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Thread: Strange fuel pressure gauge reading

  1. #1
    Authorized Vendor Turbo Mopar Vendor jeff1234's Avatar
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    Strange fuel pressure gauge reading

    I have been testing my fuel pressure gauge sending units. The gauge always reads higher than the fuel press reg setting. Awhile back I had bought a second sender but it turned out that the fuel pump was actually bad. Replaced the pump from Cindy. The reading I am getting is 60 psi (actually 55) but when I rev the car the pressure drops back 3 or 4 lbs. the car is running no differently and as boost builds I get an increase in pressure that probably reflects my boost (12 psi). Has anyone noticed this in their electric fuel gauges. The gauges act exactly the same in reading, so the sending units are good.
    The only substitute for cubic inches is cubic dollars, how fast can you afford to go?

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    Rhymes with tortoise. Turbo Mopar Staff cordes's Avatar
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    Re: Strange fuel pressure gauge reading

    That is odd. You should get less than 55PSI at idle, 55PSI at 0lbs. boost, and one psi over 55 for each lb of boost.

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    Slugmobile & MeanMini Caretaker Turbo Mopar Contributor wheming's Avatar
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    Re: Strange fuel pressure gauge reading

    Are you sure these readings don't match what the actual fuel pressure reads with a mechanical gauge?

    If a mechanical gauge reads similar, then i would say the return line is too small, too restrictive, or both.

    If a verified accurate mechanical gauge reads like you would normally expect, (55psi with vacuum line removed, and 45-47psi with the vacuum line connected and at idle) then i would say there is some error in the electric gauge. Verify the pressure range of the electric gauge. I had one once that would only read like 45psi as a minimum value.
    Last edited by wheming; 07-04-2017 at 07:34 AM.
    Wayne H.

    '91 Dodge Spirit ES 2.5L turbo 5spd
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    Supporting Member II Turbo Mopar Contributor tryingbe's Avatar
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    Re: Strange fuel pressure gauge reading

    Vacuum affect fuel pressure. Revving the engine changes vacuum/pressure numbers.

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    Slugmobile & MeanMini Caretaker Turbo Mopar Contributor wheming's Avatar
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    Re: Strange fuel pressure gauge reading

    Exactly what electric gauge do you have?
    Wayne H.

    '91 Dodge Spirit ES 2.5L turbo 5spd
    '05 PT GT 2.4T HO autostick
    '89 Plymouth Acclaim 2.5L turbo auto, "Slugmobile" yes, THE Slugmobile!
    '89 Dodge Caravan SE 2.5L turbo auto, "Mean Mini" yes, Gus' Mean Mini! (Current best 11.7786@113.04 mph! - Jan 15th, 2017 Fayetteville Dragway, NC)
    MeanMini dragracing videos: https://www.youtube.com/playlist?lis...URZLB1RxGYF6vw
    and other cars, trucks and motorcycles
    https://www.youtube.com/user/SlugmobileMeanMini

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    Authorized Vendor Turbo Mopar Vendor jeff1234's Avatar
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    Re: Strange fuel pressure gauge reading

    Cordes, tryingbe and Wheming, thanks for your replies. On my friends professional level mechanical fuel pressure gauge I set the pressure at 55 psi for a reading on the gauge of 63 psi but I see by the description above that I haven't done it correctly. I didn't take the vacuum line off. Never the less, the gauge was responding as Cordes described regardless of the error. Yesterday when I changed the senders around I noticed that the one previously installed had a loose wire. This might explain the change in readings. I have stock diameter fuel lines for Shelby Charger manufactured by hand about three years ago by a company in Ohio, so I am confident they aren't plugged or restricted. My pressure gauge is Glow Shift, sourced from Summit, I believe. The sender is made in Japan, model # GS-S03_100. It came with the gauge.
    I am in the middle of a 4000 sq. ft. grass planting war right now but I think I can get back to you with more info by tomorrow. Maybe it will solve itself, NOT, lol.
    Thank you.
    The only substitute for cubic inches is cubic dollars, how fast can you afford to go?

  7. #7
    Rhymes with tortoise. Turbo Mopar Staff cordes's Avatar
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    Re: Strange fuel pressure gauge reading

    In order to check your fuel pressure properly follow these simple steps.

