OX66 help loss of power

[Messikpe]

Need some helpfrom those who have seenit all.

I have twin OX66. Starboard turns up great, 5400rpm. Port, not so much. It will run up to maybe 4200, then after about 10 seconds drops down to 2500ish.

All new filters.
Rail pressure rock steady at 35psi, under load.
New plugs.
O2 sensor looks new, and i de-carboned the passage.
TPS is .62, and linear, up to WOT.
Spark seems fine

[Messikpe]

Forgot to add, injectors were pulled, and cleaned.

[99yam40]

did you monitor the rail pressure while the RPM dropped?
need to be check things while the problem shows up

[Messikpe]

Yes, pressure is rock steady at 35psi. I originally expected VST filter, but steady pressure even when problem occurs would eliminate this. That lead me to pull and clean injectors, and replaced all cup filters

[Messikpe]

Yes, pressure is rock steady at 35psi. I originally expected VST filter, but steady pressure even when problem occurs would eliminate this. That lead me to pull and clean injectors, and replaced all cup filters

[panasonic]

Any overheat alarms and does your alarm work? 2500 rpm is about right for rpm reduction mode. Do you have the service manual for your engine?

[Messikpe]

There are no alarms. I will confrim buzzer, but there is no indicator for over temp. T-stat were just replaced, and was water pump. Tell-tale shows great water flow. Wouldnt power reduction mode increse the idle to aprox 1100?

[99yam40]

Yes, pressure is rock steady at 35psi. I originally expected VST filter, but steady pressure even when problem occurs would eliminate this. That lead me to pull and clean injectors, and replaced all cup filters

did you clean them yourself or send off to be cleaned and flow/spray pattern tested?

something has to be changing/dropping.
spark or fuel?
how about timing?
since other is doing good you can compare readings

[Messikpe]

Im anxiously awaiting the delivery of the factory service manul which is 'in transit'

Full disclosure - Im a self employed marine tech, and i'm hoping to get a jump on this before the much needed manual arrives. I admit to knowing almost nothing about the ox66. I'm pretty handy with the 4strokers. I've been to yamaha school several times for the newer stuff, and worked for a dealer, rigging, serving, and troubleshooting.

I'm hoping to get some wisdom from those you understand these better.

I cleaned the injectors myself. I have an injector machine, and clean them often for myself, and other techs. They looked decent...volume, and pattern.

my understanding is timing is not adjustable. it moves when revved, so I assume the ECM does not have it locked.

Rail pressue is rock steady at 35PSI even durign failure.

Failure does not sound like typical VST fuel starvation failure like the 4strokes. They seem to bog, or suck air until you throttle back. This revs up, then its like someone just pulled that throttle back. Happens fast.

Plugs are new. I de- carboned the o2 passage, and cleaned the sensor. From what I read, the O2 sensor would not drop the rpm back like this, but is more of a surging. I can test more when, yes, my manuak arrives.

I have apropriate spark... these things drop one hole at idle. I dont know if a coil is breaking down at load....I'll ohm the coils, when the manul arrives (common theme here, and the USPS can't give me a date)

wondering if the ecm is pissy, and dropping out fuel or spark...thinking of swapping them and seeing what happens.

I have YDS, but that useless on this girl. I dont have the Winky blinky...but, my idle speed is not elevated, so that means I dont have any faults at idle (correct)?, and their isnt stored history



Thanks again for the help. I hate this feeling!

[DennisG01]

I'm not the most knowledgeable here, by a long shot, but... as I read through, a key thing that stuck out was the part where you said the RPM's drop quickly. That sounds electrical to me. Now, it could an electrical problem (ECM) con*****ing fuel - but fuel supply, itself, probably isn't the issue. It sounds like the ECM is seeing something it doesn't like and putting it into limp mode. The ECM doesn't store codes (no history), which means the only way to ID a trouble code is to be monitoring it with a Winky Blinky while the trouble code is actively happening.

Since you have another engine, you could try swapping sensors and electrical components one at a time - but that can get laborious - maybe swap 3 or 4 at a time to help narrow things down quicker. At least until you get a Winky Blinky, anyways.

Yes, one cylinder is dropped (#2) at idle and then from something like 800 to 1,800 (give or take) two cylinders are dropped (#2 & #5).

[99yam40]

inductive timing light will tell what timing is doing just to be sure it is in spec even at cruising speed when problem shows up.
it can also tell you if spark it hitting steady on the different plugs by moving it to the different plug wires.

I am not sure how to tell if all the injectors are fireing when they should be

[panasonic]

There are no alarms. I will confrim buzzer, but there is no indicator for over temp. T-stat were just replaced, and was water pump. Tell-tale shows great water flow. Wouldnt power reduction mode increse the idle to aprox 1100?

So this started right after water pump replacement? Tell tale can be indicating flow but still not enough flow through the rest of the block or a bad temp sensor.

[DennisG01]

So this started right after water pump replacement? Tell tale can be indicating flow but still not enough flow through the rest of the block or a bad temp sensor.

But... I believe an RPM reduction to decreased water flow would cause the buzzer to sound since it would be overheating. Or at least that's been my experience since it would be the temp sensor "complaining".

[panasonic]

But... I believe an RPM reduction to decreased water flow would cause the buzzer to sound since it would be overheating. Or at least that's been my experience since it would be the temp sensor "complaining".

I asked the OP if his alarm works and he didnt answer as far as I can see Dennis.

[Messikpe]

I agree on the water flow. Customer took it to a shop for water pumps, they told him he was good to go, and it rtan like this on the way home. He was so 'disenchanted' by the shop that he called me.
I will double check for temp issues. I have not heard a buzzer, so I'll confirm it even works, but the over temp indicator on the gauge was no showing. The telltake as amazing water flow, so I really doubt that is an issues. Also I've had the cowl off after returning to the dock, and suspect it she was 'That" hot i would have noticed