2003 3.1 L 225 VMAX fouled O2 Sensor at 14 hours run time

[Amistad Tackle]

Model VX225TLRB (2003) with 590 hours on it. It has the original sensor which I have cleaned numerous times (with Yamaha combustion chamber cleaner). Last cleaned at 576 hours and installed new plugs. Compression good at 576. Tested the sensor with the test harness at this time and it was within Yamaha manual specs although the voltage at idle was a little high, but still within specs. Perhaps overoiling at idle? This time (590 hours) the engine stopped coming out of the hole like it usually does. It was definately not running on all cylinders. Looked at the plugs and they were oily. The lower one on the passenger side was especially oily. Pulled the O2 sensor and there was pooled oil in the bottom of the tube the sendor screws in. Checked the low pressure pumps and the top one had a little bit of fuel weeping from one of the bolt holes. I rebuilt it and re-installed.

My question is why does this engine have such a problem with fouling this sensor? I have adjusted my oil rod to specs which seemed to help for a while. I end up changing plugs about every 25 hours, now its like every 14. Should I just go ahead and buy a new O2 sensor? My bet is what I've done will get her running again just fine and then 14-25 hours later I'll be pulling the O2 sensor again or at least changing plugs. I don't drive the boat very fast with the price of fuel. Usually 3-4K rpm is all I run and I do quite a bit of idling while I'm graphing spots. Could this be leading to my o2 sensor fouling and going through the plugs so quick?

[201pro]

I do not see any of the issues you are seeing. I have cleaned the sensor 1 time a year for 6 years. Plugs every 2 years. No carbon on pistons or in O2 sensor area.

What oil are you using. Are you using a carbon preventor ( sea foam or ring free) ? What plugs are you running? Going one plug hotter will help.

Not running a 2 stroke hot enough to get the oil/carbon all burned off and excessive idling will make it worse. Run that thing a little everytime you take it out and get her cleaned out.

[toledo freak]

what kinda oil are you running??? and i would perform a synk and link on your motor and also verify tps voltage

[BuzzB8]

I concur with both posts above. Switch to the BR8 HS10 plugs and you'll see they last much longer before they need replaced. For sure, check the TPS, but you need to sync it all up first.

You have double the hours I have on mine (same engine).

[Rich Z]

If you haven’t had your injectors cleaned and calibrated than it’s about time. Keihin injectors over long period of time will build up varnish inside and will flood as the return spring slows down its travel time thus more fuel is passed through the pintle. Often we need to adjust the injector to reduce the flow rate to specifications. This is because the return sprig stop is out of place or just weak return spring. At Brucato, we calibrate injectors on daily basis and no one else offers this service.

[Amistad Tackle]

I run Mercury Premium Plus. Always have. I run the BR9 HS10 plugs because that is what the manual says, although I noticed on the Boats.net site they say run the BR8 HS10 (in the schematic for replacement parts). Are most running the BR8? Was there a service bulletin on this? I have never run ring free, but I do de-carb every year with Yamaha combustion chamber cleaner. The top of the pistons have a light coat of carbon, but nothing excessive. When I clean the O2 sensor housing the tube the sensor sits in and the removable tube to the number one cylinder are usually heavily carboned. I have to clean this out with a small knife or screwdriver. The O2 sensor perforated cover is never heavily carboned. This is when I was cleaning it once per year. When I pulled the O2 sensor this time it was just oily. No big carbon build up, but it only had 14 hours on it from the last cleaning. I don't think carbon is my problem. It is more like an overly rich condition.

I'll do the link and sync per the manual specs and check the TPS voltage. With the TPS do I absolutely need the Yamaha test harness or can I get by with just a volt meter?

As soon as I have an extra $150 I'll send the injectors in for cleaning and calibration.

I am also going to check my fuel pressure at the schrader valve to make sure the VST pump is building the correct pressure.

I also may just need to run the engine a little harder. I have seen lots of posts about this engine fouling plugs and O2 sensors when doing a lot of idling. I am definately pee pawing around, but this engine absolutely drinks the fuel. Every time the boat leaves the garage it is minimum $60-70 in fuel (boat and truck combined) and that is just fun fishing. I also spend quite a bit of time idling looking for structure spots.

Thanks for responding!

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Modified by Amistad Tackle at 11:55 AM 6/21/2011


Modified by Amistad Tackle at 11:57 AM 6/21/2011

[Stratos21XL]

I think if you'll start using ring free or some other decarbing product in every tank then alot of your issues will go away. I had a 2002 Ox-66 and it was a great motor. I had problems with my 02 sensor and got tired of cleaning it once or more a year. I replaced it and started using ring free in every tank and never had another issue after that. There is also a silver valve (check valve) that is located right below the 3 low pressure pumps, if you haven't replaced it I would recommend it.

