Yamaha SX225TURY- Fuel Pressure Drops to Zero

Fuel Pressure out of VST drops to zero after power on cycle.

Fuel Pressure reads 36 PSI during initial key on cycle. As soon as fuel pump stops, 5-10 seconds, VST fuel pressure drops to zero immediately, less than 2 seconds.

Engine starts and runs almost normal, RPM's at idle =750. Cold RPM's vary to 900 RPM. Could this be a leaking injector on a cylinder that is not running below 1600 RPM?

Fuel pressure reads normal, 36 PSI, while engine running. WOT runs at 5400 RPM. Upon engine shut down, raw gas appears to run out of exhaust hub on prop.

Fuel pressure regulator tests okay with vacuum pump applied to regulator vacuum line, at 6-10 inches of vacuum, VST fuel pressure drops to 30 PSI.

My concern is leaking fuel locking up a piston and excessive fuel usage.

Is this indicative of multiple injector failures, due to the rapid pressure drop, or is there another explanation I should check first?

Should I continue to run the engine like this or fix it first?? What steps should I take first to fix this?

Thanks for any help on this one.

Re: Yamaha SX225TURY- Fuel Pressure Drops to Zero

VST pump does not hold pressure when switched off. As long as the pressure holds when running. Being there is no pressure on the system when the engine is off, kinda hard for the injectors to leak much

Re: Yamaha SX225TURY- Fuel Pressure Drops to Zero

Hello Capt. Ken, Thank you for your response.

I believe the Yamaha VST is pressurized until the pressure is released by either the injectors energizing from the engine cranking or the release of pressure via the fuel pressure gauge /schrader valve on the VST. Leaking injectors would also cause the pressure to drop.

My belief is from the Yamaha Service Manual which warns to release the VST pressure before working on the high pressure fuel system.

After I replaced the high pressure fuel pump, several years ago, I remember the fuel rail and VST being pressurized for a long period of time, several minutes, even after turning the engine off.

This dropping of fuel pressure immediately after reaching 36 PSI is a new symptom and I'm not sure what, if anything, I should do to address it.

Please let me know if your experience is different than mine.
Regards,

Greg

Re: Yamaha SX225TURY- Fuel Pressure Drops to Zero

In trouble shooting these engines, I'm use to activating the VST pump and watching the guage go up and when it stops, I watch it zero back out. Yamaha also tells you to disconnect the batteries when working on the system along making sure there is no pressure on the system. Its just a safety item. I've never seen one hold pressure very long.
I just checked my manual and it says nothing about holding pressure. Every check involves running the engine to check pressures.

Re: Yamaha SX225TURY- Fuel Pressure Drops to Zero

Hello Capt. Ken,

Any thoughts as to the cause of the raw gas coming out of the prop hub upon engine shut down?

Additionally, raw gas seems to be coming out of the intake baffle upon tilting the engine full up after shut down.

Thank you again for your input and suggestions.

Re: Yamaha SX225TURY- Fuel Pressure Drops to Zero

Check the lift pump diaphragms.