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walbro 255 fuel pressure unstable


s30kid

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I just had a quick question. I just installed a walbro 255 from summit racing that is rated up to 60psi along with a pallnet 11mm o-ring fuel rail and a fuelab boost reference pressure regulator. I have the idle pressure set to 40psi. At 15psi of boost I should be seeing 55psi of fuel correct? Anyway I'm seeing 60 psi on my gauge under boost and at cruising speed the needle is going crazy around 45psi. But under boost the neelde is stable. Is this okay or should I be concerned?

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The crazy (whatever that means) needle sounds like your reference port has turbulence or fluctuating vacuum.  Try a different hole.  Or you need a fuel pressure damper, like the stock setup.  The math problems could either be bad gauges, bad/low resolution FPR, or the measurement ports for the FPR and the dash gauge are in different spots, seeing different things.  Seems like you could run them both from a T at the same port.

 

Just some thoughts.  I don't have any of these things, but understand the concepts.  The Walbro probably doesn't have anything to do with the problem.  It just spins and pushes fuel.

Edited by NewZed
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  • 3 weeks later...

I just had a quick question. I just installed a walbro 255 from summit racing that is rated up to 60psi along with a pallnet 11mm o-ring fuel rail and a fuelab boost reference pressure regulator. I have the idle pressure set to 40psi. At 15psi of boost I should be seeing 55psi of fuel correct? Anyway I'm seeing 60 psi on my gauge under boost and at cruising speed the needle is going crazy around 45psi. But under boost the neelde is stable. Is this okay or should I be concerned?

It sounds like all of these measurements were made with the manifold reference connected to the FPR, correct?  If you are seeing 40 psi at idle, then your fuel pressure at 15psi of boost should be whatever vacuum you were pulling at idle plus 15psi.  So it sounds like you must have been pulling around 10" of vacuum (~ -5psi) at idle.  Normally you set the base fuel pressure with the manifold reference disconnected, and then idle pressure will be lower than the base pressure and boost will be higher.

 

At cruise, your manifold pressure/vacuum will move around a fair amount as you adjust the throttle, so this is likely not a huge problem.  If you are seeing big spikes in the pressure when the throttle is being held constant, then I would suspect that to be due to some lean backfiring in the manifold.

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