OX 66 RPM drop

[ManateeBWN]

I do have the following problem:
The engine runs fine for around 35 minutes at 4000 RPM. Then it starts to drop in RPM down to about 1500 and immediately comes back to 4000. This behaviour gets worseafter a couple of minutes. Once you throttle back to say 3500 all is fine again - until the same starts at 3500. And so on.
If you try to throttle up to higher RPMs the only effect is that the RPM drops are getting more violent and more frequent.
So you end up limping home with 1500-1900 RPM.
All happens only after ~35 minutes. Leads me to look for a temperature related problem...
I have changed all the fuel lines and cleaned all the filters. The impeller is brand new.
I have no idea where to start, because all the "standard" items like O2 sensor would not be temperature related...
Any help appreciated!

[99yam40]

sounds fuel related.
Monitor fuel pressures and vacuum

If no problem found with that then might check to see if you alarm system is working and monitor timing to make sure it is not changing

[ManateeBWN]

Will try to get fuel pressure test equipment.

Alarm tested and o.k.

[ManateeBWN]

Bought a fuel pressure gauge, but it did not include a schrader valve adapter (discovered that at the boat). So I opened the "fuel collector cell" that contains the high pressure pump and checked: Found the cell full, floater free and easy to move, filter with very little dirt on it, little water and dirt in the fuel cell. Cleaned everything and reassembled. Still no idea what the fuel pressure is - need to get a schrader valve adapter first.
Can anyone tell me what I should measure? 35psi correct?
Where do I measure the vacuum?

Removed the temp sensor. Measures open when removed and cold. when installed measures closed?!

Removed the oxygen sensor. Looked extremely clean to me. Engine has only 200 hours, but still very good looking. Resistance reading when removed 5 Ohms (or I am to dumb to measure) between the two white lines. That seems to be a little low?

How exactly and where do I check timing?

In short - still no idea and no hint...
Anybody can help?

[rodbolt17]

first post a model number so we know which ox66 system you have.
the vx is different from the sx and the 2.6l system is slightly different from the 3.1L.
next you dont test o2 sensors whith an ohm meter.
and if rail pressure drops below about 25 PSI the engine shut down is violent.
quick test for the alarm circuits is simply pull the stop lanyard and crank the engine, alarm sould sound instantly.
google a bit using rodbolt o2 sensor and rodbolt17 o2 sensor.
in the past 9 years I have posted on this and other forums extensivly on the ox66 engines.

[ManateeBWN]

I have the SX250TXRA year 2002.
Ohm measurement was done - ignorant as I am - believing that it would be the right way to check the sensor.
For the rail pressure I am still dumb as can be - still missing a fit for my fuel p gauge on the schrader valve. I have ordered the Yam fuel p gauge and am looking for the simple part to connect my gauge, but simply don´t have it available right now.
But then shutdown is not shut down. It drops violently from 4K to 1.5K and fires back up. Then it runs o.k. for a couple of minutes and does the same thing. Intervalls get shorter and shorter. Throttling back gives you a short break and the whole thing starts all over at lower revs. Engine never quits but forces you to throttle back.
Alarm is checked o.k. - both for oil and temp.
Did some googling already, will carry on. Hoped to see YOU getting involved!

Do you think it makes sense to try to measure all the sensors?
I tend to believe it´s either a fuel related or an ignition related thing.
Ignition would be:
- any sensor faulty and getting worse with rising temp
- ECU faulty and getting worse with rising temp
Fuel would be:
- any low press pump (or several) gone (don´t want to take them off before I received new ones)
- any high press pump issue
Need to measure rail press - agree!! Thanks for the 25psi value. Is 30 to 35 normal for "cruise"?

Thanks for caring - I´ll switch into google mode for time being.
-

[rodbolt17]

ok a saltwater series ox66 3.1L.
o2 sensor claenibg and TESTING is maint.
its not your issue.
your describing a typical fuel starvation.
the 3.1L motors eat lift pumps.
they need to be replaced about every 3 years as maint.
you will also need to check fuel system vacum at the lower pump intake.
you can tee in between the engine filter and the pumps.
max allowable vacum is 6" Hg.
thats at wide open throttle not idle.
also remove the inline check valve thats between the filter and the lift pumps.
unscrew it,dump the guts and reassemble it or use a hose mender and replace it.

[ManateeBWN]

Roger that.

Bought motor about 1 year ago - run time about 20 hrs since. No clue about last lift pump replacement - ordered as expect need to change.
Try to do the WOT run tomorrow. Would that mean lift pumps don´t fill VST - VST does not pride enough fuel - getting worse with broken diaphragm getting warm = more flexible?

Check vlv is done - guess I did not mention.

Thanks!

[ManateeBWN]

o.k.
Measured the fuel P at idle - 33psi. Sounds reasonable.
Cleaned all spark plugs, measured the gap, new ones are on order (measurements are o.k but age unknown).
Checked lift pumps - o.k.
Wanted to check O² sensor voltage and fuel press when prob occurs.

Took the boat out for 1:10 and had no probs! The only thing I really did was cleaning the O² sensor.

Another test ride in 2 days - maybe the O² sensor was the solution?

[ManateeBWN]

Sh....

Alright - test run for about 3 1/2 hours without problems yesterday.

Then we went out today for about 1 hour and had the same problem again. We ended up limping home with 1800 RPM max and occasionally dying.

I measured the O² voltage at 0,3V at idle, then the temp sensor (o.k.) and the fuel pressure at the schrader valve 33psi at idle.
Throttling up under load fuel pressure at the schrader valve broke down to < 9psi when the cutouts occured.
I tried pumping the ball at idle until it was hard and throttle up while pumping. Engine started to cut out with a hard ball. That makes me believe the problem is not filling the VST, i.e. low pressure fuel system is not the troublemaker.
I figure I do have a problem with either:
- the high pressure fuel pump, or
- the pressure regulator valve on top of the VST, or
- the high pressure fuel pump filter (which I cleaned about 4 -5 hours ago and still find it clean).

Any comments where to start or what to test would be highly appreciated!
Thanks!

[ManateeBWN]

How do I check the HP fuel pump resistor?
How do I check the main relay?
Any ideas?

[rodbolt17]

the resistor is out of the circuit above 1200 RPM.
the relay can be monitored with a voltmeter.

[ManateeBWN]

Thanks Rodbolt17,

so I am looking at a faulty relay, a faulty VST valve or a faulty HP fuel pump then?!
Is the relay a four contact standard relay (i.e. no circuit deenergized, circuit energized)?
Is the valve a common trouble maker? Or should I go ahead and buy a new fuel pump right away?

[ManateeBWN]

Took me a while to get some work done.
I cleaned the VAT valve filter, installed the VST again, checked the main relay and found it o.k. on the work bench. Did not have a chance to run the engine on the water yet.
Having those serious fuel pressure drops from ~35psi to below 10psi I figure the problem is the fuel pump (temp seizure) or the power supply of the fuel pump - which seems much more likely to me. Does anyone have a wiring diagram of the SX250TSRA or does anyone know how the fuel pump is wired?
Thanks!

[rodbolt17]

lack of fuel in the VST will create a pressure drop as well.
anytime the key is on you will have 12V between the red pump lead and engine block ground.
the operating ground path is selected by the ECU.
the ECU will ground the pump blue wire through the resistor below 1200 RPM.
above 1200 RPM the ECU grounds the blue wire internally to the ECU.
poor ECU grounds will cause the ECU to drop out.
monitor pump current draw, anything above about 6 amps will cause the ECU to drop out.