Yamaha C90 RPM problem and overheat

[JayP]

I have a 1998 Yamaha C90 (2 cycle) on a 21 foot 1998 Carolina Skiff. If I run it over 4000 RPMs the motor will go into the overheat safety mode - buzzer...shuts down to idle...etc. Seems weird - but it has always done this. I noticed over the past couple of months that the engine just didn't seem to have the power it used to - seemed like the boat was heavy. Last week I took it out and it really seemed to lack power. The RPM guage was showing 4500 RPMs - but I knew it was wrong - typically it sounds like it is struggling at 4500 RPMS - this time it sounded like 3500 RPMs.
After about 30 minutes of running with the RPM guage showing a bad reading of 4500 RPMs - the guage jumped and went down to 3500 RPMs - then immediately the overheat buzzer went off - and the engine dropped to idle. It cooled and I went back to 3500 RPMs back to the dock.

Any Ideas?

 

[robert graham]

Re: Yamaha C90 RPM problem and overheat

Same exact motor as mine. Maybe check/replace water pump impeller, cooling passages, thermostat, temperature switch on head, etc. Regarding your tachometer readings, your boat/motor should run wide open in the 5500RPM range. Have you ever seen 5500RPM on your tach? Sometimes tachs are set wrong, lose wires/connections, just a bad/worn out tach. I replaced mine a couple of years ago with a waterproof model when the 1999 original died. Good luck and let us know of your results.

 

[JayP]

Re: Yamaha C90 RPM problem and overheat

The Motor will definitely open up to 5500 RPMs - I've done it a few times just to get going - but if I keep it above 3500 RPMs it over-heats. I have a steady stream of cooling water coming out of the unit.
Are you thinking the impeller is going bad - and there is not enough water-flow?

I've also noticed when I have a couple of people (big guys) in the boat - I have to give it more power to keep it on plane - and ofcourse the RPMs go higher than 3500 and it over heats.

 

[robert graham]

Re: Yamaha C90 RPM problem and overheat

Has the water pump impeller or the thermostat been looked at or changed lately? Do you run in mostly salt water? The motor should not be overheating and a good idea to find the problem and fix it. Good Luck and let us know what you find.

 

[JayP]

Re: Yamaha C90 RPM problem and overheat

I've had it for about 18 months using it in Fresh water - and haven't done anything except add fuel / oil and grease some fittings.
I'll order a new impeller and thermostat and change them out. Thanks!!!

-Jay

 

[robert graham]

Re: Yamaha C90 RPM problem and overheat

Go to: bogueinletnc.com/yamaha for helpful instructions with photos on several repairs and maintenance procedures including impeller and thermostat replacements. Good Luck!

 

[99yam40]

Re: Yamaha C90 RPM problem and overheat

If you do not know when the water pump and thermostat was replaced last then replace them and check/replace the pressure control valve also.
This is routine maintenance to be preformed according to Yamaha

I have read that if it (PCV)sticks open it will bypass some of the cooling water and not allow the block to fill completely causing hot spots on the upper cylinder.
The thermostat can be tested to see if it is working properly in a pot of water to see what temp it opens, how far it opens, and closing temp.

 

[JayP]

Re: Yamaha C90 RPM problem and overheat

I've got a thermostat, thermostat gasket, a PCV, and a water pump repair kit on order. Hopefully one of these will do the trick-

-Jay

 

[JayP]

Re: Yamaha C90 RPM problem and overheat

There is a small part that I cant find. Its right at the point where the water shoots out the side of the unit. It clips into the housing and then the cooling hose fits onto it. I cant find it on any diagrams. Anyone know where I can find this thing? Oh yea - its actually a '97 model C90-

-Jay

 

[99yam40]

Re: Yamaha C90 RPM problem and overheat

look at this part and see if is what you are looking for.

http://www.boats.net/parts/detail/yamaha/Y-6H3-44391-01-00.html

Merry Christmas

 

[JayP]

Re: Yamaha C90 RPM problem and overheat

Thats the one - Thanks Man!

 

[JayP]

Re: Yamaha C90 RPM problem and overheat

Changed out the Thermostat and the PCV, then rebuilt the water pump - then reinstalled the lower unit. I hooked up the waterhose and cranked the engine - started...but the prop turned and no water came out. So, I dropped the lower unit again, adjusted the gear linkage back to Neutral, pulled the water pump off and found that the impeller had been reversed (bent the wrong direction)...I know I put it on right...so I think it happened when I was adjusting the drive shaft as I was installing the lower unit. Anyhow, I bumped the plastic threaded speedometer hose/nipple fitting while adjusting the gear linkage and snapped it off. Its on order $3-$4.

I've re-installed the LU while I wait for the new speedometer fitting, the gearing is right and there is an excellent stream of water shooting out of the unit. Also, I ran vineager through the unit to break-down any build-up. Not sure it did much, but I'm looking forward to getting back into the lake.

Can I run this w/out the speedometer fitting in the LU? The hose is just hanging-

 

[99yam40]

Re: Yamaha C90 RPM problem and overheat

my guess is it probably will squirt out a little water if you don't plug it off

 

[JayP]

Re: Yamaha C90 RPM problem and overheat

After changing the water pump impeller, thermostat, and PCV - I am having no more problems. The RPMs are maxing out at about 5300. The boat seems to be running better than it ever has and for some reason seems to have more power. Maybe its because I'm not worried about backing it off. I ran it a good 10 minutes at 5300 RPMs - it was beautiful.

 

[msd58]

Re: Yamaha C90 RPM problem and overheat

After changing the water pump impeller, thermostat, and PCV - I am having no more problems. The RPMs are maxing out at about 5300. The boat seems to be running better than it ever has and for some reason seems to have more power. Maybe its because I'm not worried about backing it off. I ran it a good 10 minutes at 5300 RPMs - it was beautiful.

I know this thread is 2 years old BUT I had all the same problems with my C90 and found this thread. I went through this this fall (2012) and replaced t-stat, pop off valve and water-pump. Motor runs fine and cool with no alarm going off over 4200 + RPM's like before HOWEVER last two times I started up boat cold (water temps 47*F plus) after after 5 minutes at an idle the warning beeper sounds. I shut it down for a few minutes and it does not do it the rest of the day. What I am missing here?