Yamaha ox66 issues
 

I have a 2001 Yamaha 250 hp ox66. When I run the motor between 2000 and 2400n rpm the motor will lose rpms and sometimes almost come to a complete stop. This happens while increase or decreasing the rpms. The motor runs great from idle to 2000 rpm and above 2400 rpm to WOT.
So far I have done the following

replaced all fuel filter
replace fuel line from tank to the motor
replaced vsp filter
cleaned o2 sensor
replaced plugs
bypassed the flow scan

Any help would be appreciated.

Thanks

I used to have a 2001 225 pos66. About every 20 hours, I would have to put a torch to the O2 sensor and it would run better. It would also shudder when getting up on plane and switching to six cylinders. I don't remember at what rpm that was.
I happened to be at my mechanic's place when the factory rep was there and he offered to go on a test ride. He determined that I would benefit from a prop of less pitch (19" to 17") and he swapped it out for me. It helped, but I never did like the way it idled and at slow speed.

Most of time it's fuel related. Although it's usually a little lower rpm range than what you describe, they run on all 6 cyl in neutral, "drop" two when put in fwd, then pick those two back up around 1800 or so. Sounds like you're trying to run close to the switch-over rpm. (Assuming the LP pumps are good and fuel pressure is good)

We have a customer at my boatyard who has a 2003 Pursuit with twin OX66 250's, and when she got launched last month she would not turn up over 2200 reliably.
Sometimes she would run OK up to 4600 RPM's and then fall on her face, other times she just wouldn't get up and go.
Only on one motor, other ran fine.
We did all the usual stuff, checked the fuel pumps (low and hi pressure) cleaned the VST tank, cleaned the O2 sensor, etc. etc.
Nothing helped. Eventually we sent out the injectors for cleaning and that fixed it right up.
Sea trialed her today, and she ran like a champ. Well, until I spun a hub. Doh!
Heres where we send our injectors for cleaning. Super fast and reasonably priced. Located in Bow NH.
https://www.dieselsfuelinjection.com/
FYI-OX66 Injectors are kind of a pain to remove though.

Striper56,

I've owned an OX66 250hp for several years now. Sounds like a fuel supply issue. If you haven't replaced your 3 low pressure fuel pumps recently, it is easy to check if they are leaking. You won't know if they are leaking because they are bolted to the manifold and use vacuum and positive pressure from the manifold to operate. The fuel just leaks into the manifold, causing your engine to run rich at idle and starve the engine at higher RPM's. Surging is a classic symptom. The engine will run fine until the vst tank doesn't get enough fuel from the lp pumps. RPM's will rise and fall as the vst tank fills and empties. You may not notice this at higher rpms, except that WOT might be lower than expected.

Simply loosen the two bolts holding each pump to the manifold and squeeze your fuel bulb. Do this with your engine off and cool and check one pump at a time.

If the diaphragm is shot, fuel will dribble from behind the pump. If it leaks replace it, or for that matter, replace all 3 as they are only about $30 each. Don't forget to replace the $3 gasket behind each pumps. You can also rebuild the pumps, but for $30 why bother.

Thanks for the replies Olds cool, erebus, and vancouver.
I will check the fuel pressure and LP pumps first. If that doesn't work I will dig into the injectors. I did send out the injectors three years ago, 2 needed to be replaced and the other 4 needed a little cleaning.
I will report back the results. Thanks again.

Changed the LP fuel pumps and took it for a hide. Same issue except it is now cutting out SOME of the time when pulling back the throttle from WOT down to 2400. I run it up to say 4000 + RPMS then pull the throttle back 10 times and it will only cut out once or twice. Injectors next?
Thanks

check your gas tank for a anti-siphon devices,sometimes are inside of the pickup tube similar to a little wire mesh (like on my case) or little steel ball inside the elbow fuel pickup

Anti-siphon Valve
 

I just replaced a Sierra filter with a Racor filter and vacuum gage on my '76 Seafari with an ETEC 150. On the first run I was at the BRP recommended limit of 4 in Hg. Bushwacker advised to look for an anti-siphon valve and remove it per ETEC ownersgroup recommendations to avoid running too lean & hot. Removed it and partially blocked screen on fuel tank vent. Have not rerun it yet, but fuel bulb is harder.

