Tweet
Could use some advice. The lower unit was replaced, including new impeller. The motor starts well, runs well, but when it comes back below 1000 RPM, after running, the overheat alarm goes off and the motor will not accelerate above 2500 RPM. The thermostat checked out ok. The poppit valve and spring assembly were replaced but the problem remains. Is there an existing Yamaha Service Bulletin for this problem? Anyone experienced something similar to this with their Yamaha? If so, how did you fix it? Thanks in advance for any advice/assistance.
-
-
on the hose or in the water as if on the hose it isnt getting the flow it wants. -
john,
I would pull the two thermo switches and test them. One is located in each head assy - here is the link to what it looks like and location - item #26:
http://www.boats.net/yamaha/partsyst...mponent=353022
The thermo switch is designed to close at approx
180 degrees f. and re-open at approx 160. I have personally experienced one closing at 130 and giving me a false overheat alarm/rpm reduction.
You can test them in a pan of heated water, thermometer, and ohm-meter - just monitor the two leads for contact closure.
You say "the thermostat" checks out ok - I believe there are two - one for each head just like the thermo switches. They can be tested just like the switches - should be completely open at 140 degrees f. and 0.120 inches.
The symptom you are experiencing is just opposite of what usually happens - overheat at higher rpm.
A water pressure gauge would be very handy at this point to tell you if you are dropping too low at idle - should be minimum five psi.
If a thermostat is sticking open in either head, especially the starboard since it is higher, then that cylinder water jacket might not be filling up all the way to the top at idle pressure and overheating.
Good luck [img]smile.gif[/img] ,
Ken KComment
-
Guys, Just wanted to report back on my overheat alarm problems at low RPM. Thanks so much for all of your online troubleshooting help. The problem was with the small seal on the top of the water pump impeller leaking, thereby not allowing enough water volume at low RPM's. Hence activating the overheat alarm. Replaced the entire water pump impeller and housing and the problem has disappeared. Thanks again and good boatingComment
-
john,
Thanks for the feedback - glad you got it fixed.
Do you mean the small o-ring that fits between the impeller cup and the water pump housing? Not the big o-ring that fits between the housing and the impeller plate, right?
Ken K [img]smile.gif[/img]Comment
-
Ken, actually it was the small one that was apparently causing the problem; however, we wound up replacing the entire empeller and housing assembly kit. Also, we permanently removed the two thermostats which now provides more cooling water and keeps the heads running cooler. Thanks again for your help, Ken.
Regards, JohnDaviComment
-
John,
A word of caution about running your motor w/o thermostats - it is my understanding, and I have not verified this, that when you remove the thermostats you take the chance of not completely filling the water jacket around the uppermost cylinder because of drastically reduced backpressure that the thermostats provide from their physical restriction.
If the upper cylinder (starboard side, since it is the highest) water jacket does not completely fill up, then you can get uneven heating around that piston, and that can lead to cracked rings.
I have verified a drastic drop in water pressure on my C115 when I removed the thermostats - like it was cut in half - from 5 psi at idle to 2, and 20 psi above 4000 to 10 - and this was after I installed washers in place of the thermostats!
So, with nothing in place of the stats, the psi is even less.
Bottom line, if you are going to pull the stats, at least put some washers (stainless preferred) in their place that will give you some back pressure. You want to cut the total area of the stat hole just about in half - this will give you more water flow than with wide open stats, yet will give enough back pressure to completely fill the cylinder water jacket.
Good luck [img]smile.gif[/img] ,
Ken KComment
-
John,
As an additional thought to my previous post:
Some outboard owners who do not want to run thermostats just "gut" them - cut out the spring and thermal coupling and just leave the rubber grommet. This gives you more water flow than an open stat and yet gives enough back pressure to be adequate for filling the cylinder water jacket.
Remember that there is a purpose for the thermostats - to get the motor up to correct operating temp and regulate at that temp. When the other components of the cooling system are doing their job (water pump, pressure relief valve, water jackets), then it is best to use the stats for their intended purpose.
Good luck [img]smile.gif[/img] ,
Ken KComment
-
Ken, thanks so much for your very professional technical advice regarding the thermostats. Your explanation regarding the need for them was right on!! I installed two new thermostats and the motor runs like a "sewing machine"; well, perhaps a rather loud sewing machine. This website has been so very helpful for "shadetree mechanics" such as myself. It is especially helpful to use the information from this site when at the dealership, to be able to ask intelligent questions from the $125/hour marine mechanics who make the repairs on my Yamaha outboards. Again, thanks to everyone who contributed to my "education" in getting my Pro-V and Skeeter back on the water. Fair winds and following seas.
johndaviComment
Comment