DI fuel pump what's the truth

[stout890]

So Iv been reading 2 conflicting things on here about this pump and orbital fuel injection system.

#1 the comon consensus is that the fuel pump needs to produce and does produce 107+ psi. It's seen on the pressure gauge and the manual says that's what it should produce minimum. And that's why there is so much trouble finding aftermarket pumps for the DI ski's

#2 Now if you read up in a service manual about the orbital system it says that the pump will produce 27psi with the dsss plugged in. Than when the engine is fired up the compressor produces 80psi air pressure which in conjunction with the 27psi fuel pressure produces the total 107psi you see on the gauge when the engine is running.

Now what is it? Is the electric pump really only producing 27psi at all times? Can't be, guys on here have ran factory pumps in test set ups isolated from the engine and seen 147psi. 01 di fuel pump issues

So is the service manual bulshitting us? Or is this just a mystery some bombardier engineer only knows the answer to?

What do you guys know about this system, can anyone shed some light on this, or maybe I'm just missing something that's obvious.

 

[stout890]

No one wants to chime in on this? There is vary little info about the system online. Would be interesting to hear from a professional Sea doo mech on what they were taught about this system.

 

[IDoSeaDoo]

The DI and Orbital Fuel Injection are two different systems. DI is direct injection. That's only on the DI 951 models. It produced much higher PSI because it has to squirt that directly into the cylinder. The orbital fuel injection is on the 787 RFI engines. It uses injectors instead of carbs to quirt fuel into the intake. The RFI system has a different fuel pump and runs on less pressure.

 

[stout890]

My 951 DI has orbital fuel injection. And orbital is direct injection into a cylinder as far as I know. Orbital is unique in the fact that it uses an air injector in conjunction with a fuel injector to overcome cylinder compression. RFI And DI are two different systems. As far as the RFI using an orbital fuel rail I have no idea since I don't own one.

 

[IDoSeaDoo]

My apologies, you are correct. Orbital IS DI. Here's an article that briefly explains it. How does DI work?
What I didn't know is that there was a second type of DI... never heard of Ficht. I've always read that the DI's are supposed to produce 107psi from the pump, but I've never owned or worked on this system, and will be shutting up now. Good luck.

 

[ski-d00]

To answer your questions...

#1 You are mostly correct. The DI needs to produce 107psi and there are only a couple aftermarket pumps that can achieve this. However, the ones that do seem to work okay but they have another issue. They draw considerably more current than the OEM pump which the DI is a very electronic sensitive machine. The higher current draw can cause problems if you're battery is not in near perfect condition. When the lanyard is put on, the fuel pump will prime at a lower pressure, around 26psi. Once the starter is engaged, the fuel pressure will go up to the 107psi.

#2 You do not add the air compressor psi to the fuel pressure psi. Pressures don't really work that way in that you add them together when they get combined. They actually will reach a balance between the two depending on the atmospheric or combustion pressure. I don't ever recall seeing anything like this in the manual so if you could post it I'm sure we could decipher it for you and probably just a misunderstanding.

Both the fuel pump and the air compressor can reach very high levels when they are run raw. Each has a regulator in the fuel rail that maintains the proper pressure. They are truly separate systems for the most part until entering the cylinder.

 

[stout890]

So reading up on another thread that was super informative 951 DI Troubleshooting

Seems as though the air pressure acts upon the fuel pressure regulator to to allow the fuel pressure to build above the initial 27psi.
That's why you see 27psi with the engine off and 107 when the engine is running.

 

[ski-d00]

Well, that is not completely true so be careful what you read as being accurate.

The fuel pressure regulator regulates the fuel pressure at 27psi HIGHER than the air pressure in the fuel rail. In no way is it adding to the fuel pressure so saying 80 psi air pressure + 27 psi fuel pressure = 107psi running fuel pressure is not how the system works. The back side of the fuel regulator diaphragm is exposed to the air pressure and it will maintain the 27psi pressure difference.

No engine running means no air pressure and the fuel pressure will be 27psi which is 27 psi higher than the 0 psi air pressure. When the engine is running, the air pressure is regulated by the air regulator at 80psi so then the fuel pressure shoots up to 107psi to stay 27psi higher than the air pressure.

So, fuel pressure is 107psi +/-2psi and air pressure is 80psi while running.

 

[stout890]

That's what I said right? Maybe I didn't write it out to well.
I said "the air pressure acts upon the fuel pressure regulator"
You said
"The back side of the fuel pressure regulator diaphragm is exposed to the air pressure and it will maintain the 27psi pressure difference"


So 80psi air pressure+27psi fuel pressure regulator= 107psi fuel pressure