HPDI Vst pump o'ring placement???

[Gman24]

Can someone tell me where the small black o ring at top of the VST tank pump goes. Does it just slip over the male end tube at the top of the pump. To me it appears as though it serves no purpose there. I recently replaced my VST tank filter and later realized this o ring had fallen off and I had to go back in, so to speak. I wasn't exactly sure where it went so I looked up the schematic and it just shows it above the white tube so I assumed it just slid over the end? I'm still having occasional surging problems with my HPDI after replaceing all filters. I'm thinking maybe replacing the 2 pumps beside the VST tank next???

[jethro1]

I am not familiar with that specific O ring. The SM makes the OD of the O ring appear to be the same size as the OD of the plastic collar right below it. It appears that the O ring might be inserted within the VST cover. Such that the O ring makes contact with the end of the collar. Is there a cavity within the VST cover in which the O ring can be inserted?

Does that make sense? Hopefully someone that has replaced the O ring will come along and provide better information.

For those that might be wondering what we are talking about

[Gman24]

Yes that's the diagram that I found.i'm wondering if my o ring has expanded. It will not lay on top of the tube. It actually slides completely around the tube part. That's why I said it seems to not serve a purpose now after evrything is back together.

[jethro1]

It might be designed to nest within the VST cover and then when the collar is inserted it goes within the ID of the O ring. Thereby preventing a loss of leakage and fuel pressure in that manner.

What sort of fit is there between the collar and the outlet of the pump? The outlet of the pump has to be sealed so all of the fuel pressure is applied to the fuel outlet port on the VST cover.

[Gman24]

Thanks Jethro1. I justspoke with a Yamaha Mechanic. He said exactly what you just said. The o ring slides inside of a cylinder that the pump male end slides into forming the seal to prevent pressure leakage. The o ring had likely expanded which is why it wouldn't go into the cylinder. I am hoping i can get the o ring to seat inside the cylinder. Hopefully this is what has been causing my intermittent surging.Thanks for the help.

[Charlie206]

If the o ring appears to be in good shape, freeze it, then install and see if it fits better.

[Tonyc]

You ever have your injectors serviced? I had a surging issue at wot, I had one injector running lean.

[87gtNOS]

I just had this all apart on my 250....I slid that oring over the pump tip...it took some wiggling to get it into the housing far enough that the vst halfs would go together nicely.