The engine was accidentally left on as we pulled out of lake powell, I immediately jumped in the boat and the temp gauge still read normal. It is a 5 liter, 240 HP Merc and I am wondering what damage this could have caused. What signs do I look for if the impeller in the water pump is shot next time I put the boat in the water. Thanks for the help.

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6 Responses

  1. stein_football says:

    If the temp gauge did not spike then you are most likely fine with the major operations of the engine. But the impeller can break very easily if not getting water. The impeller fits very tightly in there and uses the water for lube while running. If no water then the impeller will easily break off it’s little "fingers" and if it is bad they will get lodged throughout the water cooling system. No water equals no impeller equals no water again equals engine over heating and blowing a gasket. I don’t think it got hot enough to damage the engine itself. Do it yourself and open up the lower unit and look at the impeller. Not hard to change one yourself. If the "fingers" are bent so that it looks like a spiral it needs to be replaced but it was not broken from this incident, just wear and tear. If there are pieces broken or cracked it must be replaced because of this incident. Also try to flush out the cooling system in reverse to dislodge the pieces of broken impeller. Ideally you want teh impeller to have straight "fingers" like a star. If you have any questions go and ask at or if you have a newer outboard. Good Luck

    Also to answer your question of how will ou know if the impeller is shot? IF you take our boat out in the water and turn on the engine and if there is no stream of water coming out about half way up the leg there is a problem, shut it down. You should run the engine at most 5 seconds, you will know by then whether there is a stream of water out the back. If there is then you are OK, if not I hope you have oars.

  2. jimmykillsu says:

    Hmm i am not an expert but i say it might cause some damage to the engine.

  3. umm...Donnie says:

    2 minutes, if it even lasted that long, can cause massive damage to a boat that doesnt have a constant supply of fresh water.

    saltwater boats usea different setup than freshwater. saltwater uses antifreeze and uses the water it sucks up through the outdrive to cool the antifreeze, it would generally take a little while longer to overheat than a freshwater, but still wouldnt take very long. at all

    a freshwater will lock up/blow up/warp heads/just do bad things in a VERY short amount of time.

    running a freshwater without a constant supply of water is the same as running a car with no antifreeze.

    that said, if it didnt overheat before you cut it off theres more than likely no internal damage. however water pumps burn up sometimes even while still in the water, they’re not very partial to abuse and theres a good chance that its shot.

    if you don’t go out that much you’d probably be better off to put a pump in it just to be on the safe side before you go back out. they aren’t hard to do if you have a few basic tools, nor are they very expensive. just replace all the seals every time you do it.

    if you do go out alot, just take it and another boat out with you and keep a close eye on the temperature guage in case it starts overheating. if it still stays normal running temperature then everything should be fine.

    good luck to ya

  4. Shaun T says:

    If the impeller located in the stern drive is not all but completely ruined, I’ll be very surprised.

    Do yourself a favor and change the impeller.

    While you are at it, if you run 5 or 15W40 oil and you want to pick up some VERY noticeable performance as well as some economy, drain the engine oil and replace with 10W30 synthetic such as Mobil 1 or Royal Purple. Use Wix oil filter part number: 51060R(avail at O’Reilly).

    Good luck with the Impeller.

  5. jtexas says:

    The impeller is toast, even if it appears fine at first, the next time out, it’s just a matter of time until it fails. I wouldn’t start that engine at all until it had been replaced.

    It only takes about 45 seconds of dry running to cook an impeller – yours was wet when you pulled up the ramp, but after 2 minutes……well, downside of water pump failure is a blown powerhead.

  6. kenbgray says:

    You could burn out the water pump. Just look for the water to be coming out of the tell tale to make sure you are pumping water.

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