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Author Topic:   Yamaha F115 Making Oil

hwoodyone posted 11-02-2009 07:41 AM ET (US)
I recently read on this site about 115-HP Yamaha four-cycle engines getting fuel in the crankcase. I have been running twins for nine years and dang if my port engine just started [getting fuel in the crankcase]. What causes [fuel to get into the crankcase]? How can I fix it? Also, will this cause [engine speed] to gently drop after about a minute at WOT? Thanks for any help.
Ridge Runner posted 11-02-2009 08:40 AM ET (US)
Interesting, I have been running twin Yamaha 115's for five years and both of mine "make some oil", less if run at high RPM's, almost WOT, more when at long idle or trolling. From what I have read it's blow-by fuel getting past the piston rings. Most of these issues seem to be on engines that may not have followed Yamaha's break in procedures, so the rings didn't seat correctly. In an older engine this could be a carbon build-up problem, have you de-carbonized your 115's? Here is a thread that discusses some other folks experiences. http://www.thehulltruth.com/boating-forum/ 59569-2005-yamaha-f115-making-oil.html
hwoodyone posted 11-02-2009 09:52 AM ET (US)
I did use the man. recommended break-in even though my dealer said "just run the hell out of them". I have used SeaFoam several times so I guess the carbon is gone.
I wondered if I might have a gasket problem or worse.
I've been running these since 2000 and this is the first issue other than routine maintenance.
weekendwarrior posted 11-02-2009 10:07 AM ET (US)
The rpm drop might be due to lack of lubrication, I would limit use if it's getting that much gas in the oil.
From other posts I remember 2 things, don't remember which engins these applied to though:
(1) Some engines have the fuel pump drive in such a way that a broken diaphram will allow gas into the crankcase.
(2) A hotter thermostat can eliviate the issue with some engines.
Since yours just started doing it, I would be more inclined to look for a mechanical issue. Is your fuel pump run from a cam lobe or something similar that might allow gas from a damaged pump into the oil?
hwoodyone posted 11-02-2009 11:35 AM ET (US)
Don't know how the fuel pump works. I'll call my mech. in Florida and see what he says.
Thanks, Howard
Tohsgib posted 11-02-2009 12:15 PM ET (US)
Cold thermostat can cause this as well as too much pitch on the prop. After 9 years though I would have it looked at, could be as simple as a stuck injector.
number9 posted 11-02-2009 02:35 PM ET (US)
hwoodyone,
If so inclined you can check the lift/low pressure pump diaphragm for internal leakage per the Yamaha Service Manual. They have a procedure with the pump removed using a Mityvac attached to the inlet side to see if the pump holds/leaks pressure or vacuum. This is from a 2004 manual.
Fuel pump pressure 50 kPa (0.5 kg/cm2, 7.1 psi).
Fuel pump negative pressure 30 kPa (0.3 kg/cm2, 4.3 psi).
Bill
95Outrage17 posted 11-02-2009 10:16 PM ET (US)
The fuel pump was the problem with my 2004 Mercury 90-hp 4-stroke getting fuel in the oil. Essentially the same engine as yours with a shorter stroke I believe and carbs vs EFI. I'd lean toward a fuel pump diaphram with a pin hole in it.
- Chris

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Author	Topic: Yamaha F115 Making Oil
hwoodyone	posted 11-02-2009 07:41 AM ET (US) I recently read on this site about 115-HP Yamaha four-cycle engines getting fuel in the crankcase. I have been running twins for nine years and dang if my port engine just started [getting fuel in the crankcase]. What causes [fuel to get into the crankcase]? How can I fix it? Also, will this cause [engine speed] to gently drop after about a minute at WOT? Thanks for any help.
Ridge Runner	posted 11-02-2009 08:40 AM ET (US) Interesting, I have been running twin Yamaha 115's for five years and both of mine "make some oil", less if run at high RPM's, almost WOT, more when at long idle or trolling. From what I have read it's blow-by fuel getting past the piston rings. Most of these issues seem to be on engines that may not have followed Yamaha's break in procedures, so the rings didn't seat correctly. In an older engine this could be a carbon build-up problem, have you de-carbonized your 115's? Here is a thread that discusses some other folks experiences. http://www.thehulltruth.com/boating-forum/ 59569-2005-yamaha-f115-making-oil.html
hwoodyone	posted 11-02-2009 09:52 AM ET (US) I did use the man. recommended break-in even though my dealer said "just run the hell out of them". I have used SeaFoam several times so I guess the carbon is gone. I wondered if I might have a gasket problem or worse. I've been running these since 2000 and this is the first issue other than routine maintenance.
weekendwarrior	posted 11-02-2009 10:07 AM ET (US) The rpm drop might be due to lack of lubrication, I would limit use if it's getting that much gas in the oil. From other posts I remember 2 things, don't remember which engins these applied to though: (1) Some engines have the fuel pump drive in such a way that a broken diaphram will allow gas into the crankcase. (2) A hotter thermostat can eliviate the issue with some engines. Since yours just started doing it, I would be more inclined to look for a mechanical issue. Is your fuel pump run from a cam lobe or something similar that might allow gas from a damaged pump into the oil?
hwoodyone	posted 11-02-2009 11:35 AM ET (US) Don't know how the fuel pump works. I'll call my mech. in Florida and see what he says. Thanks, Howard
Tohsgib	posted 11-02-2009 12:15 PM ET (US) Cold thermostat can cause this as well as too much pitch on the prop. After 9 years though I would have it looked at, could be as simple as a stuck injector.
number9	posted 11-02-2009 02:35 PM ET (US) hwoodyone, If so inclined you can check the lift/low pressure pump diaphragm for internal leakage per the Yamaha Service Manual. They have a procedure with the pump removed using a Mityvac attached to the inlet side to see if the pump holds/leaks pressure or vacuum. This is from a 2004 manual. Fuel pump pressure 50 kPa (0.5 kg/cm2, 7.1 psi). Fuel pump negative pressure 30 kPa (0.3 kg/cm2, 4.3 psi). Bill
95Outrage17	posted 11-02-2009 10:16 PM ET (US) The fuel pump was the problem with my 2004 Mercury 90-hp 4-stroke getting fuel in the oil. Essentially the same engine as yours with a shorter stroke I believe and carbs vs EFI. I'd lean toward a fuel pump diaphram with a pin hole in it. - Chris