115 Yamaha Overheats only at high RPM

1992 115HP Yamaha. New impeller kit. Water tube not blocked. Runs OK all day at lower RPM (2000 RPM). When throttling up to about 3000 RPM the tell-tail (pee stream) stops. Thermostats looked OK, Overpressure valve in good condition. I'm stumped. I figured if blockage then low RPM would be a problem too but seems OK with pee-stream flowing. Any ideas?

Re: 115 Yamaha Overheats only at high RPM

Hi DanB, I had a similar problem with my 150 evinrude a couple of years ago. I never lost my tell-tale though. It turned out to be some major blockages in the water passages that blocked cooling water. It only over heated if I got up to around 3800 revs for a few minutes, there just wasn't enough water moving to handle the increased heat. The problem was some bad diverters in the heads and a lot of salt build up in the head covers. After cleaning it all up she ran fine. I don't know about the yami construction, but I'd look for something restricting the water.

Re: 115 Yamaha Overheats only at high RPM

Thanks for the feedback. Local repair shop recommended I try running without the thermostats and see if problem goes away. If that is no the answer he says the problem may be the impeller blades slipping on the bushing which while he says would be unusual for a new impeller it does seem to fit the symptom since at higher RPM the load on the impeller would probably be greater than at low. I'll let you know what happens.

Re: 115 Yamaha Overheats only at high RPM

You never said why you changed the water pump did you have this problem before changeing it?? If not I would look at the inpeller it's very easy for the key to slip out of postion dureing install and start sliping at high rpm. come back with more info or if this was the problem. GOOD LUCK

Re: 115 Yamaha Overheats only at high RPM

Yeah...sorry I forgot to mention why the impeller was changed. I bought a new 18ft Angler with the used 115 a year ago. Soon after a small crack appeared in the lower unit casting behing the prop. Turns out that electrolysis was so bad in the threads of the large nut behing the prop that it expanded the casting a cracked. I drilled a small hole at the end of the crack but it just ran in another direction. I took it apart thinking to weld it but when I saw that the threads were in such bad shape I was forced to trash the casting. A shop in Miami rebuilt a used lower unit with my gears, dogs, etc. The put in a new water-pump kit in the process so that is how I came to have a new pump. Prior to that I did not have the current problem. The motor sat for about 2 months while I screwed with the lower unit so the thermostats could have froze in that time but it is suspicious that both stats could have went bad at once. I verified a new impeller and housing sleeve and while the housing cover was off I verified the key was in place but it never dawned on me to check to see if the rubber was slipping on the drive bushing so if the check with thermostats removed don't fix the problem I'll drop the lower unit again and check this.

Re: 115 Yamaha Overheats only at high RPM

I had roughly the same as you with an older 115 Yamaha. The impeller was fine but the sealing O-ring had a very slight "nick" in it which allowed water to pass at high (3500-4000) RPM resulting in overheat alarm. Replace the O-ring even if it looks OK and Good Luck!

Re: 115 Yamaha Overheats only at high RPM

These are simple systems really. Sucking air (o ring), restriction or bad thermostats are usually the problem. Running without the thermostats is one way to check them but be sure to replace them. New would be best. Running vinegar thrugh to pee tube will break up salt deposits inside the block.

Re: 115 Yamaha Overheats only at high RPM

Simon,<br /><br />U talking about the o-ring at the base of the impeller housing (cover) or the seal at the top of the pump housing where the water tube slides into?<br /><br />HFFFD,<br /><br />How do you get the vinegar through the block. Do you force it backwards through the pee-tube or up through the water tube that slips into the pump housing?

Re: 115 Yamaha Overheats only at high RPM

The O-ring at the base of the water pump housing. <br />I run the vinegar back through the pee tube. If it is REALLY bad you may have to pull the heads and rod it out.