1999 250 hp OX66 stalling at high RPM

[bluemoon]

Hi All,

Here is my situation. I have a 1999 250 Saltwater Series on a 24-foot CC. It runs fine up to about 4000 rpm, but if I try to push it to 4500 rpm, it will run for about 30 seconds and then stall. (I'm estimating the rpm since my tach/oil/trim gauge stopped working about a month ago and I haven't replaced it yet.) I'm in Massachusetts and we have E10 gas. I've been good about changing filters, using the 10 micron filter and using addatives. I even cleaned the VST filter. I've checked the fuel/water separator and I don't see any water or gunk so I don't think it's a fuel issue. I have had someone watch the fuel line ball while this was happening and it did not collapse, so I don't think it's a blockage issue. I'm not sure what to try next. Here are a few other obsevations that may or may not be related. The exhaust seems exessively smokey at low rpm. There seems to be alot of water expelled with the exhaust at start-up. At the end of last season, I ran the engine close to wide open for a few minutes in flat, calm conditions. After a short while, I noticed that the rpms were varying. It would run at 5000 rpm for 30 seconds, then drop to 4500 for 15-20 seconds, then return to 5000 for 30 seconds and continue to cycle in this manner.

I have picked up some great info from reading some of the other topics. Any insight from anyone regarding my situation would be much appreciated.

Thanks

[Ken]

bluemoon,
I believe your motor has three low pressure mechanical fuel pumps - sounds like one could be bad (split diaphram).
You can test them by removing and blowing into them - should be able to blow in direction of fuel flow, but not the other way.
Also, your O2 sensor may need to be removed and cleaned. Not sure about that procedure - it's in the Yamaha shop manual.
Good luck [img]smile.gif[/img] ,
Ken K

[jb123]

sounds more like the sensor to me as you will get the varying rpms. clean and test it, here is some info on them:

http://www.boatsetup.com/O2sensor.pdf

and as ken said about the primaries, if in doubt , switchem out,easier to spend lil $$ in the yard than a tow bill from miles out.

[rangr02]

Nice engine, I've got the same thing on my 23' Mako. Takes a lot of maintenance, though.

Ken and JB post good advice. I dont know how much of the required maintenance you are aware of, but the O2 sensor is extremely important. It needs to be cleaned about every 100 hrs and the test procedures JB linked you to is an easy way to find out if its "clean enough". The new part number for the test harness is YB-6867 and is about $20.00 through the dealer. Ignoring the O2 sensor can result in destroying #1 piston. I tried hard to clean mine with no luck. A new O2 sensor cost $220.00 through our sponsor here, much cheaper than a melt down...

The standard advice on the 3 fuel pumps is to replace them all every 3 years. These are vacuum diaphram pumps, leakage goes right back through the vacuum port causing an overly rich condition. Easy to replace and costs about $32.00 each. You can buy all of the parts to overhaul the pumps yourself, but the cost of parts is so close to the price of a complete new pump that it doesnt make sense to me. If you dont know how long your pumps have been on the engine, I'd definetly change them out. Make sure you order the pump to block gaskets, too.

Have you checked the self diagnostic system for stored codes? The diagnostic indicator (winky blinky) is availabe through the dealer as YB-06795 for about $100.00. Kind of expensive unless you cant talk your local yamaha mech into borrowing his. Showing up just before closing time with a few cold beers and a promise to bring it back the next morning might work for you.

Keep it up on the fuel filters, E10 seems to be causing a lot of trouble for everyone. Great solvent, cuts everything loose in your tank so your filters are working a lot harder than normal. How bad was your VST filter? There are also filters on the fuel injectors, but I dont think thats your problem. My bet is youre going to find a dirty or bad O2 sensor.

Good luck, let us know how it turns out.

[bluemoon]

Well, you guys were right on. Two out of three fuel pumps were bad and the O2 sensor needed to be cleaned. Thanks for the advice.