Fuel Pressure regulators

[IRBBoater]

The mechanic said that as long as the pressure stayed the same under a slight load and did not drop below idle pressure it was good. What should the pressure be at WOT?

[99yam40]

it should stay the same all of the time,in spec

edit
Ok so the pressure will rise during initial start and acceleration.
pressure may drop during deceleration. but measuring pressure when RPM has stabilized at any point should give the same pressure reading.
is that better?

[rodbolt17]

it should be the same at WOT as at idle.

[boscoe99]

Some folks need to rethink statements recently made.

The fuel pressure varies as a function of manifold pressure. At low manifold pressure (idle RPM) the fuel pressure will be low whereas at high manifold pressure (WOT RPM) the fuel pressure will be high. Within specifications of course.

The pressure regulator is plumbed to the intake manifold. As the pressure rises and falls within the intake system this pressure results in a change to fuel pressure.

[IRBBoater]

Fuel pressure

Does a increase in total weight of the boat have a lowering effect on being able to reach max RPM or just a result of lower MPH?

[99yam40]

I would think that the change in vacuum and pressure would be during acceleration and deceleration, but once running at what ever RPM, once steady the vacuum and pressure would stabilize.

OP
At WOT more fuel is being used, and if the system cannot keep up the pressure could drop. that is what you are looking for.

if the fuel pressure stays in spec, then you need to be looking at other things.
why did you only clean one set of injectors?

[99yam40]

Does a increase in total weight of the boat have a lowering effect on being able to reach max RPM or just a result of lower MPH?

Changing load may require a change in the prop to make sure the motor can reach the proper WOT RPM
some shops will have test wheels for these motors to see if the motor has a problem.
they help in finding out if there is a motor,prop,or hull problem

[boscoe99]

Does a increase in total weight of the boat have a lowering effect on being able to reach max RPM or just a result of lower MPH?

It does both. Added weight lowers the WOT RPM and lowers the maximum MPH.

Added weight can cause a double wammy. As the weight gets added the WOT RPM lowers. If it lowers too much the motor cannot produce its rated HP. At a lower RPM the maximum speed of the boat will be even lower.

Just remember that at WOT, with the boat loaded, in summer time, with the motor trimmed just to the point of maximum efficiency (not any higher) try and have a propeller that will allow the motor to turn at or near the red line RPM.

[boscoe99]

I would think that the change in vacuum and pressure would be during acceleration and deceleration, but once running at what ever RPM, once steady the vacuum and pressure would stabilize.

OP
At WOT more fuel is being used, and if the system cannot keep up the pressure could drop. that is what you are looking for.

if the fuel pressure stays in spec, then you need to be looking at other things.
why did you only clean one set of injectors?

The fuel pressure is stabilized as long as the manifold pressure is stabilized. The manifold pressure is stabilized as long as the throttle opening and engine RPM remain constant.

As the manifold pressure changes with the throttle opening being changed, the fuel pressure changes. Throttle more open = more manifold pressure = more fuel rail pressure. Throttle less open = less manifold pressure = less fuel rail pressure.

Here is an updated test procedure from a later Yamaha SM that more clearly links the change in fuel pressure to the change in MP.

[fairdeal]

I'm inclined -as I frequently am - to throw a stone at the Yamaha "technical writer"/translator.

what is going on - and why - is pretty clear.

Describing it as they did: not so much.




"The pressure regulator changes the fuel pressure according to the vacuum pressure"

seems IMO a more "helpful" description.

[99yam40]

The fuel pressure is stabilized as long as the manifold pressure is stabilized. The manifold pressure is stabilized as long as the throttle opening and engine RPM remain constant.

As the manifold pressure changes with the throttle opening being changed, the fuel pressure changes. Throttle more open = more manifold pressure = more fuel rail pressure. Throttle less open = less manifold pressure = less fuel rail pressure.

what I was trying to get at is the fuel pressure you measure at WOT and Idle should be the same in spec while testing as long as you are not accelerating or decelerating while measuring

[IRBBoater]

Boscoe99 Thank you for the ref material. It makes more sense seeing the higher pressure at WOT.
Question 1 - Using that same ref, is it safe to say the 47.9 psi is a momentary measure of the high pressure pump output?
Question 2 - Without the use of other special load tools, is running the engine in the water up to WOT with the fuel pressure gauge installed on the fuel rail with the cowling off, the only way for me to check the pressure?

[99yam40]

Boscoe99 Thank you for the ref material. It makes more sense seeing the higher pressure at WOT.

You should not see higher fuel pressure at WOT.

did you read what all was written?

[IRBBoater]

Yes I did read it and I thought I understood it but I guess I'm confused with the following statement.

As the manifold pressure changes with the throttle opening being changed, the fuel pressure changes. Throttle more open = more manifold pressure = more fuel rail pressure. Throttle less open = less manifold pressure = less fuel rail pressure.

[rodbolt17]

google speed density EFI.

most all 4 stroke Yamaha uses this set up.

engine speed and air density.

air density is con*****ed by barometric pressure and air temp.

this is why we monitor intake air temp and barometric pressure.
at key on the ECU notes baro pressure and it is locked for that key cycle.
once running the ECU monitors air temp and manifold pressure.
manifold pressure and TPS signals give an indication of engine load for a given speed.
an increase in manifold absolute pressure tells the ECU that the load has increased.
the TPS tells the ECU the throttle angle.

now for rapid acceleration the ECU sees the sudden increase in TPS voltage and a corresponding increase in map.
now the ECU is going to increase injector ON time,some even go from synchronyous injection to an asynchroneous injection.
now with that sudden increase in pressure the regulator also allows about a 5PSI spike in fuel pressure.
this also aids in acceleration and prevents bogging on rapid throttle openings.
similar to an accelerator pump on a carb.

it is pretty simple but you must understand why the subsystem is there and what it does.
take the guy with the average center console.
to get up he rams the throttle to WOT.
map increases to almost baro.
now he is up and running and backs down to a more comfortable speed at about 3500 RPM.
TPS voltage lowers and map decreases to about 1/2 of baro.
now the regulator can sense this change in map pressure and alter rail presssure from its set point to about 5 PSI higher than its set point.

the MOST critical part of this EFI is rail pressure.
it is not monitored and the ECU always assumes it is correct.
the ECU cannot change anything based on loss or excessive rail pressure.
it simply does not know.

map pressure is basically the inverse of manifold vacuum.