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Thread: Vacuum increase at part throttle??!! Apparently It's OK in park.... Solved!

  1. #1
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    Default Vacuum increase at part throttle??!! Apparently It's OK in park.... Solved!

    I've been playing around with my fuel pressure again. The recent cold snap and grumpy startups made me get under the hood and check it. I adjusted it to 48 PSI with the vacuum line off. I carcked the throttle to about 4000 RPM and helf it there for 15 seconds. It held steady All good, right?

    OK.....I threw the line back on and noted the 40 PSI at idle she shows WITH vacuum.....again all good.


    I blip the throttle......WTF?
    I put it to 4000 RPM again (I must be pissing off the neighbors)


    IT DROPS FROM 40 TO 35 AND STAYS THERE steady until I let the throttle go back down to idle and it returns to the original position.

    The last time I even checked the pressure with the vacuum line on was with my old cam(the guage is under the hood on the rail). As I recall it went up under throttle.

    I'm stumped.....Time to buy a vacuum guage and see what the hell's goin on (the old one I had has disappeared....loaned it out and it never came back). My idle is rock solid in drive. The car has been surging in park as of late.....weird since the idle is higher then, but again that kinda goes with the issue I'm having I guess.

    Having to replace the fuel line in conjunction with the subframes doesn't help.....No visible leaks. I replaced it with rubber hose as opposed to the stock flexible plastic. In case you're wondering why, it's because I put a hole in it when doing the subframes.....
    I can't see this being a fuel problem due to the steady pressure with vacuum removed from the system......

    Anybody got any input?! Ideas?! BTW Smog pump and EGR are "ruled out" if you catch my drift.... Also the throttle was always moved open and closed slowly, never snapped open or put to WOT....
    Last edited by V8Demon; 03-28-2008 at 01:38 PM.
    -- 05 Mustang GT-Whipplecharged !! -- 87 5.0 Trick Flow Heads & Intake - Custom Cam - Many other goodies...3100Lbs...Low12's! http://www.fquick.com/V8Demon

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    With a milder cam, yes vac generally increases to a point then starts to decrease... With a higher lift, longer duration cam I don't think you'll see a increase after 1200-1500 rpm... Of course at WOT there should be very low vac, in cases where 4-5in of vac are observed, then likely the TB or carb is too small...






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  3. #3
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    I tried this all again with a base pressure of 52 without vacuum for giggles....

    44 @ idle with vacuum connected.

    A drop to 40 @ a slow rate of throttle increase.....
    Stabbing it from idle to WOT it stayed rock steady.

    All testing done with car in park. No load.

    I don't know what the pressure would be with a load as my guage is under the hood.....She runs perfect above 40* though.....Today was a good example!
    No issues when I mash the pedal. She just goes. No codes either.
    -- 05 Mustang GT-Whipplecharged !! -- 87 5.0 Trick Flow Heads & Intake - Custom Cam - Many other goodies...3100Lbs...Low12's! http://www.fquick.com/V8Demon

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    I would say that whatever problem you were having / are having isnt vac related. Generally the vac presure will jump a little and back off as you increase rpm.
    that would kind of piss me off that my fuel pres psi dropped as well.
    I wonder if there are fuel presure reg's for sefi that require no vac line. The FPR on a CFI setup is simple and ajusts with an allen head wrench.

    perhaps having a steady one presure fuel setting defeats the purpose of maf but i guess ill be learning all that stuff here soon.

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    Generally the vac presure will jump a little and back off as you increase rpm.
    That's what it SHOULD do with a load on the motor. That's what it did do with my old setup WITHOUT A LOAD (went up 2 PSI)...that was a different cam/ heads/intake/injectors.....yadda yadda.

