Fuel Pressure for 7.4L vs 5.7L

Discussion in 'Engine Performance + Maintenance' started by Crazydavez28, Jul 8, 2020.

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  1. Crazydavez28

    Crazydavez28 OBS Enthusiast

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    I have an El Camino with a Vortec drivetrain in it and already suffered thru the high mileage damp hard starting issue. Now it just won't start without a shot of starting fluid in the PCV hole in the TB hat. Once running it does run but stumbles around and if shut-off will not restart without the ether.

    Before loading the parts cannon I got a fuel pressure gauge from the harbor and it tested 43psi pulling 6.8amps, immediately went to zero once powered off.... Now I have a new fuel pump in the mail. FYI it showed 52psi once running. I'm amazed it actually ran. Everywhere I read this should be 60-65psi

    Then I went and tested my 7.4L and it was showing 43psi pulling 7.3 amps and sounded much healthier but I have some starting issues if ambient is over 90 degrees. This still seemed low to me and my thought is the regulator isn't working right. What should this actually be a for the 7.4?

    7.4 has Five-O Motorsports Ford injectors and 0411 swap. I also changed the regulator out 2 years ago but stupidly used an Amazon cheapo one.
     
  2. someotherguy

    someotherguy I'm Awesome

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    Vortec 7.4 fuel pressure spec is 58~62 psi. They will often start and run at considerably lower pressure due to the injector design; the small block Vortecs are the ones notorious for not starting when fuel pressure falls even just a little bit below spec because the poppet valve setup is very picky about pressure. (small block Vortec spec btw is 60-66 psi though it seems the MFI updated injectors operate at somewhat lower pressure)

    Your cheapo regulator could very easily be the source of trouble. Also the vacuum line leading to it...

    Richard
     
  3. PlayingWithTBI

    PlayingWithTBI Desert Old Guy

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    You're right there. It should be 58 - 60 while running and hold pressure after shutting off the engine.
     
  4. Crazydavez28

    Crazydavez28 OBS Enthusiast

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    I have noticed fuel mileage even farther in the toilet than normal. I was reliably getting 9-10 (double digits!!!) and it has dropped to 8 recently.

    I will start here and ordered a FPR The pump seemed strong and I could hear flow in the return line. Vacuum line was questionable 2 years ago so that will be new this time around.
     
  5. Schurkey

    Schurkey I'm Awesome

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    How do you know the GAUGE is accurate?
     
  6. Crazydavez28

    Crazydavez28 OBS Enthusiast

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    I don't, but it's brand new from the harbor freight store. I guess I could pipe it into something known to test it on.
     
  7. Schurkey

    Schurkey I'm Awesome

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    That's what I was getting at. Test the gauge,

    Harbor Freight sells bottom-feeder junk. They just recalled the jack stands they gave you when you turned-in the previously-recalled jack stands.

    For the record, I test my compression tester gauges now 'n' then, too. I make sure they read the same as each other, and the same as the gauge on my compressed air regulator. This is why micrometers come with standards to measure against--A person HAS to be able to trust his diagnostic and measurement equipment to be accurate and repeatable.
     
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  8. Supercharged111

    Supercharged111 I'm Awesome

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    I know you're talking 454, but just to be clear the 350 doesn't have a vacuum line. Also, the 454 line is easy to forget to hook up for some reason. :banghead: OP how old is the regulator in the 350? They're known to fail like the 454 and cause starting/running/idling issues.
     
  9. stutaeng

    stutaeng I'm Awesome

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    So what's the cause of the "damp cranks but won't fire?"

    I've got a 4.3 that has been doing this for about 2 years, maybe 3. I'm about to yank it and do a V8 swap because of low oil pressure, but would like to figure this out. Fuel pressure is good, about 58 psi on mine. I thought it was electrical?
     
  10. someotherguy

    someotherguy I'm Awesome

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    Absolutely, no vacuum line to the regulator in the 5.7 Vortec. But OP has a 7.4, as far as I can tell..

    Richard
     

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