    1. hook up gauge with car off.
    2. Run a decent size (I used 10ga) wire from the + battery terminal to the + terminal on the coil.
    3. Read gauge.

    That'll give you the static reading that the FP should be read at. You can make your adjustments while running the FP with that wire. I would still check it with the vac line disconnected and the engine running. If your battery voltage is really low compared to what it is with the engine running that could make a bit of a difference. However, I've found that it generally doesn't change anything. Most times the gauge is bouncing around enough with the engine running that I find jumping the coil to be most accurate anyway.

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    Authorized Vendor Turbo Mopar Vendor jeff1234's Avatar
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    Re: Strange fuel pressure gauge reading

    OK Cordes, I will give that a try. Thank you!
    Jeff
    The only substitute for cubic inches is cubic dollars, how fast can you afford to go?

  9. #9
    Rhymes with tortoise. Turbo Mopar Staff cordes's Avatar
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    Re: Strange fuel pressure gauge reading

    It was quickly put together, but you'll get the idea.


  10. #10
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    Re: Strange fuel pressure gauge reading

    here is a simple set up I made. i have about 25$ in the entire thing. And i leave it on.

    https://www.youtube.com/watch?v=F8y0if4kdOo

  11. #11
    Authorized Vendor Turbo Mopar Vendor jeff1234's Avatar
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    Re: Strange fuel pressure gauge reading

    Nice clean setup. In my case I want to observe pressure while accelerating.
    The only substitute for cubic inches is cubic dollars, how fast can you afford to go?

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    Authorized Vendor Turbo Mopar Vendor jeff1234's Avatar
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    Re: Strange fuel pressure gauge reading

    Update, I tested the car today, the gauge seems to be doing what it is supposed to do,.... 55 PSI static, 55 PSI at idle. When briefly operating the throttle, pressure drops slightly to about 52 PSI. Between -10 hg and beginning of boost 55PSI. At 10PSI the gauge reads 67 PSI.Thanks for the help.
    jeff1234
    The only substitute for cubic inches is cubic dollars, how fast can you afford to go?

  13. #13
    Slugmobile & MeanMini Caretaker Turbo Mopar Contributor wheming's Avatar
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    Re: Strange fuel pressure gauge reading

    Quote Originally Posted by jeff1234 View Post
    Update, I tested the car today, the gauge seems to be doing what it is supposed to do,.... 55 PSI static, 55 PSI at idle. When briefly operating the throttle, pressure drops slightly to about 52 PSI. Between -10 hg and beginning of boost 55PSI. At 10PSI the gauge reads 67 PSI.Thanks for the help.
    jeff1234
    You should see lower than 55psi when at idle. When you set or check without the vacuum line on the FPR, you should see 55psi, but when you connect the line it should drop.
    Depending on how much vacuum you have at idle, the fuel pressure should be around 47psi or so at idle.
    Wayne H.

    '91 Dodge Spirit ES 2.5L turbo 5spd
    '05 PT GT 2.4T HO autostick
    '89 Plymouth Acclaim 2.5L turbo auto, "Slugmobile" yes, THE Slugmobile!
    '89 Dodge Caravan SE 2.5L turbo auto, "Mean Mini" yes, Gus' Mean Mini! (Current best 11.7786@113.04 mph! - Jan 15th, 2017 Fayetteville Dragway, NC)
    MeanMini dragracing videos: https://www.youtube.com/playlist?lis...URZLB1RxGYF6vw
    and other cars, trucks and motorcycles
    https://www.youtube.com/user/SlugmobileMeanMini

  14. #14
    Authorized Vendor Turbo Mopar Vendor jeff1234's Avatar
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    Re: Strange fuel pressure gauge reading

    Wayne, I have my vacuum lead to the FPR located on the passenger side vacuum tap of the throttle body, is this incorrect?
    The only substitute for cubic inches is cubic dollars, how fast can you afford to go?

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    Moderator Turbo Mopar Staff Force Fed Mopar's Avatar
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    Re: Strange fuel pressure gauge reading

    The fpr should have full manifold vacuum/pressure. I prefer not to use the TB for it's source. The pressure running at idle should be 48psi with the vacuum line hooked up, 55psi with it unhooked. Can be + or - a psi w/o too much concern.
    Rob M.
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    Slugmobile & MeanMini Caretaker Turbo Mopar Contributor wheming's Avatar
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    Re: Strange fuel pressure gauge reading

    Rob's right Jeff.
    The stock vacuum tubing for the FPR is in that molded vacuum harness that comes right off the intake manifold.
    I like using the big hose barb right off the intake, and then have a reducing tee. Or have a vacuum block/manifold.
    I plan to clean this up with a manifold block at some point.
    Click image for larger version. 

