Fast Idle on '94 C115 Yamaha

[rednspeck]

First of all, I just want to say what a great site this is. Lots of great info. I have a problem that I hope someone can help me out with. Used my boat this weekend and when I started it up the idle was faster than usual approx. 1100 rpm. Whant could cause this. Normally my idle is about 800-900 rmp even when first started on a cold day. Another problem that occurred is that after running about 10 mins @ 5000 rpms the overheat alarm would go off. If I kept the engine around 4500 rpm everything would be ok. I plan on changing out the t-stats and cleaning out poppet valve. My second question is, where is the poppet valve located? I'm thinking that a t-stat is stuck. Could a stuck t-stat make the idle faster than usual?

 

[rodbolt]

Re: Fast Idle on '94 C115 Yamaha

check all your linkages,including the timer base, for binding.
the C115 has no provision for any automatic fast idle.
the overheat at high RPM can be several things from a poor water pump to scale inside the water javkets.
why woulde ya think of a stuck T-stat?
when was the last water pump complete kit installed ?

 

[rednspeck]

Re: Fast Idle on '94 C115 Yamaha

I changed the water pump about a year ago. I get 0 psi at idle and about 18 psi when I first take off, then it levels down to about 11 or 12 psi while running. I noticed that while I have the earmuffs on, the water comming out of the "pee-hole" would very faintly get stronger and weaker. I was going to change out the pump.

I assumed it was a stuck t-stat because the alarm would not go off if I stayed around 4200 rpms. I thought that the t-stat was stuck open enough to cool off the motor @ this speed but at anything faster, not enough water was going through thus the overheat alarm. Thought about scale in the jackets. Was hoping I didn't have to pull the motor apart? Would flushing the motor or pouring salt-x in the t-stat cavity help this? I wanted to service the poppet valve, do you know where it is located?

Thanks again for you help Rodbolt.

 

[rodbolt]

Re: Fast Idle on '94 C115 Yamaha

do the water pump again, use the complete kit and carefully inspect the housing for any signs of melting, any sign of melt scraps the housing, the little rubber dam and holder just under the wear plate aft end needs to be removed and the corrosion cleaned off as well.
replace the t-stats as maint once a year anyway.
the pressure control valve is located on the lower stbd side of the exhaust plate. looks like a mini t-stat housing.
remove the cover and spring, remove the grommet. carefully clean the pocket with a pick.
install a new grommet taking care not to drop it inside the cover.
if ya drop it post before attempting to remove the exhaust cover.
if the old pressure valve gasket is in good shape you can usually wipe some grease on it and reuse it.
corrosion under that rubber dam can lift the wearplate and create overheat issues, usually low water pressure at idle but can also cause them at high speeds.
oressure control valve normally gives more issues at low speed than high speeds.

 

[rednspeck]

Re: Fast Idle on '94 C115 Yamaha

Rodbolt, I changed out the t-stats and cleaned the poppet valve but the alarm still goes off above 4500 RPM's. After I changed out the t-stats and cleaned the poppet valve the water pressure greatly improved. Also there was not much scaling or salt residue in the t-stat or poppet valve housing when I did the work. I am now getting about 18 psi while running at 4500 rpm where as I was only getting 10 psi before the work. Could the overheat sensor be defective thus setting off the alarm? Is this the thermorswitch? I didn't do the water pump yet, and will, but since the water pressure is running good now, do you think it could be the switch.

 

[rednspeck]

Re: Fast Idle on '94 C115 Yamaha

Rodbolt, I need your opinion. After searcing a little more in this forum I tested the thermoswitches by unplugging each of them one at a time and finding out which one was going off, then switching the starboard one with the port one and unplugging them one at a time. After doing this procedure I determined that no matter which sensor I used, the starboard side would always set off the alarm. Knowing this I have ruled out the thermosensors. As I noted in my last post I am getting good water pressure. The tap for my water pressure guage is on the port bank though. I was considering moving it to the starboard bank to see what the water pressure is on the starboard side.

Taking everything I have mentioned into consideration, what do you think is the problem. Do I need to take off the water jackets and clean them? Or do you think it still could be the water pump. Please help I need your expert guidance.

 

[rednspeck]

Re: Fast Idle on '94 C115 Yamaha

Rodbolt, I know you're busy but can you help me on this?!?!?!? Thanks

 

[Ray Neudecker]

Re: Fast Idle on '94 C115 Yamaha

Do a compression test to confirm the head gasket isn't leaking.

 

[rodbolt]

Re: Fast Idle on '94 C115 Yamaha

the head gasket may or may not be leaking abd if it is odds are a compression test wont see it, a leakdown may but still most likly not.
the water passage to the sensor area is a blind hole. scale builds up and falses causes a overheat due to a local hot spot.
my 115 is doing it.
started out last year if I pushed it above 4500, now its down to 3800.
sooner or later I will fix it or sink it .
the only cure I know is to remove the water jackets and head and scrape all the surfaces.
on a saltwater motor its easy if ya say it fast.
but its possible to have a combustion gas leak into the cooling system without a water leak into the cylinder and the symptoms are the same.
good luck.

 

[rednspeck]

Re: Fast Idle on '94 C115 Yamaha

Rodbolt, Thanks for the advice. I guess I'll be taking off the heads and water jackets and be doing some scraping. :|