Slow loss of RPM at near WOT

[DennisG01]

In case my signature isn't available... 2000 Yamaha OX66 250HP SX250TXRY 61AX103847T

Before the forum "crash", I had a thread going about this and had noted that the vacuum gauge that was installed in the unused "out" port on the filter head was indicating hi vacuum - approximately 7" at idle and would climb another 5" or 6". I was at the end of my stay with the boat, so I didn't have time to fully diagnose.

Fast forward to now... here's the update, while skipping a bunch of the diagnosis... but the problem persists.

Last year I had a very noticeable/violent loss of at near WOT - turned out to be a single, bad LP. But when I lost RPM with that, it was very erratic/violent in the dropping of RPM's. This time it is VERY smooth - as if someone is slowing turning a dial to lower the RPM's. I can run 4,500 no problem (maybe a bit higher - but I didn't try it).

-- I plumbed a portable 3g tank directly to the filter on the side of the engine. Still had an issue.
-- I removed the vac guage from the boat-side filter head... it stayed at 7". Faulty gauge.
-- Ran with a fuel pressure gauge on the VST. When the problem happens, the pressure drops from the normal 36psi to about 28psi
-- I tested the fuel pressure regulator with a MityVac set to vacuum. The manual says that the PSI at the VST should change consistent with the amount of vacuum inches I was applying. 5" of vacuum caused a 2 or 3 psi drop.
-- The MityVac gauge held steady and did not drop.
-- Last year, I checked the VST filter - clean. I'm religious with fuel treatment/quality. Fuel recently drained from the VST is nearly as clear as water.

Gonna check the other two LP's (that weren't replaced last year) for good measure.

QUESTION... this drop in RPM's is vastly different than last years. CAN it be due to a bad LP? I'm trying to imagine how things are playing out in the VST to cause this smooth/slow drop in RPM and it just doesn't make sense to me. IF the fuel level was low in the VST, RPM drop should much more violent?

[DennisG01]

Adding info... There is ZERO loss of power to get on plane. The engine pulls as strong as it ever has. The only thing going on is a slow/smooth loss of RPM at/near WOT.

[99yam40]

is it the fuel rail pressure you are monitoring?

if the rail pressure drops when this problem shows up ,
I would think lower out of spec pressure means less fuel into the cylinders limiting RPM

[DennisG01]

Took the VST apart and pulled the fuel pressure regulator.

-- If it was blue-tinted water, you could drink the fuel - super clean
-- VST filter had only a few miniscule tidbits on it. Screen was perfect
-- FPR screen was very clean
-- Tested FPR by mouth using tip of tongue to check that it held pressure and vacuum... yup, I did that... just wanted to verify what I initially found with the MityVac

For good measure, I'll test the LP's when I get back from a boat trip with the kids.

For transparency's sake... it's actually my son doing most of the wrenching - I'm just sitting back and directing/pointing

[DennisG01]

is it the fuel rail pressure you are monitoring?

if the rail pressure drops when this problem shows up ,
I would think lower out of spec pressure means less fuel into the cylinders limiting RPM

Sorry - missed your post.

I was attaching a pressure gauge to the Schraeder valve that is on the VST tank.

I would "think" that the HP pump can't be the culprit, though, right? I mean, it doesn't know what the engine is doing - it's a "dumb" accessory - either on/off. If it was bad/failing, then the pressure would be low all the time.

[DennisG01]

I'm reading up on the O2 sensor's impact on fuel... well, "trying" to read up on this as it's hard to find factual info. But... could the O2 sensor be causing this issue at high RPM's only?

I believe the O2 test harness is just simply a connector to allow tapping into the grey & white leads to read voltage. That seems easy enough to make... where I am, it's hard to get parts in even a remotely good timeframe

[99yam40]

you need to find out why the pressure is dropping


think about it if the pressure drops and the ecu does not know that the rail pressure in not in spec, you have problems.
it depends on the pressure to stay in spec to deliver the proper amount of fuel

[boscoe99]

The fuel pressure regulator is connected to the intake manifold. It senses the manifold pressure rising and falling with variations in the throttle opening and the load on the propeller.

As the manifold pressure goes up the fuel pressure goes up. As the manifold pressure goes down the fuel pressure goes down.

Try it and see. Install your pressure gauge.

Turn the key on without starting the motor. I suspect fuel pressure will be 40 psi plus. Because the manifold pressure will be as high as it can be.

Then start and run the motor at an idle. The manifold pressure will drop. According to the manual the fuel pressure should be ~ 36 psi.

If you care to advance the throttle (manifold pressure will increase) and so too will the fuel pressure.

[DennisG01]

Boscoe, are you suggesting I do something different with measuring fuel PSI than I already did? I have normal, solid 36PSI during all RPM ranges... even WOT RPM for about 10 seconds... and then the PSI and RPM will start to drop.

[99yam40]

Boscoe, are you suggesting I do something different with measuring fuel PSI than I already did? I have normal, solid 36PSI during all RPM ranges... even WOT RPM for about 10 seconds... and then the PSI and RPM will start to drop.

he mentioned

"Turn the key on without starting the motor. I suspect fuel pressure will be 40 psi plus. Because the manifold pressure will be as high as it can be.
Then start and run the motor at an idle. The manifold pressure will drop. According to the manual the fuel pressure should be ~ 36 psi.If you care to advance the throttle (manifold pressure will increase) and so too will the fuel pressure."

does this happen?

I guess if the pressure regulator recycles too much fuel the pressure might drop or if the vst does not stay full enough it could run out of fuel to keep the pressure up.

only wat I can think of to measure the fuel in the VST is to turn the key off immediately when the RPMs drop and drain the VST to measure the amount of fuel that comes out and compare that to shutting down the motor while idling and drain it to compare

[DennisG01]

But... if the VST is getting low on fuel, then the HP pump will start pumping some air which will, in turn, produce a much different effect on the engine. It would behave as it did last year (see first post). Or... that's what seems to make logical sense to me, anyways?

I can certainly do what Boscoe said - I guess I was just curious if (or why?) that was needed given the stability of the fuel pressure at nearly all RPM's.

[99yam40]

I was just thinking what would cause low rail pressure.
pressure regulator or running a little low on fuel is all I could think of.

would not hurt to be checking the other lift pumps.
seems like I remember Rodbolt saying some motors are hard on lift pumps. But I do not remember which ones

[DennisG01]

I was just thinking what would cause low rail pressure.
pressure regulator or running a little low on fuel is all I could think of

And, truthfully, I'm not 100% sure on what I said... my "logic" COULD be flawed!

[Ferris11]

I was just thinking what would cause low rail pressure.
pressure regulator or running a little low on fuel is all I could think of.

would not hurt to be checking the other lift pumps.
seems like I remember Rodbolt saying some motors are hard on lift pumps. But I do not remember which ones

I did find the topic that you referred to where Rodbolt talked about some motors being hard on lift pumps and on certain motors. The topic was "2000 VX200TLRY Dies above 5000rpm"

https://www.yamahaoutboardparts.com/...-above-5000rpm

[DennisG01]

Thanks for fishing out that thread, Ferris. I read through it and his symptom is definitely different than mine. His engine essentially just died while mine just slowly slows down a bit.

I'll still check the LP's for good measure. I was out with the kids on a hike today at Acadia in ME. No boat stuff... but a heck of a good day!