2G Walbro 255 fuel pump looses fuel pressure when you shut it off.

[Crkilber]

I’ve been trying to figure this out for months, 1996 eagle talon tsi 16g turbo full intercooler piping and front mount intercooler, still running stock maf sensor, full exhaust.
It has a walbro 255 fuel pump and is rewired but it looses fuel pressure as soon as you shut it off and cranks awhile till it will start so I replaced injector seals and put on an aftermarket fuel rail and lines to the fuel filter and a fuelab pressure regulator. Still loosing pressure after shutting it off so I put in a new walbro 190 pump and with that it won’t even stay running, i also replaced the seal around the pump that goes into the fuel tank.
Still having the same issue with loosing fuel pressure and also now after reinstalling the 255 pump I can’t get the fuel pressure up to the 42.5 with the vacuum line removed like it is suppose to be and has been with this set up.
Now sure what else to do at this point anymore and wondering if anyone has any ideas
Thanks

 

[pauleyman]

Most of the stuff you touched doesn't affect fuel pressure. Outside of obvious leaks the possibilities are the pump, pump connection, injector itself or pressure regulator. That's it. Nothing else can cause a drop in pressure that I can think of. Lines, seals, filter etc don't cause fuel pressure drop as you describe

 

[We're on Boost]

Try one of these fuel pump grommets on the outlet pipe of your fuel pump.
These are genuine Mitsubishi. They are actually an Evo part but people have been saying they fit well (but quite tight) into the receiver tube on our DSM pump hangers. It fits onto the pump outlet pipe easy enough, like you see in my pic. Then lube up the outside of it a little bit before trying to shove it into the receiver tube.

I bought some from STM. If you are in a hurry, I would say check with them first to make sure they are in stock. When I ordered mine, STM sent them out 4 days later, so it was no problem. You could also get these from a Mitsu dealer.
https://stmtuned.com/products/fuel-tank-grommet-oem-evo-7-8-9

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[evil_eagletalon]

i dont think they are supposed to “ hold” pressure after shut down. I think your chasing another issue. Besides as soon as you “ crank” the engine it turns the pump on.

 

[pauleyman]

i dont think they are supposed to “ hold” pressure after shut down. I think your chasing another issue. Besides as soon as you “ crank” the engine it turns the pump on.

Incorrect. Fuel pressure should remain after you shut the car off. Priming the pump at startup isn't quite the same as if pressure is already there. It's also the reason why on some cars the fuel pump primes at key on. Not true for DSM.

 

[dwb]

I agree with @We're on Boost. Probably leaking out the fuel pump outlet. I used the same grommet, plus the factory o-ring. Fit nice and snug with no chance for leaks. Also, if using the stock FPR, pull the vacuum line off and see if it is wet or smells like fuel. A WB255 really should have an aftermarket FPR as it can rupture the membrane inside the stock unit.

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[1990TSIAWDTALON]

i dont think they are supposed to “ hold” pressure after shut down. I think your chasing another issue. Besides as soon as you “ crank” the engine it turns the pump on.

Mine holds pressure for a few HOURS until it trickles down to 15-20 psi. After a day, it is usually at zero. Some pumps don't have a pressure relief valve and some folks don't even like pumps that have them. I DO like having it so that it retains some pressure in between drives/starts.

 

[Crkilber]

I’m ordering one of those grommets and hopefully that works, dwb you said you use the factory o ring also with the grommet and I have a fuelab pressure regulator already installed

 

[dwb]

I’m ordering one of those grommets and hopefully that works, dwb you said you use the factory o ring also with the grommet and I have a fuelab pressure regulator already installed

Yes, and it was a tight fit. No idea if I needed both or if if I even installed it right because I couldn't find much info on it. I really just used the grommet as a replacement for the plastic spacer. Keeps the o ring straight and firmly against the lip of the pump outlet. I have no issues with losing pressure.
Good to hear you have an AFPR!

 

[Crkilber]

Did you also sill use the little white plastic piece on the tip of the pump?

 

[dwb]

Yes, the cap is necessary. I pretty much followed the 1g vfaq.
1g awd vfaq: http://www.vfaq.com/mods/pump-WB-1GAWD2-print.html
2g awd vfaq: http://www.vfaq.com/mods/pump-WB-2GAWD.html

This boxed in red is the piece I swapped for the grommet.

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For 2g, it appears to be a black plastic spacer that you would swap for the grommet:

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[evil_eagletalon]

My car fires right up not holding fuel pressure

 

[steve]

My car fires right up not holding fuel pressure

Clearly it holds pressure it just doesn't retain it when the pump is off. It should but as long as it builds pressure right away and can track manifold pressure 1:1 it's doing what's really needed.

People have uncovered various reasons why their fuel system doesn't retain pressure for very long. Sometimes its a leaking pump, sometimes it's a leaking injector, and others find a leaking seat in the AFPR. IIRC we had a run of cases where people found burrs in the seat of the return valve in their AFPR and once they cleaned those up they fixed the issue. Since the FPR is usually a controlled leak of fuel back to the tank it now sealing 100% doesn't impact it's function except in this way.

 

[We're on Boost]

I really just used the grommet as a replacement for the plastic spacer.

That's what it looks like in your pic up in post #6, where it looks like the fuel pump outlet pipe broke through the end of the grommet which then got shoved all the way down to where the spacer would normally be. This is very interesting because I wondered if this would happen. But I don't think it's supposed to be this way. In the Evo usage diagram they don't show any spacer under the grommet, or any o-ring or cap on top of the grommet. It's just the grommet. The grommet has a very obvious "'top" which seats onto the end of the fuel pump pipe and I'm pretty sure it's not supposed to slide down any farther over the pipe. It should stay on top like it is in my pic, as far as I know. I haven't tried this install on my 1g yet and probably won't for years. And I don't have an Evo. And the Mitsu diagram I have for this area is small and crowded so I wouldn't swear to anything by it.

So I'm wondering, how did it get this way on your install? Did the grommet push down like that all by itself from the force of inserting it into the receiver tube? Or did you kind of modify the top end of the grommet so it would slide all the way down easily right off the bat?

My 1-piece solution that for sure works is a piece of Viton tubing, soft (60 durometer), measuring 1/4" ID by 9/16" OD, and it's 15/16" long or a few hairs less, somewhere between 7/8" and 15/16" long. I always lube up the outside of it with Molykote 111 Compound because it is a tight fit into the receiver tube.
But I can't buy that particular Viton anymore so I bought some of these grommets just to have them and maybe try them some day, but haven't yet.

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[dwb]

@We're on Boost, I had no idea what I was doing Underneath the hood from top down I have the plastic cap, o ring, then the grommet. The grommet is in there tight against the o ring (it didn't move during install), keeping the o ring straight and secure, like how the plastic spacer was originally installed.

I think I spent over an hour trying to figure out how to install the grommet. I found no indication on how to install it in a 1g or 2g and the info I found for an EVO was extremely limited. One side of the grommet has a smaller hole than the other, which I speculated (still not 100% sure) was the top. The entire grommet fit over the WB pump outlet fairly easy. I didn't feel confident that the grommet by it's self was going to make a perfect seal or stay in place while inserted into the hood like how you pictured it, that's why I opted to keep the o ring in there. I also wasn't sure about the small cap and if it was needed to help secure the pump inside the hood.

Probably overkill to have both an o ring and grommet, but it seals well which was the goal. Plus no brittle plastic spacer to fuss with.

 

[We're on Boost]

@dwb Ok thanks for the feedback!