Trouble with a 200 hp OX66

[cpower23]

My dad has a 2001 200 hp OX66 that has some Hydro-Tec mods on it. It ran great until late last year when some how the stator burnt up on it. Still don't know how that happened but he took it to our local mechanic (not hydro-tec) for repairs. they replaced the stator on in this spring before the weather broke and he was up and going again. However all year long he has been fighting an issue where he will be running down the lake WOT, turning about 5900 rpms, and all of a sudden it will just drop down 300-400 rpms. He can back out of the throttle for a split second and mash it back down and it comes right back. It may run fine for a while and then all of a sudden it will do it again. He just happened to notice that one day he bumped the blinker trim by accident when this happened and it came right back. So he started playing with the trim when it would do it. He could bump the trim up or down just a hair, not even enough o really move the motor, and it would come right back to normal rpms. It's not a bad tach because you can hear it in the motor and you actually start to lose speed when it does it. We thought maybe it was a bad battery and wasn't getting the right amount of current all the time so he swapped batteries with the brand new one he had just put in his pontoon. It still did the same thing. He took his battery in for testing and it tested perfect (Interstate Batteries). I had an OX66 that had a bad O2 sensor that caused me to lose about 500 rpms at WOT but it did it all the time, not intermittent, so we checked his O2 sensor and there is nothing wrong with it. He found out later this year that the stator that they put in was an after market brand and not an OEM Yamaha part. My question is could the aftermarket stator be causing these problems? It's really weird, like I said he may go out one day and it never miss a beat but the next day it may do it several times throughout the day. Anyone have any suggestions.

[ClarkBird]

Have you checked the TPS?

[RL552]

I have a 2001 225 ox66 that's doing the exact same thing. I checked the low pressure pumps and one was leaking, and my o2 sensor tested bad. I haven't been out to test it. Check all ground wires.

[dls]

I have a 1999 200 ox66 I have had the same problem and it was my o2 sensor even though it checked good. That made twice my motor lost rpms and not every time out and both times even though the o2 sensor checked good it wasn't.

[CJ1]

I have a 1999 200 ox66 I have had the same problem and it was my o2 sensor even though it checked good. That made twice my motor lost rpms and not every time out and both times even though the o2 sensor checked good it wasn't.

This^^^^^. Mine did the exact same thing. When you say you back out and get back into it and it runs good, that is classic O2 sensor. You have cleaned the draw tube correct? If you suspect a stator get a winky blinky and test it. Also a ohm test. I replaced both stator and pickup on mine to find a miss and it ended up being a bad injector. As I was advised the ox66 electrical system is about as reliable as they come and are easily tested. Look at the main cable connections at the engine also. Just a thought. Cj

[cpower23]

Thanks guys we will look at the O2 sensor again. We have not looked at the TPS. It just seemed weird that if you bumped the trim it would come out of it. Maybe that was just a coincidence. Any other suggestions please let me know. We hate just throwing money at it and so far our local Yamaha mechanic doesn't have a clue whats wrong.

[clayshooter100]

I highly recommend purchasing an O2 sensor test harness. The only way to actually test the O2 sensor is by monitoring the output voltage with the engine running, fully warmed up with the gearcase submerged. I would also recommend a TPS test harness. Neither are extremely expensive and allow to to easily test these sensors without potentially damaging wires from back-probing.

https://www.simyamaha.com/Yamaha_Tes...p/yb-06867.htm

https://www.simyamaha.com/Yamaha_Tes...p/yb-06857.htm

[ClarkBird]

Just replace the O2 if that's something you're concerned with. NTK 21006 - $35. Cheaper than purchasing a test harness and considerably cheaper than an OEM sensor.

[CJ1]

Thanks guys we will look at the O2 sensor again. We have not looked at the TPS. It just seemed weird that if you bumped the trim it would come out of it. Maybe that was just a coincidence. Any other suggestions please let me know. We hate just throwing money at it and so far our local Yamaha mechanic doesn't have a clue whats wrong.

Yes, that's why I suggested checking the main power cables. Also the quick voltage surge/drop may affect the 02. Just thinking out loud. CJ

[strat751]

Same problem was low pressure fuel pump unbolt from block and pump blub any gasoline coming out of them you have a bad one.

