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What is the consequence of using a fuel filter with a higher-spec'd bypass valve opening pressure ?

Currently have a UFI ‎31.833.00 / 6Q0201051C on my 2008 City Golf - rated at 4.0 bar.
Considering a HENGST H155WK02 as replacement, but it is rated at 6.4 bar.

Would this higher pressure rating cause fuel delivery issues, if the valve doesn't open until higher pressures ?
Would it just cause lessened fuel economy ?

Or is the rating just a max-permitted before the component blows-out, and have no adverse effect at all ?

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  • The question to ask is why are you buying a different spec'd fuel filter other than for price? A higher rated pressure doesn't change the separate fuel pressure regulator's operating pressure. The higher rating is most likely for higher regulated pressures and shouldn't affect your Golf EFI system.
    – F Dryer
    Commented Aug 22, 2021 at 0:10
  • @FDryer Was shopping RockAuto. Of four options presented, 1 is out of stock, 2 list no pressure ratings at all, and this 4th option lists 6.4 bar. However, actually, I looked up "HENGST H155WK02" on other websites - and they list 4.0 bar. So the part might be correct, RockAuto might list wrong info, and my question moot. I am still curious though about what conditions cause the bypass valve to engage, and effects of using a (slightly) higher-rated one. Hengst's catalog doesn't list pressure under technical data, but does say "4 bar" in a hover tooltip, annoyingly.
    – robut
    Commented Aug 23, 2021 at 15:22
  • If specs doesn't specify a built in pressure regulator then its just a fuel filter. You'll have to verify this with either another question for someone knowledgeable about '08 City Golf, Chilton, Haynes, factory service manuals or your local VW dealer service department.
    – F Dryer
    Commented Aug 23, 2021 at 23:17

2 Answers 2

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Seeing as my other answer was not correct in this case, I would propose that the reason there is a bypass valve is to ease the load on the pick up fuel pump while there is low demand for fuel by the high pressure pump that is further down the line.

The majority of the time, the pick-up fuel pump will be able to provide more fuel than is required by the engine. Without the bypass valve, the pump will be working harder than necessary, raise the fuel pressure higher than necessary and also use more electricity than necessary. The bypass valve will allow this excess fuel to be sent back to the tank and the ease the load on the pump.

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  • Thanks, I think that makes sense! So a valve that only opens at higher pressure, would cause some/all of the effects you described of the absence of a valve ...
    – robut
    Commented Aug 26, 2021 at 1:57
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Under normal operation the bypass valve should be closed at all times, your engine will be getting protected from dirty fuel by the filter.

Without a bypass valve, if the filter becomes clogged with dirt, the fuel pressure to the engine will drop and the engine will become starved of fuel and possibly run lean. This could cause damage to a petrol (gas) engine.

The bypass valve is there to open in the event of a clogged filter. The contaminated fuel will bypass the filter, but the engine will be getting a normal supply of fuel. This is good for keeping the engine supplied, but bad that dirt is getting to the engine.

If you put a filter on that opens this valve too late due to having a higher pressure specification, you could cause the engine to be starved of fuel in the event of a clogged filter. This could be bad for the engine.

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  • Hey, thanks for the response. I don't think the bypass valve returns dirty fuel though. Check out this vid, buddy cuts a filter in half and models it - it /appears/ that fuel still gets filtered before it can ever pass through the valve. His CAD and/or his assessment could be wrong, but if it's correct, then the "bypass" valve exists to protect against some other condition than a clogged filter. youtu.be/lV7N0ZU-C2E?t=505
    – robut
    Commented Aug 23, 2021 at 15:11

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