Another F115 idle and stall issue

eroth111

Cadet
Joined
May 15, 2012
Messages
18
Hey guys, been a member for a little while now, been doing lots of reading but this is my first post. Im a marine mechanic by trade, but primarily only work on Mercruiser sterndrive go fast stuff; Fountains, Formulas, Cigarettes etc. (500hp, 500efi, 700sci, so on and so on). So my experience with yamaha outboards is limited to water pumps and oil changes for the most part so bear with me. I have a 2003 F115 that belongs to a family member of mine, that Ive been fighting a run issue on all season and its driving me absolutely insane. It runs great at WOT 6000rpm and all other rpm no issue. However at idle, in or out of gear the thing can barley keep itself alive. Coughs, sputters, and stalls frequently. And has a hesitation or stall when you nail the throttle from idle. So far, SP have been done, all filters including VST have been replaced. Lift pump replaced. Injectors sent out, cleaned and flow tested to be within spec. ISC replaced with genuine yamaha. Cylinder drop test shows all cylinders making power. Fuel pressure runs between 35 and 40psi, and will default to 40psi if i take the reference line off the regulator. TPS shows steady increase in resistance through the sweep and 0.732 reference volts at idle. Fuel is good and have run on portable tank with the same issue. The one thing I have noticed is if I unplug the TPS it seems to idle much better. I know some people have had issue with thermostats but the issue seems to be prevalent hot or cold. Used to be a saltwater boat but its in fresh now and nothing appears to be horribly crusty on the engine. Im on the verge of giving up and tossing a match at the thing but Im hoping someone here can help out.
THANK YOU!
 

tommarvin

Ensign
Joined
Nov 22, 2015
Messages
999
Welcome, this is a very interesting post because you know more than all of us and your asking for help.
We have twin 2002 mercury 115hp EFI all EFI are almost the same.
We had this exact same problem with our boat, it sat for a long time,how long is long i dont know.
Ours was simply old fuel we hand pumped it out, dirty fuel system, our injectors were dirty and the filter on the end of the injector was almost plugged .It's on you tube.
If your sure the fuel system is clean from tank to injector tip ,them it could be a wire corroded in a wire connector pull hard to check for corroded wires in connectors.
Last resort, check the error codes, you may need a laptop cable and software to check for errors.,for any bad sensors ,your motor runs it's probably not a sensor.
Put the thermostat in hot water and see if it opens and closes.
After re reading your post, it's the idle air control,and or the throttle position switch,not sure if you will see these on the error codes.
Our TSP costs 25 Bucks why not get one, our idle air control is cheap to,they can plug up.
 

GA_Boater

Honorary Moderator Emeritus
Joined
May 24, 2011
Messages
49,038
Welcome, this is a very interesting post because you know more than all of us and your asking for help.
We have twin 2002 mercury 115hp EFI all EFI are almost the same.
We had this exact same problem with our boat, it sat for a long time,how long is long i dont know.
Ours was simply old fuel we hand pumped it out, dirty fuel system, our injectors were dirty and the filter on the end of the injector was almost plugged .It's on you tube.
If your sure the fuel system is clean from tank to injector tip ,them it could be a wire corroded in a wire connector pull hard to check for corroded wires in connectors.
Last resort, check the error codes, you may need a laptop cable and software to check for errors.,for any bad sensors ,your motor runs it's probably not a sensor.
Put the thermostat in hot water and see if it opens and closes.
After re reading your post, it's the idle air control,and or the throttle position switch,not sure if you will see these on the error codes.
Our TSP costs 25 Bucks why not get one, our idle air control is cheap to,they can plug up.

You can not say Mercs and Yamahas are virtually the same. Please only offer advice if you know what you're talking about.
 
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