "bent" in the right direction when you put the impeller in the housing, or the housing on the impeller, however you do it.