OX66 Bad O2?

[Bluefish221]

Hi All,

Looking for some more help from you guys! Motor is 2001 OX66 250HP.

After not using the boat for 10 days, something felt a little off on my 8 mile run out. She got out of the hole and went on plane fine, although at 40-41RPM I was only getting 27MPH, whereas that consistently gets me 28-29MPH in same sea conditions. Not that far off, but far enough where I could tell after having this motor for a while that something was off.

On the 8 mile run back, I decided to slowly increase her to WOT (for me 52RPM). I slowly went from 42-43-44-45-46-47-48-49 RPM and once I got close to 50 she would start sounding rough, and ease back to 42 RPM on her own even though I had her at full throttle. At first I thought this could be a fuel restriction issue, so I decided to do some quick tests. From neutral I hammered the throttle down, and she seemed to "get up and go" on plane with ease just like she normally does. Weird. Once I was at cruise speed (40-41RPM) I hammered the throttle down again to WOT and was able to get 52RPM.

Did this a couple of more times and got the same results -
1) If I slowly increased the throttle up to WOT she would bog down on her own and run rough
2) If I hammered the throttle to WOT, she would stay at WOT and keep running as normal

Since I can hammer the throttle and run at WOT it sounds like its not a fuel restriction to me. From what Ive been reading this sounds like could be related to the O2? The sensor is new as of last year and was cleaned at the beginning of this season, 30 hours ago. Im going to go down to the boat today and clean it out and see how she looks but just wanted to get everyones opinion before I start tearing the fuel system apart again!

Thanks!

 

[99yam40]

how long did it run OK when you hammered down and it ran fine?

monitoring fuel pressures to make sure they are not changing when problem shows up, along with timing would be what I would look for 1st

does that motor have a knock sensor, and how old is the gas?

 

[Bluefish221]

When I hammered it, I probably only ran it for 20-30 seconds at WOT before I dropped it back again.

Monitored fuel pressure today, and it was sitting in the mid low 50s. (Higher than manual spec). Whats interesting is that when it bogged down, it dropped into the 40s and 30s.

I am not familiar with knock sensors but I will do some research on them. Gas is 2 weeks old and last time boat was run before I had the symptoms was 2 weeks ago. I usually keep the tank only 1/3 full around 30-40 gallons at a time. (tank is 126).

Tomorrow I will try and work my way through the fuel system and swap out the O2 with a used O2 from an old motor.

 

[99yam40]

running above spec might be the gauge,
but the drop would worry me

 

[Bluefish221]

Well this morning I took one step forward, but seems like 2 steps backwards.......

That used O2 sensor I thought I had, I couldn't find. So I went down to the boat, took apart the VST tank and inspected the VST and Pressure Regulator Screen. VST was little dirty but nothing major obstructing it, changed it anyways. The pressure regulator screen had some crap in it so I cleaned that out and replaced.

Decided to take it for another test run on a separate 6 gallon fuel tank to isolate bad fuel.

Took 4-5 cranks/stalls before finally got her to turn over but not without the throttle in fast idle. (This is abnormal as its usually 1 click and starts right up).

Took her out into the channel and you could tell something didnt sound right. You could tell it sounded like she was running rough. Well at least I got my fuel pressure back as it was sitting dead at 36. Tried to increase the throttle a bit and she would run even rougher and shake/vibrate the whole boat. Tried going up on plane and she was sluggish/super rough. Didnt push it above 25RPM before I decided I didn't want to do any potential damage so I turned around and went in. Went in at 10-12RPM very rough on the whole way in. Fuel pressure 36 the entire time.

Did a compression check and came up with 108-110 across all cylinders. When I pulled the new plugs (just put in yesterday) most of them were tanish, although the top one on the starboard side (remind me which # cylinder this is???) almost looked white.

Could the whitish one not be firing in that cylinder? Or running lean?

I tried uploading a photo of the plugs. Hope it works.

Tips/suggestions welcome.

 

[xltier]

if that was a 4stroke it could be water getting in the cylinder.not sure on 2 stroke.i am no means a 2 stroke mechanic.just mercruiser 4 stroke.but worth checking I would think.

 

[99yam40]

Well this morning I took one step forward, but seems like 2 steps backwards.......

That used O2 sensor I thought I had, I couldn't find. So I went down to the boat, took apart the VST tank and inspected the VST and Pressure Regulator Screen. VST was little dirty but nothing major obstructing it, changed it anyways. The pressure regulator screen had some crap in it so I cleaned that out and replaced.

Decided to take it for another test run on a separate 6 gallon fuel tank to isolate bad fuel.

Took 4-5 cranks/stalls before finally got her to turn over but not without the throttle in fast idle. (This is abnormal as its usually 1 click and starts right up).

Took her out into the channel and you could tell something didnt sound right. You could tell it sounded like she was running rough. Well at least I got my fuel pressure back as it was sitting dead at 36. Tried to increase the throttle a bit and she would run even rougher and shake/vibrate the whole boat. Tried going up on plane and she was sluggish/super rough. Didnt push it above 25RPM before I decided I didn't want to do any potential damage so I turned around and went in. Went in at 10-12RPM very rough on the whole way in. Fuel pressure 36 the entire time.

Did a compression check and came up with 108-110 across all cylinders. When I pulled the new plugs (just put in yesterday) most of them were tanish, although the top one on the starboard side (remind me which # cylinder this is???) almost looked white.

Could the whitish one not be firing in that cylinder? Or running lean?

I tried uploading a photo of the plugs. Hope it works.

Tips/suggestions welcome.

should be easy enough to check spark or voltages to that plug.
if good spark then check injector