Yamaha 90 2 strokes overheating

[threadfin]

Hi,

I have a small planing catamaran with 2 x 90 Yamaha outboards of 2006 vintage. They are both overheating above 3,800 rpm. I have changed the impellers and thermostats, checked the alarm sensors and they close at the right temp. I'm guessing some restriction somewhere? Boat is used in fresh/brackish water. Tell tale pushes a good stream of water, when running. Thoughts?

Thanks,
Cam

[ausnoelm]

Very common, you will have near blocked water passages, you will need to remove any covers and the head to clean them out, then a good drill to remove the bolts you break pulling it to bits...and before you ask, there is no spray or easy method to do this, lots will tell you to flush with vinegar and salt away, and sometimes it helps, but it is not a "fix" just a short term patch.

[cpostis]

You should check the pressure relief valves Item 38, 39, and 40 in the link page 2006 and Later 90TLR Yamaha Outboard CYLINDER & CRANKCASE Diagram and Parts. If they have either a broken spring or the valves are stuck open (partially or otherwise), you may get the symptoms you are stating. With the valve in the open position, cooling water is bypassed before the block/heads.

I had the same symptoms with a 2001 SX150TXRZ, although the boat is run strictly in salt water. I wound up doing a complete tear down and cleaning of all cooling passages. Probably not needed in your case.
This motor was religiously flushed after each use with flushing port. I since now flush in a tank. Please see my post in the attached thread post for information that may interest you in this link. http://www.yamahaoutboardparts.com/f...g-th26998.html

Good luck, Chuck

[99yam40]

PRV stuck open will cause overheat at low RPMs not high RPMs.

Do a search for rydlyme marine descaler.
I have read some good reviews of it cleaning up outboard cooling systems.

I have cleared up a couple of overheating problems by dropping the lower unit, hooking a hose directly to the water pipe going to the power head , removing the stats, and turning on water full blast while injecting compressed air into the water hose also from time to time.

It moves around some on the sediment in the bottom of the power head and helps flush it out.
this will not help one that has mineral deposits that have grown onto the passages plugging them

[TownsendsFJR1300]

Are you sure its an overheat alarm??

I find it odd, fresh water use, and BOTH overheat at the same time?

Is it possible to be an oil tank, oil pump issue setting off the alarm?

[drumbeater]

I had a bad PRV and it caused an overheat at high RPM, not low. You could idle around all day but the boat overheated after 30 seconds on plane. I would change all those parts and clean out all the junk.

You can also flush out the engine by running it in a tank with 3-4 gallons of vinegar in water. Let it run for a while, shut down and repeat 4-5 times over a few hours. I cleaned alot of corrosion out of my old C40 by doing that. Then of course plan on cleaning the thermostat and PRV again afterwards.

[threadfin]

Thanks for all taking time to reply.

The boat is in Angola, Africa and we are at the start of our inshore fishing season, so I don't want to start pulling heads and covers, with the associated nightmare of shearing bolts etc. I will do that at the end of the season if all else fails.

I can run at 3,500 RPM, but no more.

I'm 99% sure it is an overheat issue, as that is the icon that flashes on the gauge and goes into limp mode when it happens.

They don't alarm at the same time, but if you run one motor up to say 4,000rpm, it will alarm and the same will happen if you do the other side.

I have ordered water pump repair kits, even though the impellers are new, maybe there is excessive wear there? Engines only have 330 hours on them.

PRV valves and springs are free and not stuck or sticking.

I'll try dropping the gearcase and the water hose route see if any crap comes out.

Thanks again for your replies,
Cam

[ausnoelm]

Not likely to be the water pump, it is a common problem, happens almost exactly at 4,000 RPM on the dot, seen it many times.

[TownsendsFJR1300]

If the water pump HOUSING (not included in the kit), and has any warp-age / damage, that'll cause over heating as well...

[donsmarine]

Did you actually feel the top of the powerheads to get a idea of what temp is ?

[threadfin]

I am going out again tomorrow and will see how long I can hold my hand on the powerheads once it starts alarming. I believe it gets pretty hot!!

Could I gain anything by dropping the gearbox and back flushing thru the Flush addapter? Tried flushing it thru with the Tstat and PRV removed, plenty of flow and no real crap came out.

Thanks, folks!

[threadfin]

Ausnoelm,
I've read that under various searches and I guess I'm clutching at straws to avoid doing the tear down. Being here, spares are non existent and everything I need I have to buy in the USA and ship it over. So, break a bolt or whatever and it's a couple of weeks to get it here!!!

[99yam40]

read this thread to see if this Rydlyme is something you would like to try

http://www.yamahaoutboardparts.com/f...g-th19434.html