Over-heating outboard

[MoodySabre]

I have a Johnson 3.5. A few years old with low usage. It has always run fine.

Yesterday I used on a canal and after about 20 minutes or so at half throttle (the longest I have ever run it) it was smelling hot and I stopped it quickly and a bit of smoke can out from the cover. I guess this was just dirt burning off a very hot engine.

I presume this is blocked waterways. When I ran it in a tub at the start of the season there was a very gentle flow but not a very strong stream.

Is this an easy job - nothing in the user manual.

TIA

 

[dom]

First check for any obvious blockages in the water intake. When you say a "gentle flow" I presume you mean from the pee-hole; poke a cocktail stick or something into it to confirm the low flow.

If the flow recovers that will unfortunately take you down the road of a head strip down to sort/clear whatever blockage/problem may exist.

If the flow remains weak, you should replace your impeller (not a difficult job but contains a few fiddly bits which need to be handled carefully).

Good luck!

 

[Searush]

There is also the possibility of calcite build up reducing the size of the water passages, but I would be more inclined to expect that on an older engine.

 

[VicS]

I have a Johnson 3.5. A few years old with low usage. It has always run fine.

Yesterday I used on a canal and after about 20 minutes or so at half throttle (the longest I have ever run it) it was smelling hot and I stopped it quickly and a bit of smoke can out from the cover. I guess this was just dirt burning off a very hot engine.

I presume this is blocked waterways. When I ran it in a tub at the start of the season there was a very gentle flow but not a very strong stream.

Is this an easy job - nothing in the user manual.

TIA

No rocket science involved

The Johnson 3.5 looks like a bit of a hybrid consisting of a Tohatsu power head on an older style Johnson leg and gear box with neutral and forward gears

Lakesailor is probably the expert and he has lots of pictures of work he has done on both

An important difference is the fact that the 100% Tohatsu engine has a fixed drive with the water pump located behind the prop with the impeller driven by the prop shaft.

The Johnson has forward gears and a neutral. The water pump is therefore mounted on top of the gear box with the impeller driven by the vertical drive shaft.
This means of course to access the water pump the whole gearcase has to be removed.

With the small amount of use there is a good chance that the engine cooling water pathways are not blocked and that all that is needed is a new pump impeller esp if the flow from the telltale was not strong at the beginning of the season as Searush has said.

Id start with the pump impeller

It might be worth doing a compression test. If you have damaged pistons, rings or bore and the compression is low it might not be worth doing anything with it.

 

[Jamesuk]

Reading your query I related to a problem I had last season.

Same symptoms it was a screwed up impeller. It sings now and squirts out a jet of water from the tail

 

[robmcg]

I had an overheating 2.2 Mercury. The impeller was intact but the ends had gone thin and 'floppy' with wear, thus not pushing much water around at low revs. New impeller and hey presto it's OK again. Worth a precautionary change?