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150 Pro V Overheating / Alarm Sounds and engine cuts off PLEASE HELP.

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Bigfish92084

New member
This has been happening for awhile and it seems only to happen when going 4500 rpm or higher.
I can run at 3500 and alarm never sounds. I just spent $1000 dollars at the dealership where they put in a new impeller, water pump & 2 new thermostats.
Still happening. I shot video this time and noticed the psi gauge is definitely loosing pressure when up on plane. I took heat readings from each cylinder side and noticed the port side was 10 degrees warmer. The motor was peeing out the side just fine. In my research on this forum I stumbled upon this previous post. I have PM the original post member hoping he can elaborate on this fix. The post was done 5 years ago so I am reaching out to anyone else who would be willing to help. My last trip out, even at 3500 rpm the alarm went off but took awhile. When I do WOT, it alarms within a minute.

I'm going nuts trying to figure this out and the so-called $100 per hour experts are not able to figure it out.

"I have recently struck a consession of Yamaha outboards with similar cocerns to yours. After replacing t/stat and water pumps, i found in all 4 cases, yes 4, that the water tube from the pump to the lower end of the power head, was corroded where is has the rubber grommet around it. It is nessesary to remove the g/box and lower leg to replace this pipe, but it is not a difficult nor time consuming repair.
Hope this may help."

I need more specifics on how to do this "remove the g/box and lower leg to replace this pipe".

Does the lower unit need to drop?
Is the g/box the black cover on the front side of motor?
What is the "lower leg"?
 
I had a similar problem on my 1999 Sx150TXRX. Once I hit around 40 mph the alarm would sound. Water pump and thermostats were all good, engine was mounted correctly also.

My motor is a saltwater motor and I bought it in Florida where it was used often.

The problem was; the water passages had a lot of saltwater buildup, scaly so to speak. I used vinegar and water, I sprayed it, let it soak a bit, then scraped it off with a scraper and wire brush. Fixed the problem!

Take the outer cover (cooling covers) off your cylinder heads and check for buildup. You will need new gaskets for the covers.
 
Bypass or pop-it is between the cylinder heads on the back of the engine on the starboard side down low. Two 10mm bolts and a hose connect to the cover. You will see it.
 
Your problem is identical to the problem I had on my SX150 as I commented earlier. Only difference is you say it happens at rpm and I noticed mine by speed.

If the water bypass was malfunctioning (sticking closed) you would have the same issue at lower rpm/speed. The bypass would already be fully open well open around 2500 +/- rpm.

FYI, the bypass works likes this.
If it was stuck open, at lower speed you may run hot/overheat because it is bypassing more water around the cylinders than allowing the cooling water to flow up thru the cylinders.
If was sticking shut at higher rpms, you probably wouldn't notice and most likely wouldn't overheat!
At higher speeds, the bypass opens to relieve excess water pressure in the heads and helps keep the exhaust cooler, again, you probably wouldn't overheat.

In a nutshell, if wp, tstats and bypass are all working fine, no matter what you do, if the cooling passages are scaled up, the water just cannot remove the heat at higher rpms. The scaling prevents it.

Hope this helps you out
 
The scaling sounds like it may be the culprit. Even though the motor has only been in fresh water, the scaling still happens?
So, Trynhooker, did you use a small drill bit to ream out the tiny holes?
Also, on the replacement gaskets... are they dry and you need to use some type of adhesive along with the gaskets?
See photo for torque specs. Is this correct? 5.8 ft.lbs seems awfully light, but if there is sealant involved, makes sense.

Thanks again for your input. I sure hope this solves my overheating issue.
torquemotor_zps9a45a9d5.jpg
 
Until you remove the bypass and inspect it for proper operation...........do not do anything else yet. You can test the other theory afterwards.
 
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If the torque specs you listed are from the manual for your engine I assume they should be correct (don't have book with me). Just make sure you follow the pattern and torque to 2.9 following the order then go back and go to 5.8. Go slow as to not snap a bolt.
I didn't use any drill bits to clean. Use something like a small punch/etc., to clean out the holes. Preferably something like brass that's a softer metal and won't do any damage etc.

Your manual should say whether or not to use a sealant on those gaskets. I use the Johnson/Evinrude stuff in the small can with the brush attached to the cap. It's real sticky/gooey stuff and it helps make a good seal on some gaskets. Can't rem the name off hand lol

NEVER use it on head gaskets however! Your book should show which gaskets use sealants etc.

Something I didn't see in your original post. You say it's been happening a long time. Do you mean since you've had the boat?
If so, tilt the motor down straight down/horizontal, then make sure the cavitation plate on the lower unit is even with the bottom/keel of your boat. There are 2 plates on the lower unit. The one that sticks out the back is the cavitation plate. Or the one right above the trim tab..
As you go faster, the boat rides higher off the water and if the engine is too high, it will cause the water pump inlet to be higher on the surface possibly sucking in air.

And by all means, since your checking these things, check what Docks suggested! His theory may be your issue as well. Everything is a theory until it fixes the problem!

I use this site to view diagrams etc if you can't find one or need to blow the pics up to see better. Just change to your exact engine tho. http://www.boats.net/parts/search/Yamaha/Outboard/1999/SX150TLRX/CYLINDER CRANKCASE 2/parts.html
 
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