SX250TXRZ 250HP OX66 Bogs down

[ReadyOarKnot]

I am looking for thoughts and some help.

I have twin 2001 OX66 250 HP. I was running on plane (4500 RPM) and i notice my fuel consumption started going up. The starboard engine was sucking more fuel (50%). A short time later the RPM decreased and i could only get 3600 RPM max. I ran at 1500 RPM for a half hour. I then went back up on plane and the boat ran fine. After a short time the same issue occurred then repeated. This all happened on Thursday (70 mile trip). The boat sat until Sunday for the return home. The problem happened once on the way home although the starboard engine was using about 20% more fuel than the port engine the entire ride.

The only thing i did was change the water separator and check to make sure the water indicator in the engine filter was not floating.

Any ideas what to check?

LP fuel pumps change 50 hours ago
VST clean 50 hours ago
fuel filter change 50 hours ago
plugs 5 hours

Thanks in advance.

[99yam40]

might have the injectors flow,spray pattern tested
if they are not closing off properly, sticking open it might cause something like this

just a guess

[Spartan811]

Hey... ReadyOarKnot,

I got the same problem here.. on my 1998 ox66 250HP..
Doesn't want to go over 2300 RPM's coasts at 15-18rpm Ok..

You mentioned that you changed...
"The only thing I did was change the water separator....

That's weird bc I changed mine and hade the same problem, noticed its
was hissing air.. losing fuel back flow.. tightened put problem remained.

Did You figure out what the problem was? Thanks for any help!

-Maybe RODBOLT can chime in here as well....?

[rodbolt17]

typically bogging is loss of rail pressure.
occasionally to much rail pressure,clogged fuel return filter.

typically if you see fuel flow increase for the same RPM the ECU is trying to compensate for something. either the RPM is not at the map for the TPS and CPS feedback or it sees a loss of 12V and is compensating by increasing injector on time.

[ReadyOarKnot]

thanks for the input.

tested O2 sensor voltage. stuck at 0.4 v. soaked overnight in Ring Free and reinstalled. voltage varies now 0.3 to 0.6 but slow to change. this is on the starboard engine. I compared to the port engine and the range is 0.2 to 0.65 volts but the voltage change very fasts.

I was not able to duplicate the problem but the fuel pressure at the schrader valve on both engines was about 32 PSI at idle.

no codes were available on the winky blinky

i will have to look up how to test throttle position and crankshaft position sensor

[zenoahphobic]

typically bogging is loss of rail pressure.
occasionally to much rail pressure,clogged fuel return filter.

typically if you see fuel flow increase for the same RPM the ECU is trying to compensate for something. either the RPM is not at the map for the TPS and CPS feedback or it sees a loss of 12V and is compensating by increasing injector on time.

But does this lead to an increase of 50% in fuel use, but no increase in power, as the OP advised?

[rodbolt17]

the O2 sensor is simply a fuel trim.
wont cause bogging but it can cause a lean condition above 4000 RPM.
the only time a code will be displayed is if it is active.
the ECU does NOT store codes.

rail pressure at idle is irrelavent.
rail pressure at the failure RPM and load is relavent.

[ReadyOarKnot]

Thanks Rod - question about :"the O2 sensor is simply a fuel trim."

if the engine is running rich (i have black streaks out the exhaust) and the sensor was stuck at 0.4V (at time of testing not failure) doesn't that point to the O2 sensor (note - i don't know the voltage at failure since i have not been able to reproduce it)? Have you ever seen the o2 sensor indicate a lean condition telling the CDI to increase the injection length and flooding the engine with too much fuel?

"here is the snip from the tech bulletin:
When the engine is running too rich, the O2 sensor
voltage goes higher, which causes the CDI to
reduce fuel injection duration. When the engine
is running lean, the O2 sensor voltage goes
lower, and the CDI will increase fuel injection
duration. "

I can also add that the motor in question (starboard) burning between 10% and 15% more fuel than the port engine. is that normal?

on a side note - checked all compression and spark plug gaps - all is good.

the O2 sensor is simply a fuel trim.
wont cause bogging but it can cause a lean condition above 4000 RPM.
the only time a code will be displayed is if it is active.
the ECU does NOT store codes.

rail pressure at idle is irrelevant.
rail pressure at the failure RPM and load is relevant.

