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Thread: 93 SC replaced MAF now pig rich

  1. #1
    Join Date
    Dec 2002
    Location
    Omaha Nebraska
    Posts
    557

    93 SC replaced MAF now pig rich

    Hello all,
    Itís been a while, been working ridiculous hours the last couple years. Iím a proud father, my son recently purchased a 93 SC, we knew it would need some work, she was in fair condition.

    When we first bought it, we noticed the engine would cut out with higher loads. Felt to me like it was running lean, and it was getting surprisingly high mpg. Checked and fixed several substantial vacuum leaks, replaced plugs and wires, didnít help.

    Noticed the PO had the wrong MAF sensor installed (Delphi P/N AF10111 for N/A and 5.0). I was able to play with a few spare miscellaneous MAF sensors and housings until it ran pretty good. Definitely have the wrong housing (90 SC) now and the sensor is unknown. It runs real good but mpg is definitely down. 19ish vacuum at idle and right at 12 lbs boost on the factory gauge.

    We purchased a used OEM MAF specifically for the 93 SC and immediately it ran like crap. Pig rich, instantly flooding while cranking and wouldnít start. Had to hold the throttle wide open (fuel cutoff) to get it started, literally took half throttle to keep it running. Plumes of black smoke. Afraid to keep it running for too long, shut it down after 30 seconds. Yes we cleared the ecu memory. Checked the MAF for oring and cleaned the elements, no luck.

    Reinstalled the original cobbled MAF, and she smoothed out immediately. Reinstalled the correct 93 MAF, and immediately back to pig rich. Then one more time installed the cobbled MAF, running good again. Just for giggles we unplugged the MAF and while definitely it didnít run correctly, it did start, run, and idle reliably.

    Is anyone aware of a MAF bench check procedure? Afraid to keep it running for too long while flooding to check voltage. Any other ideas?

    We will pull codes of course and perhaps check the ecu for the correct number. Car is all stock. Looking for any other ideas. Might try unplugging various sensors to see if it will run at all with the correct MAF.

    Thanks for your help!
    JJ

  2. #2
    Join Date
    Dec 2007
    Location
    Salem OR
    Posts
    6,938
    I've been testing my MAF sensor output lately...but strictly on the car, engine running.

    Can you test the sensor that makes it run ok? Perhaps it's fine and there is no need to replace it (mpg down might be other issues). You'd need to backprobe the connector at the sensor with a digital VOM.

    Dk blue/org. wire with red stripe (example '90 SC) is MAF voltage:
    • idle 0.60~0.90 volts
    • 20 mph 1.10 v
    • 40 mph 1.70 v
    • 60 mph 2.10 v

    On my '90, I see 1.3v @ 1,500 rpm & 1.7v @ 2,500 rpm.

    Did you measure the housing(s) to confirm correct for your '93, which I believe is 70mm. Do you have part numbers off both sensors...maybe someone here w/a '93 can confirm. Can you see what color the injectors are?

    Strange the supposed correct new sensor isn't working correct, but I think your decision to check the ECU is a good one. Hard to match the MAF if the ECU has been changed from OE. I suppose you could try to test it on the car too, but if the engine doesn't want to run smooth that might be more trouble than it's worth.

    I'll quote an on-car testing procedure I found on the 'net:

    • Note the Volts reading on your multimeter at idle. This voltage reading usually hovers somewhere around .9 to 1 Volt DC. This reading may be stable (with only small fluctuations) or unstable with very extreme fluctuations. No matter what the instability in the reading, this will be your base reading.

    Accelerate the engine (by manually opening the throttle) as you watch the multimeter's voltage readings.
    • At about 1500 RPM's you'll see about 1.4 Volts DC.
    • At around 2500 RPM's the multimeter should register around 1.8 Volts DC.
    • The actual voltage reading on your multimeter may/will vary slightly.

    CASE 1: If the multimeter showed a smooth increase and decrease, as you manually opened and closed the throttle: then this result confirms that the MAF sensor on your 3.8L is functioning correctly.

    If the MAF sensor was defective, it would've stayed stuck at a specific voltage value no matter how much you revved the engine. Volts below 0.45 should throw a check engine light.

    CASE 2: If the multimeter DID NOT show a smooth increase and decrease, as you manually opened and closed the throttle: then this result confirms that the MAF sensor is BAD and needs to be replaced.
    Last edited by KMT; 03-19-2019 at 07:45 PM. Reason: wire color

  3. #3
    Join Date
    Dec 2002
    Location
    Omaha Nebraska
    Posts
    557

    Maybe nailed it

    Checked the ecu, correct p/n there.

    Backprobed MAF voltage with the current set up at the LB/R wire (terminal D on the MAFS), took the multimeter ground at the battery terminal. Found consistent numbers with what you posted in park idle, with a couple revs. .9ish on a warm motor and slowly decreased as the idle came down. Not EXACTLY the same as what you posted and I didnít get stabilized readings but for ballpark comparison purposes, relatively close.

    Started thinking, I wondered what the key on engine off voltage is with the MAF connected? About .7v, and responded to blowing through the MAF.

    Connected the recently purchased MAF, voltage sat at 2.5! It did also respond to blowing through it. Think you nailed it KMT! Relatively speaking, at less than 50% load the mafs would be representing full airflow.

    I was able to scoop up another 93 mafs, started doing some comparative bench resistance checks. The biggest notable difference between the suspected bad unit and my new (used) unit is between terminals A and D. Bad unit 1570 ohms, new unit 1455 ohms. All other pin checks came out pretty close, so whatever the relationship is between those 2 must be the culprit.

    Popping in my new mafs in the morning and will report back the findings. Thanks so much for your help.
    JJ

  4. #4
    Join Date
    Dec 2002
    Location
    Omaha Nebraska
    Posts
    557

    Good to go!

    Yep, confirmed the MAF I purchased was no good. It was used so to be expected. Note to anyone else having poor mileage or direvebility issues, check the voltage.

    Just to recap for historical data:

    1. Purchased the car knowing that it needed work. PO had the correct MAF housing but the incorrect aftermarket MAFS. Car had surprisingly good gas mileage and sometimes acted like it was starving for fuel. Engine would fall on its face with about 75% throttle.
    2. Cobbled together a 55mm housing and MAFS from a 92 N/A bird that I had as spares. Car ran pretty good but mileage and throttle response was down. Suspect it was running rich. Key on engine off voltage about .7. Engine running idle about .9v.
    3. Purchased a used correct MAFS, car would barely run. MAF voltage sat at 2.5v key on engine off.
    4. Purchased another used MAF, checked key on engine off voltage at about .7. Popped it in, cleared memory, started the car and took it on a drive. Nice snappy throttle response, will watch the mileage.

    Thanks very much KMT, you the man!!!!
    JJ

  5. #5
    Join Date
    Dec 2007
    Location
    Salem OR
    Posts
    6,938
    Good process, good work. Gave me some bits to add to my notes. Thanks for the follow up!

    Ken

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