Running hot at Idle & Low Speeds - 90hp 2 stroke Yamaha

[xpressflats]

I just had the waterpump/impeller replaced. I also had them clean out the water jackets(?) and head(?). It still has poor water pressure (weak pee) at low speeds and idle. I am completely new to boating and are trying to figure this out.

They performed a compression test and all looked good. They also said they removed the PRV and Thermostat and left it out. What are you thoughts on leaving out the thermostat and PRV?

The mechanic said one of the top cylinders was running 40 degree's hotter then the others. Could it be lean?

Did the mechanic miss something?

I have a Bob's Machine shop big foot nose cone on the lower unit. It seems to be leaking a little where the lower unit bolts connect. Check out this video this video link, http://www.youtube.com/watch?v=FV5xS_EjGd8 you can barely see it where the grey & black match up, but it is there. Is there a gasket that could be bad there? Could this be causing low water pressure?

What really confuses me is that the day before, while flushing, the p water was cool for 10-15 minutes! Then the very next day (when i took video) it was running to hot to touch and the alarm was sounding. I ran the boat in open water that day, so i really double anything got caught up in there.

Please help! Im going insane!

[99yam40]

You need to find a real mechanic.

No one in their right mind would pull the stat and PRV and leave them out.
They are in the motor to make sure water flows to all parts of the motor properly at all RPM, without them in place you will have hot spots and cold spots. Without the PRV the pump will not fill motor at low RPMs and the top cylinder will not have water around it for cooling

[xpressflats]

Here is another link to the motor running at close to full throttle. My guess is it is 40-45mph, ill take my gps with me next time.

http://www.youtube.com/watch?v=r691XMZ0124

[sutor623]

You need to find a real mechanic.

No one in their right mind would pull the stat and PRV and leave them out.
They are in the motor to make sure water flows to all parts of the motor properly at all RPM, without them in place you will have hot spots and cold spots. Without the PRV the pump will not fill motor at low RPMs and the top cylinder will not have water around it for cooling

Agreed. Id tell that mechanic to replace those "parts on the house". Dont let him CONVINCE you otherwise, thermostats are in motors for a reason!

[xpressflats]

thanks for the reply, what about leaving the tstat out? Is that bad as well? I am double checkign with my mechanic to confirm if he took the PRV out and left it out or if he just tested it.

Would this cause low water pressure? Or am i looking at a possible blockage? Or misalignment with the water tube when he reinstalled the lower unit?

Thanks again for your help!

[sutor623]

When you remove the thermostat it prevents the motor from reaching the proper operating temperature, typically at idle and lower speeds. Novice do it yourselfers pull T-stats out because they have gone bad and do not want to wait or spend the money for a new stat.

[xpressflats]

I just double checked and he said he left both out and it gets full flow straight through. But i am seeing such a weak P and it overheats.

This is so frustrating, the boat runs great at speeds, but just overheats while flushing, idleing or sitting at the dock. errrrrr....

all input appreciated.

[sutor623]

I just double checked and he said he left both out and it gets full flow straight through. But i am seeing such a weak P and it overheats.

This is so frustrating, the boat runs great at speeds, but just overheats while flushing, idleing or sitting at the dock. errrrrr....

all input appreciated.

Dude, just tell him to put new ones back in it. Thats the way it should be. If he cant fix the issue with the engine properly assembled, then you need to find a new mechanic.

[xpressflats]

Agreed, i will do that for sure. Any other idea's that be causing such low water pressure? I am just trying to gather as much information as possible. The shop manual is on the way, so hopefully that will offer some insight as well.

I am pretty well knowledgeable about auto mechanics (built a 450hp high compression 408 stroker), but haven't had much time to spend with the boat motor itself & i know these are two separate animals.

[sutor623]

No one in their right mind would pull the stat and PRV and leave them out.
They are in the motor to make sure water flows to all parts of the motor properly at all RPM, without them in place you will have hot spots and cold spots. Without the PRV the pump will not fill motor at low RPMs and the top cylinder will not have water around it for cooling

Again, here is your answer. See how it does when replaced and the report back.

[xpressflats]

Again, here is your answer. See how it does when replaced and the report back.

I am coming here for answers, not attitude. No need to be condescending to me.

The thermo and PRV were replaced, same problem.

After a little more research im leaning towards a mis aligned water tube? If you view my video you can see water dripping out the sides of the lower unit. If water from the pump isnt going into the water tube, where else would it be going?

I am almost certain the problem i have is lower water pressure caused by some type of blockage (which we checked and could not find) or mis aligned water tube which is next on the check list. I also have a bobs machine show big foot nose cone on it, which may also be a contributing factor.

[Silvertip]

Lets explain how water pressure work. Hook a garden hose up to the spigot on your house and with nothing on the end of the hose you would measure very low water pressure via "T" fitting in the hose. Why? No restriction. So on your engine it no longer has a thermostat or PRV. Water flows zippity quick through the engine and generally not in the paths it should flow. Has the mechanic checked the temp sensor to make sure it isn't faulty? Yes - misaligned water tubes can cause low water pressure but water dripping through gaps in the lower unit is not sign of misalignment or a leak either. Some cavities are designed that way to allow them to drain if water gets in there from any source. As was suggested stop fighting installation of the thermostat and prv. These are not budget buster costs. Replace them as the factory put them there for a reason. It is called proper engine temp and proper water circulation. To get an accurate temperature reading, the mechanic should use temperature sticks or an infrared temp gun to measure the temp at each cylinder. Thermostats when closed allow quick warm up of the engine and control engine temperature at idle to maintain idle quality. At high speed the thermostat is a restriction so the pressure relief valve opens to allow more water through the engine. Therefore and engine that overheats at idle generally has a thermostat stuck closed. One that overheats at WOT has a PRV stuck shut. Either condition may also be caused by restricted water flow somewhere in the engine. Now -- did you or your mechanic think to probe the tell tale line to see if it is simply partially plugged? String trimmer line works for that purpose.

[99yam40]

After a little more research im leaning towards a mis aligned water tube? If you view my video you can see water dripping out the sides of the lower unit. If water from the pump isnt going into the water tube, where else would it be going?

I am almost certain the problem i have is lower water pressure caused by some type of blockage (which we checked and could not find) or mis aligned water tube which is next on the check list. I also have a bobs machine show big foot nose cone on it, which may also be a contributing factor.

Make sure the water pump housing along with the kit pieces was replaced to make sure the pump is sealing properly.
Also there is metal and rubber pieces (dampener i think it is called) near the pump that helps keep exhaust gases from getting into pump, check to make sure it is still in place and not corroded.

Then you maybe looking for corrosion problems leaking water and loosing pressure.

Sorry, but I do not believe PRV opening allows more water to flow though motor at high RPM.
It dumps excess water pressure at high RPMs out the exhaust before running through the motor from what I understand. that is why low RPM overheat happens if it sticks open .
But I have never seen a flow diagram to make sure