400 RPM, loss and gain

[savanah]

Hey everybody, I have a 2003 SWS Yamaha 200 hp. The engine has been rock solid for 3.5 years, and I am a care and maintenance fanatic. It's got about 525 hrs all deepwater time. On a recent trip offshore I began to see some rpm fluctuations. Without warning it would drop 400-500 rpms and just as quickly it would add them back. It would run fine, just purring along, plenty of power for a half an hour or more and then repeat the problem for four or five minutes, and then "fix" itslef again. Oxygen sensor maybe? A sensor advancing and retarding spark? Water in fuel? Thanks Ya'll.

 

[AQUADJ]

Re: 400 RPM, loss and gain

I've had the same problem with my 98 250 fuel injected motors between 2800 and 3600 RPMs. They run great above and below that range but sometimes they'll fluctuate about 400 RPMs for no reason. Sometimes one and sometimes both motors. I've spoke with several Yamaha mechanics and even have posted here but still have not come up with a reason.

 

[rodbolt]

Re: 400 RPM, loss and gain

test your fuel system for correct rail pressure,VST for cleanliness and the low pressure pumps should be replaced as maint about every 3 years. any sensor failure would most likly drop more than 400 RPM, a failed plug cap can do that and other weired things.

 

[savanah]

Re: 400 RPM, loss and gain

test your fuel system for correct rail pressure,VST for cleanliness and the low pressure pumps should be replaced as maint about every 3 years. any sensor failure would most likly drop more than 400 RPM, a failed plug cap can do that and other weired things.

Thank you for your reply. The mech just called and said
the computer showed nothing wrong. Said he was going to run the boat to try to duplicate. I will run this by him to see if he has done these things.

 

[rodbolt]

Re: 400 RPM, loss and gain

goodthing the computer showed no codes, especially since you CANNOT hook a laptop to a saltwater series motor, the EFI sws motors you can put a diagnostic lamp on but I am telling you your not chasing electrical except possibly a fouled O2 sensor which will set NO codes.
check your fuel system thoughoughly to include fuel system vacum,low pressure and rail pressures. the rail pressure cannot vary more than about 2 PSI from 35 PSI or things get funny.
the diagnostic lamp just quicly allows me to eliminate anything the ECU actually monitors.

 

[savanah]

Re: 400 RPM, loss and gain

goodthing the computer showed no codes, especially since you CANNOT hook a laptop to a saltwater series motor, the EFI sws motors you can put a diagnostic lamp on but I am telling you your not chasing electrical except possibly a fouled O2 sensor which will set NO codes.
check your fuel system thoughoughly to include fuel system vacum,low pressure and rail pressures. the rail pressure cannot vary more than about 2 PSI from 35 PSI or things get funny.
the diagnostic lamp just quicly allows me to eliminate anything the ECU actually monitors.

Thanks again for your reply. I probably expressed that poorly. He did say his light was good or something to that effect. We are checking pressures and pumps in fuel system as the fuel/ water sep. It was new in the spring, one of the Yamaha 10 micron jobs, the blue ones. A buddy of mine had a similar problem once when the f/w sep was full and was burping a little water into the system at wierd intervals. Ahh... the perils of outboard diagnostics. I will echo a previous post when someone said they wished the could clone you (Rodbolt) and spread you around all the mech shops in the country. Thanks again and I will up date next week as things develop.