overheating w/o thermostats

[dorado23]

I'm having problems with my Yamaha 1995 ProV 200hp. I've been running it without thermostats for about the last year (bad idea) and lately the engine has been overheating, alarm goes off and the limiter kicks in. Impeller is new and works fine, water coming out is very cool, engine also feels cool to the touch. Also, when the alarm/limiter kick on I notice a large amount of steam coming out of the exhaust, this slowly dissipates until there is no longer any more steam and that's usually when I can resume running but not long before it all happens again. The fish are biting and I need my boat......any help would be greatly appreciated. Thanks in advance.

[Clownomon]

Hi Everyone From NY!

Hi fellas!

I've just signed up www.yamahaoutboardparts.com and wanna say hi to all the guys/gals of this board!

[pappastratos]

How old is the water pump ? over 1 year ? try replacing it & check inlet for trash.
You might want to test the temp sensor. I have seen the steps outlined here on the forum.

[dorado23]

help

water pump is brand new, it was the first thing I replace. just got new thermostats going to try tomorrow. if doesn't work I'll test the sensors and check the poppet valve.....

[Hollywoodfish]

If the water coming out of the motor is not blistering hot then you are getting a good flow. Sounds like a bad overheat sensor, or worst case a blocked water jacket close to the sensor. let us know. Good Luck

[dorado23]

Could it be the poppet valves?

[99yam40]

PRV will normally cause low RPM over heat from being stuck open and dumping too much water and not letting block to be filled all the way up. higher RPM with a good pump will give enough water flow to not overheat.
running without thermostats will not let the water flow properly though out the motor.

[Groovius]

I had a similar steam problem with my '91 225. Wound up that water was leaking around the exhaust gasket and causing the steaming issue. Also had exactly the same overtemp issues (minus the missing thermostats). Never figured it out, but water pressure was erratic.

[Hollywoodfish]

Groovius, How did you diagnose the leak in the exhaust gasket? One of my motors has intermittent water flow from the outlet that leads to the pressure guarge (top of the block), but the water coming out of the pisser is still cool.

[dorado23]

At this point I've replaced the impeller and the water pump housing , water shoots out at force. I also checked the thermostats and removed the cylinder head cover, cleaned out all the salt and crud (they were pretty clean) and replaced the gaskets. Put it all back together and the engine starts right up and sounds great, boat jumps right up on a plane but unfortunately after running for a couple minutes it over heats and the limiter kicks in???? Everytime it happens I get the steam coming out of the two exhaust ports and as soon as it cools the steam disappears and I can resume normal operation.....but not before it all happens again. The water coming out the pisser is cool to the touch. I'm certain that the steam is related to the problem but I just don't know how????

[130mckee]

The water that comes out of the telltale is always going to be cold. it's raw water. check the water that exits about the prop that's will be steaming hot I'm sure. sounds like your water passages are clogged. pull the heads and you should find the problem. but to me it sounds like your water pump housing is not sealing or is bad. did you only change the impellor or did you change all of the gaskets and housing? at high speeds if the housing is bad or not sealed most of the water will exit the exhaust causing your steam. put therms back in because the motor will not operate to capacity.

[130mckee]

replace the head gaskets and water jacket gaskets. you could have a bad gromet at the base of the block sometimes that fails and the telltale still has pressure but the block still isn't getting enough water. steam sounds like water either getting into a cylinder caused by bad head or water jacket. there are not many more reasons for overheat.

[dorado23]

130mckee, pardon my ignorance, but what do you mean by "bad gromet" ?

[130mckee]

where the water pump tube meets the bottom of the block. I have a mercury that was produced by yamaha and it has a grommet where the water pump tube meet the bottom of the block and it failed and plugged the water passage. I still had alot of pressure at the tell tale but was not enough to keep the block cool at high speeds. but tell tale was pissing like normal. took me about a week to figure out I changed grommet and everything has been excellent since. inspect everything on the cooling system.

[ROSSI]

hey mate i would replace the poppet/PRV it gets stuck from salt water build up and could get jammed stuck or open causing over heating at high rpms