[rodbolt17]

at .4v it is lean.
.9v is pig rich.
but the O2 sensor is not your main issue.
the O2 sensor is NOT monitored for any codes.

the O2 sensor feedback is simply used to "trim" the fuel delivery.

[ReadyOarKnot]

Thanks -

will look at the sensors above and 12V supply

at .4v it is lean.
.9v is pig rich.
but the O2 sensor is not your main issue.
the O2 sensor is NOT monitored for any codes.

the O2 sensor feedback is simply used to "trim" the fuel delivery.

[Spartan811]

typically bogging is loss of rail pressure.
occasionally to much rail pressure,clogged fuel return filter.

typically if you see fuel flow increase for the same RPM the ECU is trying to compensate for something. either the RPM is not at the map for the TPS and CPS feedback or it sees a loss of 12V and is compensating by increasing injector on time.

Thanks for the input... Rodbolt..!!

Spartan811

[ReadyOarKnot]

so i purchased the two test harnesses so i could test the TPS and CPS
YB-06857 TPS TEST HARNESS
YB-06867 TEST HARNESS

the TPS was easy and i wanted to compare the differences between the two engines. Here are the results:
Starboard
Neutral 0.624 V
Full Open 3.897 V

Port
Neutral 0.628 V
Full Open 3.830 V

Is this difference enough to cause 15-20% more fuel consumption in the starboard than the port engine?

I need help with measuring the CPS. I only get a few mV. Cranking i am supposed to see several volts and at 1500 RPM under load 3 V, i think (from memory). What am I doing wrong?

[rodbolt17]

well or starters quit chasing wabbits and hunting ghosties.

the TPS wont cause a bogging nor a surging.
neither will the CPS.

lets look at how that system monitors fuel flow.

unlike modern EFI with command link this system uses a physical paddle wheel.
the faster the paddle wheel spins the quicker the 5V square wave and the more fuel flow is indicated.

now how can we increase flow?
bust a fuel lift pump diaphragm.
lose some battery voltage to the ECU.
clog the VST return filter(located under the fuel regulator).
have some injectors not closing.

that is about the only ways to increase fuel flow on that setup.

remember the O2 sensor is simply fuel trim, it overshoots and undershoots constantly.
the feed back voltage should stay rapidly changing between about .35V and .7V at 3500 RPM or so.

my testing would be checking spark and compression.
test the spark plug caps.
carefully test/replace the lift pumps.
test fuel rail pressure at the failure RPM and load.
at or near WOT that motor can guzzle 25+ gallons an hour.
if rail pressure goes much above 35 PSI at or near WOT look for a clogged return filter or a faulty regulator.
if it drops much below 32 PSI suspect a fuel restriction damaged lift pumps or an air leak.

[ReadyOarKnot]

thanks Rodbolt. I will check it out.

I did find a loose ground on the battery for the starboard engine. was not able to take the boat out to test. will report back.

I thought you mentioned, above, the TPS and CPS could cause this if the CPS or TPS was outside the ECU map.

well or starters quit chasing wabbits and hunting ghosties.

the TPS wont cause a bogging nor a surging.
neither will the CPS.

lets look at how that system monitors fuel flow.

unlike modern EFI with command link this system uses a physical paddle wheel.
the faster the paddle wheel spins the quicker the 5V square wave and the more fuel flow is indicated.

now how can we increase flow?
bust a fuel lift pump diaphragm.
lose some battery voltage to the ECU.
clog the VST return filter(located under the fuel regulator).
have some injectors not closing.

that is about the only ways to increase fuel flow on that setup.

remember the O2 sensor is simply fuel trim, it overshoots and undershoots constantly.
the feed back voltage should stay rapidly changing between about .35V and .7V at 3500 RPM or so.

my testing would be checking spark and compression.
test the spark plug caps.
carefully test/replace the lift pumps.
test fuel rail pressure at the failure RPM and load.
at or near WOT that motor can guzzle 25+ gallons an hour.
if rail pressure goes much above 35 PSI at or near WOT look for a clogged return filter or a faulty regulator.
if it drops much below 32 PSI suspect a fuel restriction damaged lift pumps or an air leak.