Fuel pressure

[Cplsteve]

I have a 2001 Yamaha f115TLRY at idle I’m getting about 31 psi. When I turn the key on the fuel pressure starts dropping. I have been barreling this issue for a while and have checked everything I know. Had injectors cleaned and checked, replaced low pressure pump, replaced high pressure pump, new timing belt due to time and hours on motor. At idle yds is reading 4atdc. Any and all help is appreciated

 

[boscoe99]

How can there be fuel pressure dropping with key on but the motor is running at 31 psi?

When the motor is running the key is in the on position.

 

[Cplsteve]

How can there be fuel pressure dropping with key on but the motor is running at 31 psi?

When the motor is running the key is in the on position.

Yes but also when the motor is running the mechanical fuel pump is pumping unlike when the motor is off

 

[boscoe99]

Fuel pressure is measured downstream from the high pressure HP pump. HP pump being located in the vapor separator tank.

The mechanical LP pump merely keeps the VST filled with gasoline.

The motor runs on HP pump pressure. Not on LP pump pressure.

I am not understanding how mechanical pump pressure has anything to do with the high pressure pump pressure. The primer ball can be used as a substitute for the mechanical pump if and when needed.

 

[Cplsteve]

Fuel pressure is measured downstream from the high pressure HP pump. HP pump being located in the vapor separator tank.

The mechanical LP pump merely keeps the VST filled with gasoline.

The motor runs on HP pump pressure. Not on LP pump pressure.

I am not understanding how mechanical pump pressure has anything to do with the high pressure pump pressure. The primer ball can be used as a substitute for the mechanical pump if and when needed.

With the engine off key on the pressure drops. When u first turn the key on the pressure jumps to 40psi as you hear the hp pump activating and then it starts to drop off with psi. After water testing with the fuel pressure gauge it runs at 36 psi and when you go to full throttle it drops to 20psi.