03 F115 loss of rpm's

[Mueller08]

My 03 F115 that is on my Lund explorer 1800 will only get to 5000 rpms and 33mph when I used to get 42 mph. I have check the vst filter, clean except for a little sandy grit that rinsed off easily. Had injectors cleaned and tested, they were ok,they went from a flow of 228 to 234 after cleaning. I checked low pressure pump diaphram ,it seemed fine. I checked fuel pressure at the fuel rail, it was 40 psi when I turn on the key,at idle,and when under a load it jumps up to around 45 psi. I ran it with an auxilary tank and had the same result. Am I missing something? I have the original prop on the motor that it came with. Other than the loss of top end rpms the motor runs good, idles fine, dose not die or hesitate when accelerating. Thanks in advance for any replies!!

[99yam40]

what is the fuel rail pressure at WOT where it is having the problem?

[boscoe99]

My 03 F115 that is on my Lund explorer 1800 will only get to 5000 rpms and 33mph when I used to get 42 mph. I have check the vst filter, clean except for a little sandy grit that rinsed off easily. Had injectors cleaned and tested, they were ok,they went from a flow of 228 to 234 after cleaning. I checked low pressure pump diaphram ,it seemed fine. I checked fuel pressure at the fuel rail, it was 40 psi when I turn on the key,at idle,and when under a load it jumps up to around 45 psi. I ran it with an auxilary tank and had the same result. Am I missing something? I have the original prop on the motor that it came with. Other than the loss of top end rpms the motor runs good, idles fine, dose not die or hesitate when accelerating. Thanks in advance for any replies!!

The pressure regulator and or the input to it is out of specification.

The pressure regulator is plumbed to intake manifold pressure. As the manifold pressure rises and falls with the throttle valves opening and closing the fuel pressure should change accordingly.

With the key on only (maximum manifold pressure), the fuel pressure should be nominally 44.1 psi (its maximum).

With the motor running at an idle RPM (lowest possible manifold pressure) the fuel pressure should be nominally 35.6 psi (lowest fuel pressure).

As the throttle is advance the manifold pressure rises and so too should the fuel pressure. As the throttle is retarded the manifold pressure decreases and so to should the fuel pressure.

Not sure in the pressure regulator not being within specification is your problem or not. Inspect the plumbing between the pressure regulator and the intake manifold. Inspect the fuel return line from the FI rail back to the VST.

[Mueller08]

I am not sure what my rpm was at 42 mph , I would guess around 58 to5900 but I can tell from the sound of the motor i should be getting a lot more rpm's, this motor used to scream! I have also change the canister fuel filter at the back of the motor. Are there other filters I should check? I do not have a water separator filter.

[rodbolt17]

well lets see.
at key on engine off manifold pressure is at its peak.
so we see about 44 PSI on the rail.
at or near WOT manifold pressure is again at its peak.
so I would expect about 44 PSI on the rail.
that vacuum line is simply part of the accelerator function.
with a vacuum applied it lowers rail pressure a bit. 5-8 PSI .
at rapid acceleration manifold vacuum drops(or pressure increases) and rail pressure spikes.
at a given injector on time if you spike rail pressure you get more fuel.

[99yam40]

Op looks like you need to check calibration on your pressure gauge to make sure it is accurate and retest.
make sure you read at WOT too this time as that is what you are saying the problem is.

IF key on motor off pressure is in spec but WOT pressure is low then I would look for a restriction.
If idle and acceleration are off then look at pressure regulator maybe.

I may be wrong

[Mueller08]

Thanks for the replys! I'm thinking about replacing the fuel lines on my motor. A auto mechanic buddy said my fuel line on the motor feels soft. Could soft fuel line cause this lack of top end rpm's? Any tips you could share before I get into replacing these line? Also the fuel line I've seen lately has a clear liner in side the fuel line that likes to bunch up when you push a fitting into it. Where can I get good OEM Yamaha fuel line? Thanks.