MAF voltages at idle?

  • #1

    MAF voltages at idle?

    So i have a factory FSM for my car, but it doesn't list operating specs at idle or 2k rpms, etc for MAF. My car fails smog due to high HC and sometimes CO (HC is 600-800, CO is 0-1.2%). I was able to get my car registered due to a waiver, but registration is coming up again in a couple months. And so far i've only been able to fix my idle issue, but i've still got a fuel mixture gremlin.

    What has been fixed/adjusted so far:

    15* BTDC timing
    Adjusted idle at IACV
    New air regulator
    New CHTS w/ connector (nissan)
    New fuel filter, plugs/wires/cap/rotor (all nissan parts)
    fuel pressure 34 at idle, 42 unplugged
    Has an aftermarket catalyst
    Vacuum is 22" at idle

    I think i have a sticking lifter on cold start in warm weather (its noisy, and vacuum stumbles on start-up until the lifter isn't noisy). After 5 seconds or so though, i have good vacuum. I've sprayed for intake leaks, and cannot find any. I'm suspecting maybe a leaky or plugged injector, or a "lean" miss, but i'm not sure.

    Anyway, onto my question, was just giving you a little backstory on what i'm diagnosing.

    I was checking my O2 voltages today, and fully warmed up after a drive and letting it idle, my O2 sensor was jumping from .04v to 1.22v, it didn't really sit long in the lower voltages indicating consistently running rich, but it would spike out to 1.1-1.2v, generally meaning lean, which seems alittle odd to me. Normally O2 sensors don't go above 1v very often, but this is a titania sensor i believe, so i don't know if its got a bit more tolerance. So i'm wondering if i have a bad sensor, or if i have an exhaust leak (i think i have a small one around the driverside manifold). I unplugged the O2 with the engine running, and it actually seemed to run a bit smoother, then i restarted the engine it started running like crap. So i reconnected the sensor, no change. Did a battery reset, and was still kinda rough but smoothed out shortly after.

    Anyway, i then checked MAF voltage at the ECM, and it shows 2.5v fairly steadily. Problem is, i don't know if thats normal or not, since the FSM doesn't list it, just 0-12v or something like that. I mean, other MAF's i've tested show voltage from .99-1.4v at idle. But those are fairly modern MAFs from the past 3-5 years. So, does anyone know what their car's MAF voltage is sitting at idle?

    Also, this comes more as a suggestion that anything, but perhaps we could make a chart/post of known good sensor readings/voltages on various years for diagnostic purposes? Mainly involving those still using the factory ECM. If you don't have emissions/idle components installed, maybe just state you don't for comparison.
  • #2
    High CO usually means bad cat… high HC generally means you're running rich or suffering some ignition problem, possibly you have 1 leaky injector that's dumping excess fuel too.

    Your fpr should always see vacuum reference, or you will always run rich at idle/low-rpm, and consequently fail
  • #3
    Z31s use a 2-7v MAF signal, versus modern cars 0-5v
    - VG30DET (HE341) 86 300ZX - 1982 280ZX Turbo - Headered NA 1986 300ZX 2+2 - 2000 Xterra -
  • #4
    Originally posted by michaelp
    Z31s use a 2-7v MAF signal, versus modern cars 0-5v
    So if 2v is the low end of range (low air flow), then 2.5v would sound about right at idle. Has anyone measured voltage of a MAF at idle in a normal working vehicle? Does it also show 2.5v?

    High CO usually means bad cat… high HC generally means you're running rich or suffering some ignition problem, possibly you have 1 leaky injector that's dumping excess fuel too.

