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Engine missing/hesitation (?) around 4K

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  • Engine missing/hesitation (?) around 4K

    Hello,
    I've just upgraded my 1985 GL 5-speed with a Stage 1 turbocharger from PureTurbos and am experiencing a slight but noticeable hesitation occurring when I rev up hard (WOT) from about 2K through to over 5K --- it always occurs at around 4K, but doesn't happen each and every time. It happens in 2nd or 3rd gear and maybe others, but I haven't taken the car that high in speed with this issue. Otherwise, the car dives fine. I'm listing all the engine mods I've had done so that perhaps someone might know what to look for ..

    JWT Engine ECU with Mustang MAF sensor
    Euro Cams
    1mm oversized intake and exhaust Ferrea valves
    Ported heads
    Bosch 440cc Green Top injectors
    Adjustable NISMO fuel pressure regulator set to 3 BAR (fuel pressure gauge installed in line)
    Denso/Kyosan Fuel pump (no dampener)
    Gutted intake plenum
    1 BAR bypass actuator preloaded to 0.4 BAR
    Larger 240SX throttle body
    Recirculation surge valve installed

    This issue only started to occur when I replaced the Turbocharger and fuel pump (done at the same time). There are no issues above 4K as the car continues to run strong and hard up to redline. And if I don't go to WOT but still accelerate hard, there's no hesitation at 4K. This makes me suspect that the fuel pump is not the issue, and I might benefit from a fuel damper. Perhaps some sort of pressure resonance is occurring and the demands at WOT aren't being met by one or more injectors due to a low pressure zone forming in the fuel rail. Can such an issue occur? I'm also wondering if it might not be a bad idea to remove the fuel pump's voltage from ECU control and have it go at full voltage all the time --- the amperage difference is not all that much. I can replace the pump with a relay to fool the ECU into thinking it's controlling the pump's voltage (when in fact it's varying voltage to the relay) and run a steady 12V through the relay to the pump. (A Bosch relay will work within the voltage swing the ECU allows.)

    I'll be more than happy to answer any other questions you might have regarding the car and what I've done. Thank you.
    Last edited by NewJerseyZ; 04-20-2018, 10:41 PM.

  • #2
    Well, in case anyone's interested, today I did the fuel pump relay circuit mod (as mentioned above) where a relay is put in the circuit to act as a load in place of the fuel pump for the ECU. The relay switches a steady 12V from the battery to the fuel pump. (The relay is a Nissan standard 30A blue top 4 contact relay.) Did it prevent the hesitation from occurring at heavy load? ... No. It's still there as it was before, but the car does seem to drive more smoothly and the idle is more steady. Except for that hesitation issue at 4 - 4.5K, everything works just fine.

    On a side note, a great way to run a wire from the engine compartment to the rear of the car is to snake it through a wire harness grommet just below the battery. It will come out just above the ECU ... from there you can run it along the existing harness of wires just below the passenger door sill. (Removing the battery makes the job very straightforward.)

    Comment


    • Dunkine
      Dunkine commented
      Editing a comment
      Will running your fuel pump that way shorten it's life span?
      If there's nothing wrong with your fuel pump, could it be a tuning issue?

  • #3
    Could be many things....

    Sounds maybe like a wideband O2 sensor would be beneficial to see what it's doing at a hard pull 4K. Going lean, or rich (or even no change) might help narrow down a cause.


    84 AE/Shiro #683/Shiro #820/84 Turbo

    Comment


    • NewJerseyZ
      NewJerseyZ commented
      Editing a comment
      Thank you for the reply ...
      I'm beginning to think I might be exceeding my fuel injectors limit. It occurred to me because everything else is just fine, and my only major change of late (before this issue) was increasing the turbocharger's capacity. I did some rough calculations of what's required under my conditions and it showed that 440cc injectors was pretty much at the limit of handling things. Add to that my Euro cams and larger valves, and I'm using a best guess for my turbocharger's mass air flow ... well, I just might be. Anyway, I'm going to try another approach first, and that is that I might have an unbalanced pair of fuel rails. You see, in order to get 3 BAR of fuel pressure, I needed to go with an adjustable FPR, so I have the NISMO unit. This eliminated the stock front FPR, so I decided to reverse the flow into the rails, using a Tee up front. But a Tee up front only fits if the 2 90-degree legs each feed one of the rails, and the fuel enters from the other leg. Hence one rail gets the fuel straight through the Tee while the other gets it after first making a 90-degree turn. This may be starving the second rail somewhat from a Bernoulli effect at the Tee. So I'm going to replace the Tee with a Y-connector with 2 60-degree legs which will give a much better (and hopefully balanced) split of fuel to each rail. (I am still using the OEM stock fuel rails that came with the car.) I'll update things after I try out the Y connector, which should be here in a week or so; it seems no one here in the US has one available that I can configure into my plumbing, so it's coming from Europe.
      Last edited by NewJerseyZ; 04-28-2018, 09:36 PM.

