Yamaha 140Hp Overheating

[Rapson]

I have hit a brick wall.

My 1987 Yamaha 140Hp 2 stroke is overheating. I can cruise at 3800rpm with no worries, go over 4000 and after about a minute the alarm starts screaming its head off.


I just put a whole new water pump kit in the leg, new impeller, housing seals etc. Put the boat in a Fingle and headed out to the 36's with out the alarm going off. I though you beauty its fixed until it went off comming in and now its back to just how it was.

I have taken off one of the plates around the head and it looks ok, going to take the other off today to see if there is any obstruction in there.

The lower leg that was on this outboard had a leak in the gearbox and had a dodgy weld repair and Garry amd I couldnt get the prop off with sledge hammers so I got a second hand leg and new prop. The only other thing that is different is the spark plugs are not an exact match to the old ones but I got them from the Yamaha dealer.

Has anyone had anything similar to this problem? It has got me buggered!

[jiggin joe]

Check /change thermostat?-sledge hammer not required.

[WET-BOX-MARINE]

t stats and poppet

[axolotl98]

As you said, sounds like an obstruction somewhere in the water flow. In an old Rude 85 v4 with similar issue, when the engine was stripped, it was as if it had been plastered in solid cement (salt). The owner never flushed it, and within a few years the salt from the seawater was built solidly within the engine...

[Rapson]

Took the thermostats out as soon as the overheating issue started, and they were brand new. How can I test the poppets? or can I take them out?

Took the water jackets off and the heads and couldnt finds any obvious blockages, just a bit of muck. Have ordered new gaskets and will put it back together and hopfully go for a run next weekend.

[99yam40]

why don't you just pull all the pistons out, that will keep it from over heating.

No, seriously you need to have properly functional thermostats and pressure relief valve installed on these water cooled motors other wise it can cause hot spots and overheating.

[AvidAngler]

Had the same model and problem and it was due to a non-functional thermostat. Bump for you.So you asked for links to free picture hosts them so I thought I'd post my own Fish pictures gallery
You can check out most on www.google.com

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[Tucker]

Good pics Raps. Like to see those same pics after a flushing with Salt Away. I use that stuff but don't know if it does anything.

[Rapson]

After removing the heads I thnk my problem is a small piece that has corroded off the side off the cylinder wall, which happens to be right where the temp sensor is. Could possibly try toge tit welded but I have concerns that it could start a snowball effect with the remaining wall.

Can anyone recomend another fix such as a metal mend type product? I know it will be a temp fix but I dont expect to get to much more life from this engine.

[Michael j]

Is the bad spot your looking at thru the sleeve into the cylinder?
If not did a big piece fall off that could block water passage?
After you get tit fixed can you send pics??

[Rapson]

Pics of the heads all cleaned up. Dont think I am going to attempt to get the chip welded as I think the head liner will just start to disintergrate. Hopefully a new head gasket should do the trick.

[Michael j]

I agree, I wouldn't weld that. Put a straight edge across the block & make
Sure the cylinder sleeve is not low at the bad spot. You could wire wheel
The aluminum & marine Tex it to slow down the corrosion? May not help
At this point.
Some gaskets say use no sealer but in this case it may be a good idea?
What is in that bottom spark plug?

[Rapson]

The stuff in the bottom spark plug is just from cleaning, dont worry new plugs will be part of the reconstruction and I was thinking along with you and using sealer when putting it back together, hope this works!

[Rapson]

Update on the overheating issue.
Todate I have replaced the water pump (new housing, impeller, seals the works), new thermostats, taken off water jackets and cleaned, taken off heads and cleaned and replaced with new head gaskets, tested thermo switch (both trigger at 85c) and tested the warning buzzer circut by the book (get engine to over 2500rpm, disconnect thermo switches and connect the wires, buzzer sounds, engine drops to below 2000rpm, disconnect thermo switch wires and buzzer goes off.
After all that I go out the other day, cruising at just over 4000rpm and the buzzer goes off. I pull off the cowling and find that I can comfortably put my hand on the heads for atleast 5 seconds with out being burnt.

Can anyone shed some light on this or do I have a new anchor?

[axolotl98]

Bugger...
My next move would be to scrap the thermo-switches for new ones. You mentioned you tested them OK, but just to be sure that the electronics do not play ****s with us folks...
There are 2 heat-sensitive crayons you can paint the head with, just to be sure that your hands are not too insensitive to heat.