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87 Pontiac 2.8L FI

Discussion in 'Electronic Repair' started by [email protected], Jun 28, 2006.

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  1. Guest

    Hi;

    This one has an IAC error (code 35) and it says it's lean I think (code
    44). I don't think the mixture is that far from stoich, as it passes HC
    and CO. Problem is it fails NOx. EGR ports are good. It also does not
    purge the EEC, there is a whoosh when you open the gas cap.

    The canister purge, EGR solenoid and TCC run off the same line, which
    is up, has 12 V. The main problem has something to do with EGR so we
    went there first. The convertor locks up we believe, but the system
    never goes into closed loop operation. After a several mile drive the
    light still flashes fast in run test so I'm not so sure about the TCC.
    I haven't personally driven it.

    Afterward, we grounded the gray wire to the EGR and it did not actuate,
    later we found that it is fed with ported vacuum, and there was on
    vacuum at idle. So we left the gray wire shorted to ground and revved
    it, sure enough it actuated and the engine stumbled.

    We moved on to the IAC, removed and inspected it. It seemed to be at or
    near maximum extention. Retracted the plunger and reinstalled leaving
    the electrical connector unplugged. Of course the engine ran at a high
    idle, and slowed down when the IAC was plugged back in, so we know it
    goes one way.

    It is surprising to me that ported vacuum is fed to the EEC, not
    manifold vacuum, I thought it purged right at startup. Either I'm
    mistaken or they changed it.

    There are four vacuum hoses at the throttle body The placement seems
    logical, The other two do have direct manifold vacuum.

    Is this correct or is there a clog ? The vacuum I can feel is indeed
    ported vacuum, not simply there when the RPMs are up, so I don't think
    it's a clog.

    The EGR has an air bleed, when it is not actuated the ported vacuum is
    there. This may be normal because there is an air filter. Is this
    normal ? A leak in that spot could concievably be taking the IAC out of
    range, throwing the code 35. Or is the IAC the more likely culprit ?

    With a code 44 though, I guess the possibility exists that it needs an
    O2 sensor. Should I go there ? There should be no reason an IAC or EGR
    malfunction should throw a code 44, or is there ? Also an O2 error
    should not cause it to throw a code 35, or should it ?

    Usually I can handle these things, but this one is a bit wierd. It
    might not be unique so therefore I post. I see where maybe it could be
    one simple thing with this combination of symptoms, or of course it
    could also be the ECM.

    Any ideas ?

    Thanks in advance.

    JURB
     
  2. Phil

    Phil Guest

    Swap the lambda (O2) sensor & retry....
     
  3. D_Zoot

    D_Zoot Guest

    First, some background information that might help some of this make
    sense:

    Idle air control is handled by the ecm and position is indicated in
    counts. 0 counts would be fully closed, 255 would be fully open.
    Generally at a hot idle counts average 15 to 45. You need some manner
    of scan tool to see what the counts are. The ecm will set a code 35 if
    the ecm reaches 0 counts or 255 counts and the idle speed is still not
    within the desired idle speed.

    Code 44, lean condition, is determined by monitoring the oxygen sensor.
    Bias voltage on the o2 circuit is 450mV. Anything below 450 is
    considered lean, anything above is considered rich. Since we can't
    hold fuel control exactly at 14.7:1 all the time we vary the fuel
    rich-lean-rich-lean and hopefully it will average out at stoich. If
    you tap into the o2 line with a scope or meter, you should see this
    happening, above 450, below 450 and so forth. The ecm sets a 44 when
    it has reached a certian point in it's fuel control compensation table
    and the o2 remains below 450mV.

    So, based on the code 44, it's likely that the o2 is stuck below 450.
    This could be either a bad sensor, which is common, or you could
    actually have a lean fuel mixture. One of the things that could cause
    lean fuel mixture is a vac leak. Something that bears creedence to a
    vac leak theory is the code 35. A vac leak could also cause higher
    than normal idle that the ecm cannot compenstate for. So, check
    carefully for vac leaks (don't forget to check in the engine base,
    since on a 2.8 an intake runner leak will be under a valve cover or in
    the lifter valley), if no vac leaks turn up, try an o2 sensor.

    Egr - I'm not convinced you actually have an egr problem at this point.
    Lean fuel will cause your NOX to be high due to the elevated
    combustion chamber tempurature. Get the fuel trim issue sorted out
    first. The small amount of vac you noted at the egr "filter" is
    actually the solenoid bleeding off vac to the valve when the ecm
    commands the egr closed.

    Purge - Purge is ported. Generally we don't want purge at idle as it
    can cause idle quality issues since we don't really know how much fuel
    vapor we are going to pull from the canister at any given time. On
    older vehicles (pre-OBDII) off idle purge is commanded on via the
    solenoid and the purge amount is controlled by the ported vac. The
    higher the throttle rate the more purge we can allow without noticing
    any change in the way the engine runs. If it's pulling a good vac in
    the fuel tank it sounds like your fuel cap may not be venting to allow
    fresh air to enter the system. But there are several venting schemes
    used, and since you didn't state what this is for a car I can't be more
    specific.

    Good luck!
    D
     
  4. Bob Kos

    Bob Kos Guest

    PCV Valve?

    It's a common source of vacuum leak and very often overlooked. Especially
    if it's recently been changed with the INCORRECT part ( oil change shops are
    good for this ).
     
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