OX66 Odd top-end RPM problem. Rodbolt can you address?

[steamin53]

First let me say this is a repower from a 93 200 to a new to me 99 OX66 250 Salt Water Series. I've only had it in the water twice since installation.

Initially I experienced a drop off in RPM after about 10 min high speed running which on inspection once back at the house I attributed to a leaking drain seal on the water separator filter. I replaced the filter with a non-draining type as an expedient fix.

Yesterday I tried the engine again. It seemed ok, but after about 15 minutes of ~5K to 5.6K rpm operation it seemed to bog down at full throttle but ran fairly well at mid range RPM.

Now the weird problem.

At high speed ~5K up if I hit the power trim switch up or down it instantly will significantly increase engine RPM as long as the trim motor is energized then drop once the switch is released.

Could it be a bad ground causing the RPM problems?

I've double checked battery connections and found them clean and done a visual of all ground connections I can see on the engine without removing component covers. The tach shows 14+ volts at high rpm.

 

[rodbolt]

Re: OX66 Odd top-end RPM problem. Rodbolt can you address?

not hard, you just need a fuel rail test guage, can rent one at most auto parts stores and a digital voltmeter.
slip the meter pins up the red and blue wires on the pump.
attach the pressure tester.
start the engine.
fuel pressure should be about 35 psi and voltage about 9v.
take off and run, pressure should stay about 35PSI at all RPM's and voltage above about 1200 will increase to about 11V or better.
now when it falls on its face obseve both readings.
should still have 35PSI and above 11V.
if less than 11V check all grounds and hots between everything.
if 11V or above and less than 35 PSI check all fuel system components for restrictions,proper lift pump outlet pressures and the VST screen for debris.
if all check out then send your injectors out for service.
the OX66 motors can sense low battery voltage(big current draw with the trim) and increase the injector on time allowing more fuel.
the ign system is basically independant of battery voltage.

 

[steamin53]

Re: OX66 Odd top-end RPM problem. Rodbolt can you address?

Rodbolt thanks for your reply. Very helpful.

Since I posted I did clean the o2 sensor and it's tube (joint) and found it to be what I would consider in pretty good shape without as much carbon on it as others have shown in pics. The engine does seem to run a little better afterward but did still tend to bog down at or near full throttle position. At one point it seemed to stall somewhat and lost several hundred RPM (kind of like a carbed engine with a partially blocked high speed jet) but corrected with several rapid throttle position changes while underway. It wound up running around 5K RPM but would bog down above that; initially I could easily get 5.5K to 5.7K.

Battery voltage does indeed drop with the trim motor energized but not below 11.5V. However, that's read by my GPS unit and not at the pump which of course could be much different. The engine didn't react as violently to trim changes in the last test run as it had prior to cleaning the O2 sensor.

Would you suspect fuel rail pressure problems if the engine does at least some of the time run up to full RPM? I do note that after running at high rpm for 10 min. or so that the idle drops a good bit; down from ~800 to 500 in neutral. It sure sounds like fuel starvation doesn't it?

Tonight I undertook to check and clean the VST filter. Interestingly enough I found that there were a couple of cover screws rather loose. There were no apparent fuel leaks however. Now I have no idea if that would have affected top end performance or not since as I understand it the VST system is pressurized at least to the extend that the low pressure pumps feed it. While the filter was slightly "discolored" it had no significant amount of debris on it. Just the slightest small particles almost impossible to see individually with the naked eye. A hot soap and water wash followed by some carb cleaner and a blow off with dry air may have improved it...hard to tell really except by color change.

I'll be testing the engine afloat tomorrow. If it doesn't seem better I'll next rebuild the low pressure pumps and follow with spark plugs although I was hoping to decarbonize before changing plugs. I hope I don't have to do the fuel rail pressure/voltage test as I don't have any hands to help right now and I'll bet that's a tough nut to pull off by yourself while underway at high speed.

I have also considered the injector screens (filters) but it seems that if they were the problem the problem would be constant and I'd never be able to achieve the full engine RPM. Would you agree?

It's tough when you have a used engine and no maintenance history to rely on. I guess all pm is appropriate to do just for peace of mind. Someone has pulled maintenance on this engine as it has the new type O2 sensor tube and those cover screws on the VST surely didn't get loose by themselves.

Steve

 

[steamin53]

Re: OX66 Odd top-end RPM problem. Rodbolt can you address?

Well after cleaning the VST tank and injector pump filter it ran better but still would occasionally loose RPM and would consistently loose power and RPM at full throttle. More troubleshooting revealed 1 low pressure pump leaking when the primer bulb was compressed tight and held. I replaced diaphrams in that pump and ran the engine again. Now she will run at 5700 and most of the time will hold the RPM but only if RPM is very gradually increased. If at ~5700 and I advance the throttle more it consistently looses power and bogs down. Strange. Is there a governor on this engine? That's how it acts...as if the ECM were somehow lowering rpm to keep it from going above 5700.

Could the throttle position switch cause such a problem?

I will rebuild the other two pumps...who knows it may actually be the problem.

I'll also buy a fuel rail pressure tester next week and check pressure underway.

The increase in RPM with the trim motor engaged seems to not be an issue now.

Steve

 

[rodbolt]

Re: OX66 Odd top-end RPM problem. Rodbolt can you address?

there are 3 lift pumps on a 3.1L for a reason. if one is bad odds are high the other 2 are weak/failing as well.
as you have noticed that engine is a tad thirsty at or near 5500 RPM which is all its supposed to turn.
I try to prop them for 5300 max and let them ride.

 

[steamin53]

Re: OX66 Odd top-end RPM problem. Rodbolt can you address?

Thanks again Rodbolt. You're right the spec is 5500 top-end max RPM. I have heard from others that 5600 to 5700 is good but I'm not trying to blow up a 1999 engine.

All of my tests have been without fishing gear, with one passenger, and 1/2 of less tank of fuel. I'm confident with a full load of fuel, ice, gear, and fishermen it won't turn 5700 and even if it would I wouldn't/couldn't in the Gulf...too rough. I sort of think that it might be better to be a little on the light side with the prop than just right at high speed with a less than full load. That way I'll have more power in swells etc. Speed is NOT my fascination.

Still though, it does seem odd that it drops off in the final bit of throttle advance at the high end. Maby it is actually a fuel pressure issue to the injector pump.

I will say that today we did a 50 mile round trip in the Tennessee River without a hitch. Even there I rarely exceeded 4700 RPM and more often was at around 4400.

I sure do agree on the fuel addiction of that engine. Much less economy than the 1993 200 it replaced.

I'll post back once I've solved the problem or run into a brick wall.


Hey one more question. I need to replace the tach and speedo. Do you have any idea what the correct part numbers are for a 93 engine. I adapted the harness at the engine using yamaha adapter cables so I still have the original 1993 hook-ups at the other end in the console. I've been searching ebay but I'm not sure what part numbers are going to work.


Steve

 

[ctrock29]

Did you ever figure out what changed rpm with trim switch? My 2000. Ox66 250 is doing the same thing