2004 F115C won't rev past 3000rpm

[Hokie 59]

I've been working at solving this problem with my 2004 F115C Yamaha and can't seem to put my finger on it. After doing all the service stuff - new plugs, fuel filter, lower unit oil change, engine oil and filter change - I can't seem to get it to rev up to 6000 rpm like it has always done.
Then today it occurred to me that the Schrader valve may be there for the same reason it's on my Duramax diesel - to purge the air from the fuel rail.
Does this seem like a reasonable remedy? If so, back to the lake I go and try to purge the air. - Al

[pstephens46]

I believe these systems purge themselves. I took mine apart and drained all the fuel while doing maintenance awhile back. I primed with the fuel bulb and it started normally. That valve is used by most to check fuel pressure at various rpm. Sounds like you need to do exactly that when the problem occurs. Sure sounds like you have a fuel issue.

How long has it been sitting since it last ran correctly?

[ausnoelm]

I wonder how changing the oil and filter, and the lower unit oil was hoped to fix a low RPM issue?

[fairdeal]

I wonder how changing the oil and filter, and the lower unit oil was hoped to fix a low RPM issue?

I also wondered - but was too polite to bring it up.....

[99yam40]

I do not think he said that caused the low RPM, it just happened after he did all of that service.

OP
I think I read the valve is there to allow you to let air in while draining the VST, otherwise it would not drain all of the fuel.
also to get a pressure reading.

I think you need to be looking for something else causing the reduced RPM.

look for codes, wires unplugged, and monitor fuel pressures to start out with.
Draining VST, servicing the VST screen and then maybe injectors if pressures are staying in spec

[boscoe99]

The schraeder valve is there moreso to check the fuel pressure when the motor is running. It does help to speed up draining of the VST.

Put a fuel pressure gauge on the motor and test for fuel pressure. See if the system maintains fuel pressure at all RPM's. If the fuel pressure falls off at or about 3000 RPM's find out why the pump is being starved for fuel. If the fuel pressure does not fall off at 3000 RPM then I would have the injectors cleaned.

I would first however verify that the control cable is fully engaged and that the throttle valves are actually fully opening when the throttle handle is fully advanced.

[Hokie 59]

Sorry I wasn't clear on the service stuff.

99yam40 is correct. I did the services 'cause it was time to do them, but it had been running fine prior to the services.

I did go to the lake today, bleed the schraeder valve and got gas. So my theory was wrong there.

Now on to the recommended stuff like draining and cleaning the VST screen. That appears to be complicated - I have gotten as far as removing the "silencer" and the flywheel cover. Seems that one of the threads here had a video step-by-step of how to get to the VST. Will try to find it.

Thanks ever so much for your comments. You folks are the best!

[ausnoelm]

OK, just so we are all clear, the motor was OK, then you did routine maintenance, now it won't rev over 3,000RPM? are you trying it in the water, or in neutral on the hose?

[pstephens46]

OK, just so we are all clear, the motor was OK, then you did routine maintenance, now it won't rev over 3,000RPM? are you trying it in the water, or in neutral on the hose?

I asked previously how long the motor had been sitting. Who knows?

And I hope the op is not revving the motor in neutral.....

[TownsendsFJR1300]

OK, just so we are all clear, the motor was OK, then you did routine maintenance, now it won't rev over 3,000RPM?

Great catch and EXTREMELY important for proceeding forward.

Op, did the engine run FINE BEFORE your maintenance work?

[99yam40]

Sorry I wasn't clear on the service stuff.

99yam40 is correct. I did the services 'cause it was time to do them, but it had been running fine prior to the services.

I did go to the lake today, bleed the schraeder valve and got gas. So my theory was wrong there.

I believe he alread said it was

[pstephens46]

Betting it was 6 months prior to working on it.

[boscoe99]

Sounds to me as if the motor was running poorly and all of the service work was done in an attempt to shotgun a solution.

[99yam40]

Well some people may wait for something to go wrong before servicing their motor, but he said it was just time to do it and it was running well before he put his hands on it.

So if this is true he may have did something to something that is causing the RPM problem.

YDS hooked up would probably point him in a direction if the was a wiring or sensor failure do to damage to it while doing some work under the hood.

Or start looking for damaged wiring,connector, or sensor.

[TBrutlag]

I had the same problem last year with my f115. Mine turned out to be the screens in the injectors.