2003 f115 fuel pressure regulator

[beachpoint]

I have a 2003 f115 I have serviced the fuel system,injectors cleaned new high pressure fuel pump and flushed entire system. I have owned this motor since new and cleaned the fuel system yearly, but it will not get full power like the screen is clogged in the pump. I checked the pressure on the rail got about 40 psi idle to about half throttle as I advance to WOT it drops to about 20 psi if I disconnect the vacume line to the pressure regulator the pressure go to about 42 psi all the way to WOT and the boat runs fine. My thought is the regulator is bad and bypassing at high RPM

[boscoe99]

The pressure regulator has a sense line that is attached to the intake manifold, to measure manifold pressure. As the manifold pressure decreases and increases so too does the fuel pressure.

With the motor running at idle speed the fuel rail pressure is supposed to be approximately 36.3 psi. As the throttle opens the manifold pressure rises and so too should the fuel pressure.

Yamaha does not specify a test procedure to compare fuel pressure to manifold pressure. The check they specify is only to apply a vacuum gauge to the pressure regulator to verify that as the manifold pressure lowers so too does the fuel pressure.

Based on what you are saying (that the fuel pressure decreases as the manifold pressure increases with the throttle being opened) it would seem that something is amiss with the pressure regulator.

[rodbolt17]

actually, not quite right.

the vacuum sense line is part of the acceleration control.

as the throttle is rapidly opened the ECU increases injector on time and goe to a batch injection on a cycle.

as the throttle opens rapidly intake pressure increases(vacuum drops)
and the rail pressure should increase by about 5 psi .
this keeps rail pressure steady while both synchroneous and asynchroneous injection is happening.

doesn't matter if its carbed or EFI, open the throttles rapidly and you MUST add fuel rapidly.
carbs use an accelerator pump, EFI uses the TPS signal to the ECU and the fuel pressure regulator vacuum sense line.

I have never seen a regulator failure, doesn't mean it cant happen.
but I have seen the lift pump cannot keep the VST full.

Yamaha is proud of that regulator.
I have replaced a few due to rusting away.

[boscoe99]

actually, not quite right.

the vacuum sense line is part of the acceleration control.

I'm gonna have to "not quite right" back at you.

The variable pressure regulator is not part of the acceleration control system. The regulator is simply designed to maintain a given pressure differential between fuel pressure and intake manifold pressure. Let's assume an absolute fuel pressure of 55 psia with the motor not running. The intake manifold pressure will be nominally 15 psi which results is a pressure differential of 40 psi (55 psia minus 15 psia = 40 psia).

The motor is started and run at an idle in which case let's say the intake manifold pressure drops to 6 psi. The variable pressure regulator being tied to the intake manifold pressure drops from 55 psia to 46 psia. Therefore, the pressure differential between the fuel pressure and the intake manifold air pressure remains at 40 psia.

The ECU is expecting the pressure regulator to make the necessary adjustments in order to maintain a certain pressure differential. The ECU then considers manifold air pressure, engine RPM and throttle position angle openings and rates of change, to vary fuel delivery and timing for acceleration control.

All of this does not help the owner of the motor. What would have helped is if Yamaha had a range of values for fuel pressure as a function of varying manifold pressures in the service manual. All they basically say is that the fuel pressure should go lower as the intake manifold pressure goes lower.

He states his problem as "as I advance to WOT it drops to about 20 psi". This is contrary to the way a fuel pressure regulator tied to an intake manifold is supposed to work. As the motor is advanced to WOT the intake manifold air pressure should be increasing and so too should the fuel pressure. His dropping to 20 psi indicates a problem with the pressure regulator IMO.

[99yam40]

where"s the pop corn

[boscoe99]

I will not be partaking of any "peeing" contest. Said my points, that's it from me.

[99yam40]

Not trying to get anything going , just wondering.

But the drop to 20 PSI at WOT is the key I believe

[rodbolt17]

and Boscoe is wrong. gotta call ya out on this one.

ther ONLY reason the regulator has a sense line is for RAPID acceleration.
after that its simply out of the loop.

but other than that the sense line serves NO function.
leave it off and it will run a bit rich at idle but no noticeable difference at cruise, other than fuel consumption will be slightly higher.

the REGULATOR does what you describe.
the sense port is ONLY for rapid acceleration.
it adds about 5 PSI to the rail when vacuum dumps.

but the ONLY function of that vacuum line is acceleration.
the rest is by the spring loaded regulator.

as the injectors fire and rail pressure varies the regulators opens and closes to maintain rail pressure.

however at RAPID throttle opening the vacuum on the regulator diaphragm drops, this allows about another 5 PSI of rail pressure.

don't belive me, simply hook up your pressure tester, start the motor, take a reading, UNPLUG that sense line and watch pressure increase.

this stuff aint rocket science.

and I do have a few weeks experience on most all the instructors, field service reps and the 3 jap engineers that I had to spend a day with last month.

[beachpoint]

2003 f115

The part I was not sure of is if I remove the sensing line I get about 41 psi boat runs like never before. So my thought was plenty of fuel getting to the vst I did change the lift pump 2 years ago, seems to be working well, also checked prime ball move fuel it stays hard. I was leaning toward the regulator since with the sensing line removed it holds pressure and runs fine, as soon as I reconnect it the pressure drops and no power. I figured I will give the regulator a try the old one does not have much left anyway from rusting. I was also wondering where the return fuel goes Thanks everyone for all your help

[BoatmanNH]

I know this is an old post. What resolved your problem?

i Have the exact same problem. I remove the sense line and engine runs great. Connect the line and the engine falls on its face.

[boscoe99]

Five plus years later I still shake my head when I read this thread. Don't understand it.

[boscoe99]

I know this is an old post. What resolved your problem?

i Have the exact same problem. I remove the sense line and engine runs great. Connect the line and the engine falls on its face.

beachpoint has long since checked out. Has not posted in over 5 years.

[BoatmanNH]

My situation is the same as beachpoints except I have a 2004 F150. Did the same thing as him in regards to the fuel system. I have a rough idle and it lacks power. If I remove the sense line the engine smooths right out and runs normal. I checked the FPR and it seems to be working correctly. As soon as I hook the sense line to the intake it runs like crap. I checked the pressure sensor valve and that seems to be in specs. There’s no codes and everything seems to be in spec. Scratching my head on this one.

[boscoe99]

Did you put a vacuum pump on the regulator? For every psi drop of air pressure there should be a corresponding drop in fuel pressure.

[BoatmanNH]

Yes, vacuum test the regulator and fuel psi went down.