  • #4
    To Dunkine:
    Thank you for the reply; here are some thoughts regarding your comments ...
    As for the fuel pump going full tilt 100% of the time ... yes, it might shorten pump life, but I don't think it will be all that much. Plus I think it's worth the price as the ECU would be having to handle a greater current with the upgraded pump, and I might take that out much sooner --- not a good thing. And others on line have done this mod with no mention of early pump failure. I do like the fact that I now have a steady, reliable amount of fuel pressure (over manifold pressure) under all conditions. Also, don't forget that I'm using a Denso pump; modified yes, but still none-the-less very well made. (Hey, we modify our cars, I'm sure that somewhat shortens their lives.)
    As for the tuning issue, please read my reply to Z_Karma above, as I mention things regarding fuel delivery.

    Comment


    • #5
      Have you pulled your plugs to see how they've been burning? Like Z karma said you would benefit tremendously from installing a wideband 02. Not knowing what your air/fuel mixture is introduces a lot of guess work and unknowns when troubleshooting.
      Originally posted by Andrew84zx
      tell her your car is so fast it will make her panties fly off
      545 RWHP & 540 RWTQ
      VG30ET.com Member

      Comment


      • NewJerseyZ
        NewJerseyZ commented
        Editing a comment
        Thank you for replying, it's much appreciated ...
        Funny you should mention the plugs, because just yesterday I replaced them with colder ones ... Denso IQ24. I did this because I realized I was now producing a higher heat range within the cylinders than stock. Anyway, I examined them and saw that each one appeared textbook for plugs that were experiencing ideal fuel/air mixtures. No excess carbon or other signs of abuse. A slight brown color around the tip, as being normal. Just as they should appear. (They were NGK Irridiums with a 6 heat range number.) After installing them I fired up the engine just to be sure everything was secure ... I'm sure you know just how hard it is getting to some of those plugs. The car ran fine and idled smoothly, with only the normal variation in rpms (+/- 50 or so). No missing or unusual hesitation when blipping the throttle. All normal.
        I am curious however about having/using a wideband O2. Might you (or anyone else) suggest which one would work best given my modifications?
        Last edited by NewJerseyZ; 04-29-2018, 09:19 AM.

    • #6
      I highly recommend the zeitronix wideband O2. I've been using it for about 12 years and after using one I don't want to drive without one. It's literally the most important gauge in my car next to water temp.
      Originally posted by Andrew84zx
      tell her your car is so fast it will make her panties fly off
      545 RWHP & 540 RWTQ
      VG30ET.com Member

      Comment


      • NewJerseyZ
        NewJerseyZ commented
        Editing a comment
        Sorry, I don't have a photbucket account and can't see your image, but are you sure I can use such a device? Please note from the OP, I have a JWT ECU and don't believe I can do anything with it, tuning-wise. About the only changes I can make are fuel pressure and ignition timing.

    • #7
      No, you can't adjust the tune on the JWT but the Wideband will still tell you the exact health of your AFRs. Without a wideband your driving completely blind could be anywhere from pig rich conditions to the verge of blowing up from running too lean. The symptoms your car has will almost always show in the air/fuel mixture and once you observe an unsafe condition you can decide to actually stay out of it or avoid driving until its corrected. I can't possibly stress enough how important running a wideband O2 is.

      The image is just my signature and theres a photobucket plugin for Chrome you can download that fixes the broken photobucket links.
      Originally posted by Andrew84zx
      tell her your car is so fast it will make her panties fly off
      545 RWHP & 540 RWTQ
      VG30ET.com Member

      Comment

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