[shadyman]

I had a 1999 VMAX 225 for awhile and understand the X066 pretty well. I usually do my own maintenance, troubleshooting, and repairs. Don't like to wait and repair cost at the dealers are often way too much for the little they do. Your problem really sounds like the 02 sensor fowling. Before I replaced the 02 sensor mine was intermittant, it would lose RPM for awhile, then maybe let off the throttle a second and mash it back down (as you described), and RPM's would come back normally - but not all the time. How did you check it? Have you cleaned it and how much oil and carbon builds up on it? If you have an over oiling problem, plugs never look brown in color and look oily, joint between block and o2 sensor gets heavily carboned and really oily, and O2 sensor gets fouled with carbon frequently. If so, see below. I posted on Texas Fishing Forum recently. Once I fixed the over oiling and finally changed the 02 sensor, it worked flawlessly and 02 sensor didn't foul all the time. As far as checking 02 sensor, if you simply checked it with a meter, it probably showed OK. The only real way to test it, is to meter while it is running on the lake which requires wire it to a meter with long leads and watch the voltage output while on the water. If the sniffer screen is clogged, which you really can't see inside, it cannot properly read oxygen level, therefore send erroneous voltages to the CDI. The operating voltage is between 0 and 1 volt. The small voltage changes tell the CDI to adjust full/air ratio. I tried cleaning mine several times, but they get clogged inside the very vent holes on the sensor, it is impossible to clean correctly. I didn't want to buy a new Yamaha, so I bought a NTK from the auto parts store I believe is p/n 21006. It has 4 wires same color as the Yamaha NTK sensor. 2 grays, one white, and one black. Two are for the pre-heater, and two for the voltage signal. But it has only one connector, cut and splice the connectors from the Yamaha sensor. Did a lot of research on the NTK. There are two types of 02 sensors, 0 to 1 operating volts, and 0 to 5 operating volts. X066 uses 0 to 1 since the operating output to the CDI is less than 1V. The 21006 is 0 to 1 operating volts. It is slightly bigger in diameter, but will fit into the housing. I used it for over a year and ran like a champ - again once the over oiling was fixed. There are several posts on the forums about other guys successfully using the 21006 NTK. Since then my VMAX broke a ring, grenaded the piston, and cracked the block. So, I am parting it out. I have the 02 sensor and all the assy parts on Ebay, item 302994766067. I promise you it works. Also, I have a good stator if you find out you need another. I also still have most other parts for your motor if you need anything.

"The VMAX has the O2 sensor which needs to be kept clean, inlcuding the small dogbone shaped transfer tube. Mine was clogged shut with carbon and had to drill out. When the O2 sensor gets really clogged it cannot be cleaned properly. I eventually replaced mine. When they get clogged and voltage to CDI will fluctuate and fall outside voltage parameters, the CDI will default to a premapped program to allow motor to run and function - but will lose about 5 or more mph, and several hundred RPM. It might switch back and forth between normal mode and pre-mapped mode. O2 sensor in a 2 stroke is just not a good idea, but it works just have keep clean. If its carboned up and clogging frequently, and is really oily when removed, and the plugs are oily and not brown in color, and it smokes at idle alot - IT IS OVER OILING. Common problem: there is a link between variable ratio oiler VRO that connects to the middle throttle plate of the throttle body. The more the thottle plate opens the more it oils, simple. What I have discovered: there is a plastic clip that fits in the hole of the throttle plate link, this rotates and locks the link from falling out, the plastic clip will wear and get lose in the thottle plate hole allowing link to move with spring loaded tension of VRO and away from the specified positon. The plastic fitting is p/n 6R5-41237-00-00. On crowleymarine.com parts look up your motor, it will be item 20 in "OIL PUMP" section view. Most people will try to adjust the plastic threaded link on the other end that connects to the VRO but find the link runs out of threads and will grind off a few threads of the rod to get more adjustment - not realizing the plastic link is worn on the other end. I would replace that platic link joint as well since it is normally cracked around the threads. Item 23 p/n 6H1-41237-00-00 ($14.95). Again, if signs of over oiling leads to lots of problems, including excessive engine carbon, fouling 02 sensor, etc, a few dollar fix makes that engine run very well, and 02 stays clean for a long time - which is critical for the older VMAX."

[BulletV]

What's the voltage, when you are running WOT? The rectifier and aftermarket stator could be causing an over charging situation. I had a similar problem with my Promax and I changed out the battery with the one in my pontoon and it did the same thing. Both were Interstate 24MXHD 1000MCA. I am an Interstate dealer and about 7 years old. I put in a brand new one and it worked fine. I don't know what the difference is but with both the old batteries, my motor was charging 17.5V at idle. Both the old batteries work fine on the pontoon motor
(Merc 150 4S).