1996 Yamaha 40hp c40plru Overheating

[joshl]

Have been chasing an overheat condition last couple trips to the lake. The overheat warning buzzer originally started activating at about 1/2 throttle. Eventually it began to activate at idle. Since then I have replaced the impeller and housing, the thermostat, and what I've read is a poppit valve or pressure control valve and my overheat condition continues. Tell tale seems to be pumping a good stream of water through. At this point I've run out of possible culprits.

Does anyone have any advice on what might be causing it to overheat?

 

[99yam40]

Cooling passages plugged up would be my thought.
Do a web search for Rydlyme Marine descaler to try cleaning passages without having to take motor apart.

Are you sure there is not a blown head gasket or hole letting combustion gasses into the cooling system?
a leakdown test may show this
Did you change the water pump cup and wear plate also? or just the impeller and housing?

I have had some luck with blowing compressed air into water being circulated through the motor to stir things up to help flush sediment out of the motor cooling passages.
But you have to drop lower unit, hook up a water hose to the pipe heading into the power head, remove the stat and PRV.
rig up a way to inject compressed air into the water going in, turn on water,and blast with air off and on to see if anything comes out besides water.
If that does not do any good try the Rydlyme flush

 

[robert graham]

Maybe remove/inspect/clean that temperature sensor...just to be sure if that could be the issue...

 

[joshl]

99yam40,
I tested compression this afternoon. Psi was equal on both cylinders. I know the leak down test is more revealing but that is probably beyond my skill level. I did notice what looked like dried water marks down both sides of the motor at the head gasket. I don't know if that is connected to anything? Is it possible to get even compression on both cylinders but still have a leaking head gasket?

I did change the cup for the water pump but not the wear plate.

Dropping the lower unit is really easy on this motor so I will do that and try to flush it out. I assume that if I free something up, whatever it is should fall out of the exhaust correct? I will also check into the Rydlyme Flush.


I did remove the sensor and cleaned but with no success.


Thank you all for the help!

 

[joshl]

[FONT=&quot]Wanted to bring this post to conclusion in hopes that it may help someone in the future. My overheating issue is solved. This motor is new to me in the last year and the overheat issue continued to get worse over time. It is a freshwater only outboard so I was not convinced that I had clogged passageways in the cooling system but after replacing the normal cooling parts (impeller, stat, etc) I decided to remove the cylinder head. Cylinder head was very clean along with head cover passageways. After finding no issues with the cylinder head, I removed the exhaust cover and found a pretty big piece of rubber (shredded impeller!) in the passage way behind the exhaust cover. The passageways for water flow are smaller than I had imagined and this piece of impelled was blocking the water flow. I also took a coat hanger and pulled another 3 pieces of impeller out of the water cooling tube reaching up from the bottom of the lower unit with the lower removed.

In hindsight I wish I had fished around with the coat hanger in the water tube coming up from the impeller at the beginning. It probably would have saved me some time because I did not need to remove the cylinder head but I learned a lot regardless. At some point the previous owner must have shredded the impeller but I had no idea.[/FONT]