Overheating Outboards

[thatone123]

Yesterday after I started a late model OMC 15 2 stroke circa 90's or early 2000.s I had the motor completely overheat after a 1/2 mile run. Nursed it back to shore, with little or no water coming out of the tell-tale or exhaust. Engine VERY hot. Then it started pumping again after throwing much very cold water on the engine and got me back fine another 5 miles. Inspection at home seems that it was run dry at one point by previous owner as the impeller was melted on the outside of the vanes and a general mess. Or it got clogged with mud and weeds and/or ice. Very surprising it got me home.

I attribute that it seems not to have affected the motor, due to synthetic oil as this same problem happened with another outboard a few years ago and it was fine after that. I might be wrong?

In general what kind of damage can one expect in such a scenario as the motor seems to run fine (for now). It was not overheated to the point of peeling the paint off the head area, but very hot and steaming bad. You could fry the proverbial egg on the motor. I don't believe these outboards (late OMC 15's) have a overheat shut down feature, but it just stopped running. Maybe I was lucky it shut down and avoided further damage as I know rings, pistons and etc. can get affected in such a scenario.

 

[Scott Danforth]

You most likely scored the pistons and probably blued the crank.

Do a compression check

 

[roscoe]

It shut down because it got so hot, that piston/rings expanded to the point that it could not move, seized, all the lubricating oil was burned, and any residual oil in the crankcase is probably no longer protecting any surfaces.

It may run ok for years, but damage was done, and performance will diminish.

 

[82rude]

Happened to me in the 1990,s with a 57 18hp Johnson also.Exact same scenario except at the end mine was seized solid.Freed the pistons with a hardwood dowl and it was used for another 10 or so years.Hope you get lucky and get more years out of it.

 

[Sea Rider]

In such a case will need to know why OB overheated badly, if impeller was melted inside water pump liner shurely OB picked anything at lower leg which you were not aware of. It's good practice from time to time to look back and check peeing port is working, but if at speed temp will rise abruptly when water pump runs dry.

In wanting that OB to work healthy again, replace impeller, thermostat, plugs and torque head gasket back to specs for mating surfaces to seal properly.

This OB is used on salt or fresh water ? Would you say that has low or high hours use ? Did powerhead smoked heavily, fumes where seen when overheated ?

Happy Boating

 

[444]

I agree with all the above comments. This is why you always read posts from guys saying to change the impeller and thermostats on any engine after you buy it. If it ran enough to get you home, it might still be salvageable. BTW, I wouldn't credit any of that to synthetic oil vs conventional.

 

[Lightwin 3]

I agree withthe others but how hot is hot?

I'v e been lucky in the past and just popped a couple head gaskets. Both instances were because of debris in the water. By changing them I was able to inspect the bores. Good. However, I replaced impellers too.

I was lucky.

 

[Sea Rider]

On those OB's you can have an overheat due to extreme salt building inside water passages along thermo failure independently if lower leg caught some debris when at wot and fried the impeller inside the water pump liner. Those thermos are Jurassic ones compared to simple modern one piece thermos found in newer OB's.

Have had 3 extreme overheats while boating with a 15 HP Evi OB to the point of having seized the pistons in the cylinders, on all 3 occasions managed to fully recuperate the wounded OB by changing impeller, thermo, head gasket and flatten cylinder head at a machine shop.

Happy Boating

 

[thatone123]

Thanks for all the good info!!! It got hot enough to shut down, but it did not take long on the starter cord to turn it over again. I was in a rush to get out of there as there was an approaching storm and gunned it in more shallow water than usual. I'd say it has average use whatever that means, but seems to have high compression 110 both. I think it was run in salt also. Fumes? I guess as there was a lot of steam if that is what you mean. I could smell not a good smell like slightly cooked plastic or some such. It ran wide open almost back to the landing for 5 miles. I could not believe it pumped good water all the way back on a melted and very set and brittle impeller. I will check out the thermos and head gasket.

 

[thatone123]

Checked the thermos out and there is quite a bit of salt residue in there, but I have seen lots worse. Plugs have become a pewter color instead of bright shiny from the heat. Same compression of 90 psi on 3 pulls and 110 on 4. I am thinking/hoping this old tough bird came through alright. The head bolts are tight not loose.

 

[jimmbo]

That pewter colour may be Aluminum from the pistons. You said the engine did slow to a stop, that would imply the pistons had expanded too much to continue moving. Best thing to do would be to pull the head, the transfer port and exhaust covers to inspect the bores and piston skirts for scuffing/scoring.
I doubt throwing water on the engine cooled the cylinders down much, as the water jackets when empty don't/can't conduct heat very well. The impeller may not have melted until yesterday. A few seconds of being deprived of water at high speed will melt them. Plastic bags are notorious for blocking water intakes.

 

[Sea Rider]

Yep better remove entire powerhead and pull exhaust cover, cylinder head, thermos, squirt pure vinegar on all water paths, let penetrate on salt layers, crust. Clean all water passages mechanicall scrapping them with a wooden sushi stick, take advantage to decarbonize pison heads and internal head, ckeck cylinder cond. Will need to buy a new impeller, plugs and different gaskets such as : powerhead base, thermo, cylinder head, exhaust cover, torque to specs.

Anything such as a banana peel, algae, piece of newspaper can clog lower wates intakes and fry any OB in a very short time period while running wot. If with no issues on cylinders, OB will work again as a champ. Retorque all gaskets soon after first 10 worked hours.

Happy Boating