Need Someone's Help

[howitzer]

The other day we went out to go fishing, both engines worked fine...turned the one engine off for about 3 hours, when we turned it back on, there was no water coming from the exhaust and the engine started to overheat. We have twin 1988 Mercruiser 7.4 litres. Turned out the impeller crapped out on the motor.

We replaced the impeller today, started the motor up and still overheating with no water coming from exhaust. I have a feeling there is a block somewhere in the cooling system either from basic crap build up, or from the impeller's left over pieces im not to sure.

Does anyone here have an idea what could be causing this, and if you think there is a block somewhere, any idea where it most likely will be? I'm thinking it would be best to undo all the water lines and see if there's a block anywhere, just limited work space to do so.

Any help would be greatly appreciated. if you need more info i can reply with it

[Knotlost]

Start by going to the hose where the water comes from the outdrive, and take the time to hook up a garden hose to back flush it toward the outdrive. I assume you have the outdrive off, and have check and are certian the outdrive to the mount is clear and free.... Then see if the flow comes out the outdrive yoke, if it dose flow free then you know what side your plug is on, and go from there.

[NitroMusky]

It is most certainly a piece of impeller. I've heard of coincidences and this isn't one...

As said try to flush it and go from there.

[Paul Czarnecki]

I've seen just one small piece of impeller cause an overheating problem. It takes a lot less than you would think. I've also seen the old plastic wrappers from cigarette packs overheat motors and "cook" impellers. Check everything.

[howitzer]

I'm only 22 and no mechanic but its an inboard motor so I believe its a little different. I'm going to take the hose off the seacock and off the thermo housing, hook a hose up and try and flush that water pipe backwards. The oil and tranny coolers are on the same line so maybe some pieces of the impeller got lodged in the coolers, I don't see any harm with flushing them backwards. Shouldn't do any harm to the new impeller by going backwards either

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[G-Daddy]

Mercruiser service manuals will tell you to backflush your system to make sure you get all the impeller pieces out. Raw water or closed loop cooling for the engine?