Both engines overheating after new impellers

V

vzljcf

New Member
Mar 1, 2011
5
Michigan
Boat Info
2005 Sundancer 280
Engines
Twin Merc 5.0/Bravo III
We recently had impellers replaced on twin Merc 5.0s. Within 15 minutes into our first run after, both motors overheated within a minute of one another. After research we figured out there were some bad impellers (too small) on the market and the mechanic re-replaced and ran the motors for 25 minutes with no issues. The next day we started it and again both engines overheated within 15 minutes. It feels like water is running through the hoses so clogs seem unlikely, and it also seems unlikely that anything like a blockage, water pump or thermostat would coincidentally go out on both motors at the same time, immediately after replacing the impellers. We are stumped. Anybody have ideas? Please and thank you!!! We want to be floating for the Fourth!!


sorry, boat info is wrong in our profile. Currently have a 2005 Sundancer 280/twin 5.0L/bravo III
 
What did they install? Did the housings get destroyed? Who is your mechanic? I am surprised a skilled boat mechanic would install the wrong impellers.
 
I'll get in line under the "how could a skilled boat mechanic install the wrong impellers?" sign. I'm not a boat mechanic, but as a plain old' DIY'er, I doubt an under-sized impeller would get past me.

I also wouldn't assume that I could "run" an engine for 25 minutes and assume the cooling system is 100%. Temp means nothing until the engine is under a load.

I think it's too early to rule out a clog/blockage. At idle or even at higher revs without a load, something like a broken vane from an old impeller won't be an issue, but could easily cause the overheat you're seeing once you actually run the boat.
 
First, Welcome to CSR! I would look at the lower unit. While I have not worked on a BIII I have worked on quite a few of the others. You need to make sure the water tube was placed correctly when the lower unit was put back on.
 
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BIII pump is on the engine not in the drive.

Any chance the hoses were put on backwards when he put the pump back on (assuming he pulled the pumps to do the job)?
 
If the impellers were broken, and the mechanic did not get all the pieces, then those pieces could be causing a blockage... I would ask to see the old impellers and see if all the vanes are there...
 
The mechanic is a boat mechanic we've known for years. Not throwing anybody under the bus there... Shoulda, coulda, woulda :)

Thanks for the ideas. One impeller was whole, the other had a couple small pieces out but no fully broken vanes and the pieces were retrieved for the most part. That said, we will try flushing just in case, and also check the hoses and belts. We are revisiting this later today, so I'll update later but open to any other suggestions between now and then!

And, thanks for the welcome, Gary Hill!
 
I pretty much have that same boat. Things I have noticed are, it's impossible to get the 2 hoses reversed, Putting a new impeller in an old grooved up brass pump will cause it to run hotter than with the old impeller. How hot did the boat run before this? I would assume you have thru hulls on your boat and they are open? The water line from the B3 is not enough to cool your setup
 
It seems like we have identified the problem. For some reason when we lift the boat out of the water on the hoist an air lock is forming, so suction and water flow don't re-establish the next time we drop it in and start it. That's why it's worked both times for the mechanic and overheats for us on the next time out. All he had to do was pull a plug that let air in and water started pumping through. Ever heard of anything like this?
 
No never heard of any plug pulling. Have him show you where this plug is located and share the info. thanx
 
It's the drain plug that drains the water inlet hose on the top of the water pump. Big air bubble is forming every time we pull the boat out of the water.
 
if that is happening, you might have a bad hose allowing water to leak back out.
 
I keep mine on a lift. Does the same thing every time.. Once you start an engine, goose it a few times and that will get the flow going....check/listen for the water flow. Then do the same for other engine..... First time it happened to me I too had brand new impellars and I called the mechanic right away. First question he asked was do you keep it on a lift?
 
That's really interesting, Mark, sounds like the exact same deal. Thanks for the tips. It's not ideal to have to open the hatch every time we start up, but we are just really relieved it's resolved before the weekend!
 
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