Overheating question
 

I have an 02 mercruiser mag 350 mpi. A few weeks ago, I had a full service done - impeller included. I took the boat on the water last weekend and ran all day no problems. Yesterday, I launched and by the time I came back from parking the truck the boat was overheating. I shut the engine off, pulled it out of the water, and inspected the water pick up for any blockage. Thinking it was clogged somewhere (the water was full of small plant debris), I relaunched it and it overheated again. Frustrated, I took it to the shop who hooked it up to the hose. Boat ran great, no overheating. The shop thought it was an air pocket (I started it out of the water a couple times for a few seconds).

I took it back on the water immediately after - overheated again! Why would it overheat on the water but not on the hose?? The shop thought the impeller was fried, so I took it home and pulled it off. Impeller looked perfect, no blockage at the inlet or outlet.

My thoughts are that the boat sucked something up. I'm considering sticking a shop vac on the hose where I can. How do you go about unclogging the line? My second thought is a bad thermostat, but wouldn't it overheat on the hose?

Cats? Also check trans cooler line for clogs

Cats?

He doesn't have catalytic converters. Check the small outlet pipe from the water pump to the transmission cooler. Stuff gets stuck there, like chunks of impeller especially. Take off water pump and stick fingers in that hose or peer down it with flash light

Do you have a freshwater strainer in the boat? Are you sure the impeller is on correctly and isn't slipping? To unclog lines you can try to snake it out, if anything is there. If you have doubts about the impeller crank it with the cap to the house off to make sure its working correctly.

Is it a Vdrive, Direct Drive or I/O?

Vdrive

Just to make sure, no kinks in the hose and no obvious leaks?

when you say you "started it out of the water a couple times for a few seconds"...you mean you started it without any water at the impeller? Don't do that at all, ever.

check that you haven't got the hoses switched on the pump.

If it doesn't overheat on the hose, but does in the water it makes me think you have an impeller/pump issue. The hose is making enough pressure to overcome your lack of suction from the pump and keep the engine cool. When you drop it in the water the lake isn't forcing water into the engine like the hose and your not getting enough water flow to cool the engine. If you had a restriction/blockage it would also be blocking the hose water. I think you have a pressure/ flow issue when the hose isn't helping force water into the engine.

You are correct, that if the thermostat was stuck it should also overheat on the hose.

Check the tranny cooler!!

Yep.. check the inlet to the trans cooler.. If the boat ever burned up an impeller and the pieces were not removed, then they are stuck on the inlet screen to the trans cooler. The hose will force the water past it but the impeller has a hard time sucking water past them.. The dealer should know that. Good Luck..

Depends how long they ran it on the hose too... If it didn't get up to operating temp then the thermostat wouldn't have opened and he wouldn't have known...

It sounds like a blockage at the tranny cooler.

Did you use the boat before you had it serviced? Why did you have it serviced? What did the dealer say your old impeller looked like?

I had it serviced because I had just purchased it. Old Impeller was fine. After service, I took it on the water for 8 hours no problems. It's an 02 elite v. Where's the trans cooler located?

was it run in salt water by its previous owner?

Follow the hose from the output of the pump.. it will lead to the trans cooler.. usually on the starboard side of the engine below the head..

Isn't this the repo boat from your buddy? Short sale thread? Maybe your buddy didn't take that good of care of the boat. You should be able to check with him and see if the boat has a history of this.

He's got a v-drive so it will be port side.

Trans cooler is the first thing the water pump pumps water to. There is a short - probably 3-4 inch section of hose that goes to it...

How are you hooking it up to the hose? If it's a fake-a-lake thing, you will be using the same inlet as the lake. However, if there's a flush kit on it, check the lines between the intake on bottom of the boat and the flush kit/fitting. Maybe there's a clog there that you wouldn't hit on a hose?

I did, never had an overheating problem, impeller always changed, etc. Which makes me think a blockage. He offered to bring it to his shop to figure it out.

hose is hooked to the lake intake at the bottom

Thanks wakeworld!! I took off the hose at the transmission cooler and there were 2 broken pieces of impeller, clogged with algae! I talked to the original owner who told me that the impeller fried 3 years ago, they must have been in there all that time. He never used it much, and my guess is that when we launched Friday, the mucky water clogged at the impeller pieces. Nevertheless, I pulled all the gunk out, pulled out all the hoses and shop vac'd all the old water out. For good measure, I pulled the thermostat and tested it, cleaned the housing and reinstalled. Ran fresh water through for a good 15 mins, no overheating issues (yet). Im 99.9% my problem is fixed. Thanks guys!

On a second note, I'm a bit disappointed that the shop didnt pull the hose at the transmission cooler when I did a 100 hour service. I specifically told them I just purchased it, and would like to make sure everything is seaworthy. I also noticed the trans fluid was at the low end of the dipstick, which annoys me since I just paid to have the fluid changed.

Awesome that is all it was. Maybe you learned something about the shop you used. Lesson learned might be to not go back there!!

Lesson learned : you want something done right, you do it yourself... Bizarre how shops are not thorough. Boat shops get away with it with the mindset that is likely that they are exclusive... Good thing for you, Mercruiser dealers are dime a dozen.

And this was a shop recommended by 5 people on wakeworld. I also paid for them to test a fuel pump which they weren't able to diagnose, but I was and replaced myself. Oh well. Reminds me why I do my own work