Car running lean at idle / under light throttle

  • easarcher
    easarcher
    Member
    • 96
    #1

    Car running lean at idle / under light throttle

    First off, let me start by saying I know that most cars idle a bit lean, that's not what I'm talking about.

    My Z is vg33et swapped, using stock Z31T injectors, turbo, and ECU. Fuel pressure is adjustable, but set at 35 psi idle. I just got through installing a wideband, and found some curiousities. At idle, the car runs rich, anywhere from ~15.3 or so to maxing out the gauge. Under light /medium throttle, it wavers around the 14s to 15s, and under boost it goes to about 11. Any ideas as to what could be causing it to idle so lean? I tried unplugging the o2 sensor, and that brought idle AFRs to 12.2 for a while, then it went back to the mid 16s by itself. It also bounces between 14.4ish to the mid 16s when under normal throttle, seemingly at random.

    I have never dealt with a wideband before, so I don't know what the range of normal is for our cars, but maxing out the gauge to lean sounds quite wrong to me. Any thoughts?
    09 Lancer ES 5 speed
    86 Z31 VG33E(NA2)T.

    "I want it to sound like the pits of hell are going to engulf your face!"
  • nitroman21
    nitroman21
    Senior Member
    • 135
    #2
    You might increase the fuel pressure a bit.. maybe 42psi? Being that the new motor has larger combustion chamber it would require slightly more fuel than the stock motor. The ECU can't tell that the engine is bigger and would be running the 3.0 fuel map.
    '86 2+2 NA2T: Completed and sold unfortunately.
    '86 GLL turbo. Still can't believe I got it.
  • michaelp
    michaelp
    Senior Member
    • 9384
    #3
    nitroman21;n764784 wrote: You might increase the fuel pressure a bit.. maybe 42psi? Being that the new motor has larger combustion chamber it would require slightly more fuel than the stock motor. The ECU can't tell that the engine is bigger and would be running the 3.0 fuel map.
    Combustion chamber size has nothing to do with requiring more fuel. More air means more fuel is required - but the stock EFI can compensate for this. Every VG33 swap I've done has run equally as rich as the VG30 did. Its a MAF system not MAP.

    Also. Combustion chambers are identical sizes - displacement is .3L larger.
    - VG30DET (HE341) 86 300ZX - 1982 280ZX Turbo - Headered NA 1986 300ZX 2+2 - 2000 Xterra -
  • 300zxturboftw
    300zxturboftw
    Senior Member
    • 3709
    #4
    Set your fuel pressure to 37 psi static. Do a leak test in the intake system using smoke or pressurized air. Sounds like a vacuum and boost leak to me
    Usual Z31 suspect: Garage Queen (aka broken)