    Your fpr should always see vacuum reference, or you will always run rich at idle/low-rpm, and consequently fail
    If i unplug the FPR with the engine running, should i notice any change? Because i can unplug it and i don't really notice any change in idle. I have vacuum at the FPR as well.
  • #5
    yeah, I was getting those voltage readings…its correct....I used to have a Fluke MM mounted in the interior which was connected to the AFM…I would hit the max/min record feature to see what it was doing at idle and upper rpms at full throttle boost…I was getting 2.48 at idle[700rpm] with a very smooth idle....your fine.
  • #6
    Originally posted by mcx
    yeah, I was getting those voltage readings…its correct....I used to have a Fluke MM mounted in the interior which was connected to the AFM…I would hit the max/min record feature to see what it was doing at idle and upper rpms at full throttle boost…I was getting 2.48 at idle[700rpm] with a very smooth idle....your fine.
    Thanks! Thats the answer to my question. So then.. my MAF voltage is normal, which would mean i'm probably lookin at an injector issue, or maybe an O2 sensor? Don't suppose you have o2 sensor readings at idle? lol. Ever see your O2 spike 1.2v?
  • #7
    no ,I never measured o2 voltage with MM ..I just put the ECU in the correct mode and made sure it flashed the correct number of times in 10seconds..or made sure the red and green flashed in synch as it idled and through 3000rpm or so.
  • #8
    The O2 sensor varies depending on your cars year and if its NA or turbo. Some Z31s use Titania, some use Zirconium.

    This means some create their own voltage reading for the signal, from 0-1v…while others work off a reference and vary resistance to feedback 1-5v.
    - VG30DET (HE341) 86 300ZX - 1982 280ZX Turbo - Headered NA 1986 300ZX 2+2 - 2000 Xterra -
  • #9
    Originally posted by michaelp
    The O2 sensor varies depending on your cars year and if its NA or turbo. Some Z31s use Titania, some use Zirconium.

    This means some create their own voltage reading for the signal, from 0-1v…while others work off a reference and vary resistance to feedback 1-5v.
    Mines a 87 turbo and as per fsm is a titania sensor. I never saw it go above 1.2v. It normally read like .05-.6v but would randomly flare up to 1.1-1.2v. Which if it reads like a zirconia type would mean its lean spikes.. but dont know if titania sensors read the same way. unless PO Installed a zirconia sensor not knowing. Would a zirconia sensor cause any damage to ECM if that was the case?
  • #10
    Originally posted by mcx
    no ,I never measured o2 voltage with MM ..I just put the ECU in the correct mode and made sure it flashed the correct number of times in 10seconds..or made sure the red and green flashed in synch as it idled and through 3000rpm or so.
    I did that as well previously and the LEDs flash as per ESM indicating stochiometric, but I find it hard to believe I'm stochiometric but failing smog due to high HC. Which is why I'm digging into diagnosing this.
  • #11
    before NY ditched the emission testing on pre obd2 cars, I used to knockback my timing to 10..change the oil…I'd run it on e85…and pass with flying colors....I always cleaned the posts on the distrib cap/rotor and checked the egr......I never failed or came close....it doesn't answer your problem but give it a try and be done with it.
  • #12
    Originally posted by mcx
    before NY ditched the emission testing on pre obd2 cars, I used to knockback my timing to 10..change the oil…I'd run it on e85…and pass with flying colors....I always cleaned the posts on the distrib cap/rotor and checked the egr......I never failed or came close....it doesn't answer your problem but give it a try and be done with it.
    The fun part is Upstate NY never had emissions testing other than visual on pre-obd2 cars. haha
    - VG30DET (HE341) 86 300ZX - 1982 280ZX Turbo - Headered NA 1986 300ZX 2+2 - 2000 Xterra -
  • #13
    I know…I know…upstate perks.
  • #14
    ^ Incidentally Mike, I work on the side in Yonkers doing NYS inspections. If you need to get the 2-hundo through just let me know, pre OBD stuff is simple. OBD-II stuff can be problematic but I've learned some techniques which can sometimes help (sometimes not).
  • #15
    Originally posted by Gregmatic
    ^ Incidentally Mike, I work on the side in Yonkers doing NYS inspections. If you need to get the 2-hundo through just let me know, pre OBD stuff is simple. OBD-II stuff can be problematic but I've learned some techniques which can sometimes help (sometimes not).
    Thanks Greg.