    By raising the fuel pressure a second time and getting about the same numerical drop just enforces my feeling if that there is a problem, it aint the fuel delivery. The fuel rail loses only 10 PSI in 30 minutes after shutting the car off. Well within limits.

    perhaps having a steady one presure fuel setting defeats the purpose of maf but i guess ill be learning all that stuff here soon.
    Explain.
    -- 05 Mustang GT-Whipplecharged !! -- 87 5.0 Trick Flow Heads & Intake - Custom Cam - Many other goodies...3100Lbs...Low12's! http://www.fquick.com/V8Demon

  6. #6
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    Quote Originally Posted by V8Demon View Post
    Explain.
    I dont think i can.
    I "think" that SD systems operate on a different type of PWM program than MAF and therefore, MAF is more co-dependant on Vac parameters and thus effecting fuel / air ratios. If your FPR were not Vac driven, the fuel presure would be locked in and unable to decrease or increase unless there was a hard failure.,,ie- like the FPR i pointed out. As you already know, my information is less than helpful and you are more advanced in this stuff than i am.

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    Actually SD is MUCH more dependent upon vacuum parameters. Think about it..... SD relies on the BAP/MAP to figure out how much of a load is on the motor while taking into account TPS, IAC,EGR settings while also measuring RPM. Since the Mass Air Sensor measures volume and the BAP/MAP is disconnected and reads only the atmosphere as a guage while taking the readings from various other sensors, the total dependence on vacuum is reduced.

    It is the reason the myth of needing to switch to Mass Air to run any cam lives.....While you can't run any cam you want with SD, there are plenty that you can. The trick is keeping the vacuum level high at idle.
    Last edited by V8Demon; 03-28-2008 at 01:26 PM.
    -- 05 Mustang GT-Whipplecharged !! -- 87 5.0 Trick Flow Heads & Intake - Custom Cam - Many other goodies...3100Lbs...Low12's! http://www.fquick.com/V8Demon

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    whats confussing is in your post you siad you adjusted the FPR to 48 with no vac line.
    you reconnect the vac line and observe 40psi and called it good.


    looking at this from another angle, perhaps the vac line / path is collapsing.

    In any other cercumstance i would say the pump is getting weak but since your symptoms went away (i assume) with no vac line, your right and its vac related.

    Check for line blockage or easy to squeeze lines leading to the fpr.

  9. #9
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    At idle while vacuum is high with no load it will drop pressure slightly. At WOT with a load on the engine vacuum is almost non-existant and full pressure or very close to it should be seen.

    In any other cercumstance i would say the pump is getting weak but since your symptoms went away (i assume) with no vac line, your right and its vac related.
    And that's why I double-checked it the second time with higher pressure and no vacuum. To confirm no fuel starvation at RPM.

    Check for line blockage or easy to squeeze lines leading to the fpr.
    That line to the FPR IS soft.....Appears to be in good condition though.....
    Last edited by V8Demon; 03-28-2008 at 01:27 PM.
    -- 05 Mustang GT-Whipplecharged !! -- 87 5.0 Trick Flow Heads & Intake - Custom Cam - Many other goodies...3100Lbs...Low12's! http://www.fquick.com/V8Demon

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    4000 RPM in park is not WOT, its is very little throttle and you will actually have more vacuum at 4000 RPM than idle. It shouldn't change by 5psi though, maybe 2 or 3psi. Maybe more if it's cammed for more lower torque.

    Did you put a vacuum gage on the engine when you test fuel pressure? Your fuel rail pressure minus manifold pressure (absolute) should always be the same at all conditions.

    48psi - 14.5psi = 33.5psi differential at WOT
    40psi - 6.5psi = 33.5psi differential at part throttle

    And SD is completely dependent on a good vacuum signal to the MAP sensor to determine fueling. All other vacuum leaks are relatively harmless unless the affect a fueling device. Vacuum leaks affects MAF by allowing air into the engine that the computer doesn't know about since it didn't enter the engine through the MAF sensor.
    Last edited by xjeffs; 02-14-2008 at 02:34 PM.

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