[21XDC]

My 2003 250 ox66 has been running amsoil hp injector oil, seafoam and br8hs-10 plugs and I still have never cleaned or replaced the o2 sensor, and the motor never had a hick up yet... Maybe I shouldnt have said that.

[bassfinder1]

If TPS, oil adjustment and FP are all good start running ringfree or seafoam in the required amounts in every tank and if you do extensive idling, run the snot out of it. What is occoring is on ox66 engines they are oil hogs, if you do alot of idling the oil builds up and your combustion chambers are not hot enough to burn out the excessive oil. Put some new plugs in it and clean the sensor and tube and drive it like you stole it. Almost forgot, check your fuel line for any soft spots from the poor quality fuel, you could possibly have your feed line collapsing.

[Amistad Tackle]

Will do. I am going to check everything over from top to bottom and then once I have it all checked, and replaced what I need to, I will start using ring free. I believe if I buy the winky blinky tool I will get a code on everything, except the O2 sensor, if it is malfunctioning. Time to spend some more money.

Mine doesn't use as much oil after I adjusted the oil rod to specs. It still smokes big time at start up.

[Amistad Tackle]

New plugs (BR9 HS10). Had already bought the plugs before the BR8 HS10 recommendation. Cleaned the O2 sensor per the service bulletin. Rebuilt the top low pressure fuel pump and as expected had top end of 65 mph at 5500 rpm on the Ranger 492VS in upper 80 temps and high humidity. Now will it last. Need to get the ring free and I am still going to run the winky blinky to check for any codes.

[Amistad Tackle]

Did everything in the last post and it still crapped out right at 13 hours. Went to plan B as described below.

Check compression. Good. I tested TPS and adjusted. Checked fuel pressure. Good, but fluctuated around 35. Changed plugs. Did link and sync. Rebuilt one leaking low pressure fuel pump. Bought new NGK 02 sensor and retrofitted with old wiring harness. Started running ringfree all the time. So far so good. Smokes on start-up, but overall performance is great. Steady WOT and good idle. I think the 02 sensor may have been the main culprit. It was still functional, but maybe a little tired.

Now the possible exhaust leak in my other post.

[classic242]

Amistad Tackle,
I will have to agree with some of the others somewhat, first of all Mercury Premium Plus does contain more molybdemum than the Yamalube 2M, a little more than 300 ppm more. If you choose to use Yamalube 2M and run Ringfree in every fill up and use either the Yamaha gas treatment or Startron gas treatment, if you are having to use gas that contains ethanol, your problems should go away for good. The reason that I mentioned the Mercury Premium Plus, which the newer Yamaha manuals say you can use, is because I know a few Yamaha owners were having some carbon problems using the Premium Plus and the problems went away when they switched back to the Yamalube 2M but, one will hear several different stories about this. I have no dog in that race, just stating what I was told by some other Yamaha owners but, the Mercury Premium Plus is a very good outboard motor oil by anyones standards. Hope this helps !!

[Amistad Tackle]

I have run Merc Premium Plus since day one and never had any trouble until lately. We have had 10% ethanol in our gas here for a long time. I also never ran ring free, but decarbed every year with Yamaha combustion chamber cleaner. The top of my pistons look like every other OX66 I have seen as far as carbon. The oil I ran wasn't the issue. Yamalube isn't easily found here. The 02 sensor was likely the major issue. I cleaned it many times, but I think it got lazy at idle and over oiled. Over 600 hours on the same O2 sensor isn't bad. I'll report back if this wasn't the final fix.

[ZXD]

You need to open that thing up ,at least once, every time you take it out .

[Amistad Tackle]

This is an old post, but as a follow up after I put some hours on the motor, and to help others, what is described in my last post solved my issues. 668 hours and runs perfect. Haven't changed plugs once this summer, and I spent a good portion of the time idling with the side scan. Quick out of the hole and full power at the upper end.

[Northeast Bassin]

Had you ever tested the performance of your O2 sensor? Curious to know if it was reading ok, but not working ok

[Amistad Tackle]

Yes I did test the old O2 sensor with a test harness and digital volt meter while the boat was running on the water. It was reading within specs, but as I remember the voltage readings at idle where higher than they should have been, resulting in a rich mixture at idle. I believe this led to fouling of the plugs. I'll be interested to pull the new O2 sensor and see what it looks like compared to the old one. The old one was always oily and heavily carboned.