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Thread: Fuel pressure drop

  1. #11
    Registered Member chevynut's Avatar
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    So looking back a year ago it looks like this could have been the start of my fuel pressure regulator problems. My brother came to CO in April, and I fired up the engine for him and it was popping out of the exhaust and running rough. I didn't do much with it after that until I tried to get it running better to charge the A/C. Now I know the fuel pressure regulator vacuum line was pouring fuel into the intake.

    That brings up a question. I have been told to remove the vacuum line and that you really don't need vacuum to the regulator. I know the vacuum changes the fuel pressure with respect to ambient pressure, but keeps it tied to manifold pressure. So as manifold vacuum drops, the fuel pressure rises. But it seems like a constant pressure relative to ambient would be better, and you can just tune the system to the correct for the added pressure at idle.

    I'm not sure whether to hook up the vacuum line or not, and I read conflicting things online about it. if I didn't hook it up it would prevent the problem I had with the broken diaphragm and flooding of the engine which would be nice.

    Any advice or comments on this?
    Last edited by chevynut; 01-14-2024 at 11:28 AM.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


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  2. #12
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    First off, you have it backwards. More manifold vacuum (less manifold pressure) on the pressure regulator the lower the fuel pressure.

    As a general statement, the effect of having or not having a vacuum referenced pressure regulator can be dealt with in the fuel map, and with the fuel trims using the O2 sensor.

  3. #13
    Registered Member chevynut's Avatar
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    Quote Originally Posted by Rick_L View Post
    First off, you have it backwards. More manifold vacuum (less manifold pressure) on the pressure regulator the lower the fuel pressure.
    I meant that as manifold VACUUM drops, fuel pressure rises. I corrected it.

    As a general statement, the effect of having or not having a vacuum referenced pressure regulator can be dealt with in the fuel map, and with the fuel trims using the O2 sensor.
    That's what I've been told, but there are conflicting opinions on various forums. To me it shouldn't make any difference what the actual pressure is as long as the fuel map and O2 sensor deals with it. Some guys are of the opinion that the vacuum line makes the regulator act like a "accelerator pump" by increasing the fuel pressure when the vacuum drops. Not sure I buy that, especially with 60 PSI of fuel pressure. I think I'll plug the vacuum port under my intake and remove the vacuum line. I don't ever want my engine flooded again and I don't see the downside to not having it.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1962 327/340HP Corvette
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

  4. #14
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    Biggest thing here is what one is starting with both hardware and fuel map.

    Many OEM GM setups used vacuum reference - TPI, LT1, Ramjet do. If you're starting with one of these and an existing fuel map it probably is much easier to keep it. The problem you had is probably a freak. Lots of miles put on these regualators without failure.

    The Edelbrock Pro Flow 4 uses vacuum reference. Since it's not as adjustable as some others, if one had one of those it might be best to keep it.

    On the other hand, LS engines don't use vacuum reference. I don't think Holley intends for it with Terminator X. whether it's LS or not.

    I don't blame you for not wanting to have that failure again.

  5. #15
    Registered Member chevynut's Avatar
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    Quote Originally Posted by Rick_L View Post
    The problem you had is probably a freak. Lots of miles put on these regualators without failure.
    I actually did a web search and found a lot of instances of this happening with these regulators. It's probably a small percentage, though.


    I don't think Holley intends for it with Terminator X. whether it's LS or not.

    I don't blame you for not wanting to have that failure again.
    I think I'll just put a rubber cap on the manifold vacuum fitting so I can put the vacuum line back on later if I think I need it. If it's not necessary I'd rather not have it. Thanks for the input on this.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1962 327/340HP Corvette
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

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