overheating when stopped but cool when moving

[lammbn]

I have a strange overheating problem that just started suddenly on my 2000 20i with a 350 competition ski engine. I noticed today when pulling a boarder that water temp was at 100 degrees, way lower than its ever been. it usually stays steady right at 185 and doesn't move. when i came to a stop. the temp skyrocketed right to 250 and stayed there. we stopped and checked everything out and could hear water boiling in the block. we got the boat back up to speed and when we hit plane the temps dropped right back to 100 until we stopped again. we immediately got off the lake and came home. this sounds like an impeller to me but the thing that has me thinking it could be something else is that i made the dealer i bought the boat from this spring change the impeller before i bought it. would a new impeller go out in about 80hrs? could this just be a thermostat? it changes temp very quickly when i hit plane which makes me think the pickup is forcing water into the engine. any help is greatly appreciated!

[chpthril]

Is it a closed cooling engine? Sudden temp swings are just not feasible. It takes time for a huge hunk iron to heat up or cool down. I would suspect a temp gauge or sender to be the cause of rapid temp changes, if it wasnt for the sounds of water boiling. That only happens when the actual water temp increases above 212F un-pressurized.

[lammbn]

the sender isn't measuring the temp of the block though. it is reading the temp of the cooling water. so if the water wasnt moving in the block it would get hot really quick and the thermostat would be open because of the temps. it seems that as soon as i get up to speed the water is being forced into the motor and thus the large drop in temps as the cold water is forced past the open thermostat and onto the temp sender. I work at an auto repair shop but the cooling system on these boats is a little different than a car i know.

if anybody knows, why do we have a separate impeller on these boats and a water pump on the front of the block like a normal chevy 350 as well? do they serve different purposes? is the impeller just a lift pump of sorts?

I think im going to just go out and pull my impeller housing to check it as a first step.

[chpthril]

Is your boat engine closed or open cooling? The temp of the water in the block is whats referred to as "engine temp" and is related to the actual temp of the of the iron block. heat transfer from the iron block to the water passing through the block, is how the engine is cooled. So their temps are related.

The water pump on the front of the block, as the same as a truck motor, is called the circulation pump, on a boat motor. This pump circulates the water through the engine. The impeller pump housing is what draws fresh water from the lake, to the circulation pump on an oped cooling engine. If your motor is a closed cooling engine, the block is filled with normal antifreeze like a car system and the raw water impeller feeds the water exchanger. This act as radiator in a car. If your engine is open cooling, then its 100% cooled by lake water.

[lammbn]

oh. then i am not sure what i have. i just looked at my engine for a few minutes but had to get the cover on quickly because of a big rain storm that is starting. I can explain part of my system and hopefully it will help identify what i have.

The water pickup in the bottom of the hull goes to the impeller housing. from the impeller housing outlet it runs up to a distribution block of sorts mounted on top of my intake manifold. that block has four hoses going into it, two large hoses and two small hoses. it also seems to have the thermostat mounted underneath it and the temp sender mounted in the rear portion of it. the two smaller hoses go to the exhaust manifolds and i cannot recall where the second large hose goes to. that is all i can remember of my 2min of looking at it. hope that singles it out enough.

[kx250frider617]

Maybe your impeller blew and doesn't suck water at slow speeds, then at high speeds, water is forced through the impeller from the water pressure. I would start at the impeller first, because it the easiest to change and because its the cheapest.

[lammbn]

i agree. Now i know that there are two different impellers and most people figure out which one they need by the engine serial number which is usually located on the top of the flame arrestor. my serial number seems to be missing from there. but it is a 2000 5.7L competition ski mercruiser. any idea on how to figure out which one i need without the serial number? maybe buy one of each and once i figure out what one i need return the other and get a spare of the right one? I want to order one today if there isnt one in town.

[lammbn]

just wanted to update so there was some closure to all of this. the impeller was shredded. almost completely gone. I recovered almost 90% of the pieces from the screen in the transmission cooler. I installed the new impeller and did a test run tonight. everything ran flawlessly and I couldn't be happier! glad that's over!