Fuel Pressure Drop on Throttle Tip-in

Discussion in 'High Tech Retrofits' started by redriderjf87, Sep 14, 2018.

  1. redriderjf87

    redriderjf87 Veteran Member

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    Jan 23, 2016
    Quad Cities, IL
    So, in follow-up to my other threads on the FiTech lean-spike that I *thought* I solved, I have replaced my mechanical fuel pressure gauge to a transducer that I am reading out to an Arduino. Strictly by reading the voltage coming from the transducer, I am seeing about a 5% pressure drop consistently (2.13V -> 2.03V, according to my transducer spec that is about a 3 psi drop). That is blipping the throttle at idle, so I imagine it's worse under load (will see when I get the Arduino mounted).

    I am dropping a line to FiTech and Dr. Google, but I'd like to get a sanity check, should I be seeing any pressure drop going through the FiTech TB regulator? I was under the impression that the ecu is especting 58 psi constantly to get the correct amount of fuel pulsed out. Sorry I am racking my brain at this point, thanks.
     
  2. 70RS_L48

    70RS_L48 Veteran Member

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    Mar 4, 2010
    Seattle Area
    The pressure should drop a little at tip-in since the injectors are opening more to provide fuel. The question is, does the pressure stay lower if you blipped the throttle and held it open? (just a little, to hold at 2500rpm or so) The pressure should move back to ~58psi (or the 2.13V you're seeing on your transducer.
     
  3. dave@ztech

    dave@ztech Veteran Member Lifetime Gold Member

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    Mar 27, 2016
    NY
    I would think, however, that based on how fast electronics react , you would have trouble seeing that drop, it should be so ms, that it would be seen only if you played it back in slow motion, just my thoughts!
     
  4. redriderjf87

    redriderjf87 Veteran Member

    117
    11
    Jan 23, 2016
    Quad Cities, IL
    Sorry for the late response. To answer 70RS's question, it seems like it always blips low but recovers to hold 2.13v, even if I keep the rpms up. Its annoying though since the car goes lean consistently everytime I stab the gas. I just can't tell if it's a software or physical pressure issue.
     
  5. ULTM8Z

    ULTM8Z Veteran Member

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    May 19, 2000
    Los Angeles
    not sure how your calibration is set up... but on a GM TPI system that would usually point to needing a little more fuel on the accelerator enrichment (basically more pump shot in carburetor-eese)....

    i guess I'm skeptical it's a pressure issue.

    are you going lean at sustained high load? if not then I doubt the the fuel pump is having problems on tip in.
     
  6. redriderjf87

    redriderjf87 Veteran Member

    117
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    Jan 23, 2016
    Quad Cities, IL
    I'm starting to agree on it not being a pump issue (or regulator).

    I should have added it's a Tanks Inc / Walboro 255 lph, going through a Russel filter and running mostly through the factory hard lines to the FiTech's regulator and back. Vented above the filler cap per Tanks' spec. Fuel pump wire tied into the FiTech orange wire and PWM set to 100% per FiTech.

    Under any rpm at constant load I'm able to meet my afr setpoints up to 6000. I've also upped the accell and fast accell pump, to the point that the car will bog/choke when i blip the gas. It almost seems like I'm changing the amount of fuel on tip-in, but it's not getting there soon enough. Which made me question my intake, but I don't really see anyone with an Edelbrock air-gap having issues.

    I am going to talk with FiTech again tomorrow, and probably re-scour the Chevelles thread again in the meantime haha. I will keep you guys updated and if theres anything you think would be good to check I'm all ears. Thanks for the help
     
    Last edited: Sep 16, 2018
  7. ULTM8Z

    ULTM8Z Veteran Member

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    May 19, 2000
    Los Angeles
    My Miniram was like that... it needs the fuel RIGHT AWAY because of the log style plenum. Any delay in delivering the added fuel resulted in tip-in stumble. In the $8D mask for TPI code, there's a factor that speeds up the delivery of the fuel, so I increased that considerably.

    Like you, before that I kept adding more and more fuel trying to defeat that tip-in, but it never quite got there. Once I found that factor and increased it, I discovered that I didn't need nearly as much quantity of accel enrichment.

    One thing I also did was to idle in open loop. That really nailed the tip-in crispness. It also allowed me to modify the fueling table to add just a little more fuel at/near idle speed at map values just above what I'm idling at. The open loop prevents the computer from fighting against me on that extra fuel by keeping it from pulling it back out. I also idle at about 13.5:1 AFR (as opposed to being forced into 14.7:1 in closed loop). For me, the little bit of extra fueling at these low speeds really got the Miniram reigned in.
     
  8. redriderjf87

    redriderjf87 Veteran Member

    117
    11
    Jan 23, 2016
    Quad Cities, IL
    Thanks, that sounds a lot like my situation. I see one setting in the FiTech setup that seems similar to what you're describing, I'm going to ask them about that (i forget the name off hand).

    In my recent troubleshooting, I am doing something at least kind of similar to open loop, in that when I most recently reset my fuel learn tables, I limited how much the system could learn out to keep it at 75% of the initial table, instead of ~65% that it would normally learn to. I then turned the learning off and have used that in the meantime. Definitely not ideal but it seems to give me somewhat a buffer of fuel to make it not *as* lean on tip-in. The problem I've seen is if I don't do that, the FiTech sees a lean stumble and richens up the fuel learn tables way out of wack.

    Fortunately I do not see too many issues off idle, it's mainly if I'm cruising at 1500-2500 at step in it at all. The other variable is my cam is fairly low vacuum (9" at 900 rpm idle) so I'm wondering if that's making it significantly harder to pull in fuel quickly. I plan to re-cam it this winter anyways since I'm trying to make it more streetable and make the components match up a little better from how the last owner built it.
     

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