2002 OX66 sputters/surges between 1300/2200 rpms

[lswjth3@yahoo.com]

Hello, new user here and thanks ahead of time for everyone's help. I have a 2002 Yamaha 250 OX66 Model # SX250TXRA with 560 hours. Took out the boat and noticed that it was surging at top end, would go from about 5400 rpm's to 4200, to 3900 then it would go back up to 5400, etc.

I went ahead and did the de-carbonizing procedure, replaced the OX sensor with the NGK equivalent and replaced all 6 plugs with the NGK's, went out and boat ran like it was new, came back washed the boat hooked up the water hose to the ears, started it up when i gave it gas it would surge from about 1200 to 2300 rpm's up and down like I was playing with the throttle.

I then went ahead and did the following:
Checked compression, 125 on all 6 cylinders
Checked fuel pressure, stays steady at 34psi even when it surges
Checked the stator coils each bank is at 300 Ohms, my meter says that they are putting out 60 V each, (I dont have a meter that measures peak voltage or the DVA adapter)
Checked pulser coil, I have 300 ohms between each of the 6 wires to gnd
Checked the spark plug caps, all are within specs
Checked the Air ambient sensor for correct voltage, ok
Checked the other sensor stbd side by the fuse box for correct voltage, that's ok
Unplugged the CDI box bottom connectors to see if there were any broken or pushed pins and did not find any.
Checked for voltage going into each coil pack at 3000 rpm's, 125 volts on each pack, also measured for resistance on each coil pack on primary and secondary winding's, they all were good.

Cleaned all the grounds that I could find and those were nice and clean
Unplugged the yellow "Get me home" wire under the CDI box and this did have some corrosion, took it apart and cleaned it put it back together.

I have now replaced the following parts:

3 LP pumps, Yamaha oem

VST filter and gasket Yamaha oem

Pressure regulator filter Yamaha oem

Water temp sensor, verified it worked with meter and heat

USED TPS Sensor, adjusted to .5 volts with linkage disconnected, goes to 3.7 volts at full throttle, engine in the on position not running, no change still surging at low rpms, hitting the primer bulb while it surges does not do anything either.

Replaced poppit valve Yamaha oem

Replaced water pump with sea choice kit

Replaced fuel injector filters

T-stats are 1 year old and the gaskets were also replaced

Engine mounted fuel filter is 1 year old, no bubbles and I don't see the volume of gas drop or increase when it's surging

New Racor filter

Primer bulb is 1 year old Yamaha oem

Oil rod has the white connector on it

Wiggled all the harnesses while the surging is happening and no change.

I have yet to take out the boat again however with the ears on it, if I go past 22/2300 rpm the surges ends and the engine seems to run ok? This thing has me stumped, I am not a mechanic by any means but I know how to remove a screw and put it back(most of the times).

I did notice that If i disconnect my TPS sensor, the surging goes away? Any and all help is greatly appreciated, Thank you! Rick

[DennisG01]

2-strokes often don't run smoothly when on a hose. Plus, that engine has cylinder deactivation somewhere in that RPM range you are talking about. Could those two things be the reason? I would try again in the water before doing anything else.

Curious... what is this "yellow get me home wire"?

[Tucker]

2-strokes often don't run smoothly when on a hose. Plus, that engine has cylinder deactivation somewhere in that RPM range you are talking about. Could those two things be the reason? I would try again in the water before doing anything else.

Curious... what is this "yellow get me home wire"?

The emergency yellow wire in the back that bypass's the ECU.

Yea, I found that out with the the hose attachment and even a test drum. Get different readings when the engine is on the boat in the water. Don't know why...

[lswjth3@yahoo.com]

Thank you both! I am planning on going out on Saturday and will report back..At least I'm learning!!!

[DennisG01]

The emergency yellow wire in the back that bypass's the ECU.

How does one use that? What does it do? Does it somehow allow the engine to run if the ECU craps out? Do you just jump 12V to it?

[lswjth3@yahoo.com]

As far as I know you just unplug it, I think that it then puts you in limp home mode. I don't think that the engine will rev past 2000 rpm if this is disconnected. Hopefully someone with more knowledge then me will chime in, thxs.

[rodbolt17]

with the TPS disconnected it defaults ign timing,stops CCS and top speed cannot be reached and the idle goes to about 1100 RPM.
that engine uses CCS.
means it drops 1 cyl at idle and two cylinders between about 1200 to about 2200 RPM.

[lswjth3@yahoo.com]

Thank you Rodbolt, If I remember correctly, the rpm's stayed at 700 when I disconnected the sensor and i could rev it past 3k no problems...I am taking it out on Saturday and I will report back with what happens. In the meantime have a great day/week sir and many thxs for your help!

[lswjth3@yahoo.com]

All, I was not able to take the boat out today, will try again for next weekend and i will post what happens, in the meantime thank you all for your help!

[99yam40]

from what rodbolt wrote it should have affected the running. something is wrong

[lswjth3@yahoo.com]

I have to agree with you, but the "something" has been hard for me to find out what it is, I will report back next weekend, thxs!!

[rodbolt17]

just remember, if running as designed it drops spark on cyl 5 above about 700 rpm.
from about 1200 to about 1800 it drops spark on cyl 2&5.

[99yam40]

is that only in neutral or also in gear?

[lswjth3@yahoo.com]

Both, but I have yet to test it again out on the water, thank you for all the help, it's very much appreciated!

[rodbolt17]

as the OX66 motors that use CCS have no shift position switch then CCS works based on RPM inputs from the pulser coils.
the pulser coils are mounted in a physical 7*BTDC position.
that is what is used for default timing.
revving on a flusher is a teat on a boar hog.
what is base ign timing once the engine temp is over 113*F?
does the ECU know it is above 113*F?
that is where the diagnostic test lamp come in mighty handy.

it doesn't cost much.
way cheaper than chunking parts at it.