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    Wayne H.

    '91 Dodge Spirit ES 2.5L turbo 5spd
    '05 PT GT 2.4T HO autostick
    '89 Plymouth Acclaim 2.5L turbo auto, "Slugmobile" yes, THE Slugmobile!
    '89 Dodge Caravan SE 2.5L turbo auto, "Mean Mini" yes, Gus' Mean Mini! (Current best 11.7786@113.04 mph! - Jan 15th, 2017 Fayetteville Dragway, NC)
    MeanMini dragracing videos: https://www.youtube.com/playlist?lis...URZLB1RxGYF6vw
    and other cars, trucks and motorcycles
    https://www.youtube.com/user/SlugmobileMeanMini

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    Authorized Vendor Turbo Mopar Vendor jeff1234's Avatar
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    Re: Strange fuel pressure gauge reading

    I will find some way to get direct vacuum to the FPR. Thank you for the information. I will update when done.
    The only substitute for cubic inches is cubic dollars, how fast can you afford to go?

  18. #18
    Slugmobile & MeanMini Caretaker Turbo Mopar Contributor wheming's Avatar
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    Re: Strange fuel pressure gauge reading

    The FPR and the MAP were tee'd together from the factory.
    The best is to have a "clean" (dedicated ) signal line, so that other vacuum users like cruise control, vacuum canister, or boost users like wastegates, bov, etc don't affect the signal to the fpr and map. Those are two main things your cal is dependant on.
    If you have less fuel pressure than you should have in boost you'll be lean.
    If you show less manifold pressure than you actually have you will be lean.
    Both at the same time compounds the error to a point where it could be a costly learning experience.
    Wayne H.

    '91 Dodge Spirit ES 2.5L turbo 5spd
    '05 PT GT 2.4T HO autostick
    '89 Plymouth Acclaim 2.5L turbo auto, "Slugmobile" yes, THE Slugmobile!
    '89 Dodge Caravan SE 2.5L turbo auto, "Mean Mini" yes, Gus' Mean Mini! (Current best 11.7786@113.04 mph! - Jan 15th, 2017 Fayetteville Dragway, NC)
    MeanMini dragracing videos: https://www.youtube.com/playlist?lis...URZLB1RxGYF6vw
    and other cars, trucks and motorcycles
    https://www.youtube.com/user/SlugmobileMeanMini

  19. #19
    Authorized Vendor Turbo Mopar Vendor jeff1234's Avatar
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    Re: Strange fuel pressure gauge reading

    I re installed my line using intake manifold pressure at the pipe coming out of the intake, no vacuum lines off anywhere, fpr gauge reads 55 psi with the hose on and 60 psi with the hose off. Does that indicate a bad diaphragm or some other problem.
    Last edited by jeff1234; 07-09-2017 at 04:45 PM.
    The only substitute for cubic inches is cubic dollars, how fast can you afford to go?

  20. #20
    Slugmobile & MeanMini Caretaker Turbo Mopar Contributor wheming's Avatar
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    Re: Strange fuel pressure gauge reading

    If it is adjustable, i would reset to 55 with the hose off.
    It is a stock looking, non-adjustable regulator, then i would say someone has increased the spring pressure by slightly compressing the case in a vise.
    Post a pic of your regulator, and of the hose barb end.
    Wayne H.

    '91 Dodge Spirit ES 2.5L turbo 5spd
    '05 PT GT 2.4T HO autostick
    '89 Plymouth Acclaim 2.5L turbo auto, "Slugmobile" yes, THE Slugmobile!
    '89 Dodge Caravan SE 2.5L turbo auto, "Mean Mini" yes, Gus' Mean Mini! (Current best 11.7786@113.04 mph! - Jan 15th, 2017 Fayetteville Dragway, NC)
    MeanMini dragracing videos: https://www.youtube.com/playlist?lis...URZLB1RxGYF6vw
    and other cars, trucks and motorcycles
    https://www.youtube.com/user/SlugmobileMeanMini

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