OX66 Issues
 

Hey Striper56,

Just in case you don't have this link:

http://www.boatinfo.no/lib/yamaha/ma...003yamaha.html

Section 3 deals with fuel and there are some good write-ups on OX66 EFI engines. On 3-58 and 3-59 there is a discussion on CCS also. The engine only fires on 4 cylinders between 850 RPM and 2000 RPM and 5 cylinders below 850 RPM. There is also a shift cut switch that if actuated (for example by moving the throttle into neutral from WOT) causes the engine to drop two cylinders, no matter what the RPM's are. This is to avoid shock to the drive train.

Here is the update on the still not running correctly motor. I installed 3 new LP pumps. The motor sounds better but still cutting out when increasing or decreasing the rpms from 3000 rpm down to 2000. I took a 6 gallon tank with me and connected it to the LP pump. Still didn't run correctly.
One time I shut the motor off and could not get it to start. It sounded like the kill switch was disconnected but was not. Tried a few more times still nada. Went back pumped the primer ball switched the battery to both and it fired right up and ran fine.
Great website VancouverPotter23. I will add it my favorites.

Thanks

This afternoon I was testing the TPS the motor would not start. I checked the primer ball it was soft more like empty. Pumped the ball for a few minutes until the ball was hard and it fired right up.

Still sounds like a fuel supply issue.

I agree with oldscool. Sounds like you've got a fuel delivery problem between your bulb and the vst. When you pump your bulb you are filling your vst and pressurizing the line between the vst and your bulb and the engine behaves. Then the line loses pressure for some reason. Try 3-100-101 in that link to boatinfo. If you have a vacuum gauge, you can stick it in line at various points to check for faulty pumps, air leaks, etc. If you have an air leak, some clear plastic line might show bubbles.

I did see some small air bubbles in the clear filter on the engine block when it was refilling. I was running the motor and noticed the filter would go down half an inch then when filling I did see air bubbles.

Not for nothing, but, I had an issue with the downstream check valve in a primer bulb. I could pump the bulb up tight, but at some point the ball would stick cutting fuel flow off. I was seeing bubbles and some gas through the in line filter but motor wouldn't stay running. Replaced the bulb and haven't had a problem since.

I learned a long time ago that an aftermarket bulb can appear perfectly fine. Now ONLY OEM! One place not to play with every other Chinese rubber available. Cheap starting point, especially if you are using same fuel line and primer bulb when switching to six gallon fresh tank on deck. Maybe this will help.

Vezo

I bought the primer ball from West Marine $29.99. I think they had a West Marine brand that I did not will not buy. I think it was a Mercury primer ball. I was having the problem before and after replacing all of the fuel line and primer ball.
I hooked the 6 gallon tank directly to the LP pump and the engine still would cut out when increasing or decreasing the rpms mostly between 2000 and 2400 rpms.

Anti-Siphon Valve Removal
 

As an update to my 5-18-18 post removing the anti-siphon valve and partially blocked air vent screen dropped the vacuum gage reading from 4"Hg after a partial throttle run to 2"Hg after a WOT run. I concur with some of these other remarks, after issues through the years I only use OEM fuel bulbs. And even with that replace them before you think you need to. Once couldn't keep my Merc running back to the ramp without continually squeezing the bulb.

First let me thank every one for all the great info and help trying to fix figure out my issue. I learned a lot about the fuel system. It ended up being the little filter on the high pressure pump. After reading about the HP pump I tested for the correct voltage 9v at idle which was good and testing the pressure which was good 35. I found the issue. The little filter on the HP pump was clogged. I washed it off with carb cleaner reinstalled the filter put the VST back together and she ran perfectly. Good for another 1700 hours. Thanks again!

Good going! I replaced that one last year, along with the fuel filters, etc., but my problems were related to the lp pumps, of which 2 out of 3 were leaking into the crankcase. I don't think they had ever been replaced. I never needed to pump the ball to start the engine, but when it had been sitting, the ball was always soft.