Technical aspects of automobiles

High emissions at idle

I have problems with high HC emissions at idle. The engine runs fine,
ignition timing is at specs and other emissions are below the limit. When I
run the engine at fast idle, HC readings fall but at idle they slowly creep
over the limit.

This is a friend’s 2001 3-series BMW with 90k miles, has new plugs and the
engine idles smoothly.


TIA,
Leon

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posted by admin in Без рубрики and have Comments (5)

5 Responses to “High emissions at idle”

  1. admin says:

    Leon wrote:
    > I have problems with high HC emissions at idle. The engine runs fine,
    > ignition timing is at specs and other emissions are below the limit. When I
    > run the engine at fast idle, HC readings fall but at idle they slowly creep
    > over the limit.

    > This is a friend’s 2001 3-series BMW with 90k miles, has new plugs and the
    > engine idles smoothly.

    > —
    > TIA,
    > Leon

    Possibly a vacuum leak that would cause leaning at idle just to the
    point of slight misfire.  This will raise HC only at idle, and may not
    be severe enough to feel – especially with that 2.5 I6 engine.

    Toyota MDT in MO

  2. admin says:

    - Hide quoted text — Show quoted text -

    Comboverfish wrote:

    > Leon wrote:
    > > I have problems with high HC emissions at idle. The engine runs
    > > fine, ignition timing is at specs and other emissions are below the
    > > limit. When I run the engine at fast idle, HC readings fall but at
    > > idle they slowly creep over the limit.

    > > This is a friend’s 2001 3-series BMW with 90k miles, has new plugs
    > > and the engine idles smoothly.

    > Possibly a vacuum leak that would cause leaning at idle just to the
    > point of slight misfire.  This will raise HC only at idle, and may not
    > be severe enough to feel – especially with that 2.5 I6 engine.

    On a related note, what would cause high *CO* at idle?

    (Assume all other numbers are well within limits at any speed, and CO
    is low at speed as well.)

  3. admin says:

    Hugo Schmeisser wrote:
    > On a related note, what would cause high *CO* at idle?

    > (Assume all other numbers are well within limits at any speed, and CO
    > is low at speed as well.)

    CO is  produced from an excessively rich mixture.  Unless you are
    dealing with a carburetted or old FI system, it’s not likely that you
    will see high CO *only* at idle RPM.  I didn’t say impossible, just not
    likely.

    Toyota MDT in MO

  4. admin says:

    Comboverfish wrote:

    > Hugo Schmeisser wrote:

    > > On a related note, what would cause high CO at idle?

    > > (Assume all other numbers are well within limits at any speed, and
    > > CO is low at speed as well.)

    > CO is  produced from an excessively rich mixture.  Unless you are
    > dealing with a carburetted or old FI system, it’s not likely that you
    > will see high CO only at idle RPM.  I didn’t say impossible, just not
    > likely.

    You imply that this *may* be possible with an "old FI" system. If so,
    why would that be the case?

  5. admin says:

    Hugo Schmeisser wrote:
    > You imply that this *may* be possible with an "old FI" system. If so,
    > why would that be the case?

    Older Bosch type systems which most manufacturers adopted or copied had
    little control over emissions at idle.  They could very well run
    extremely rich at idle yet fairly clean at cruise.  There was no use of
    feedback info from the O2 sensor at 0% throttle.  Really early units
    including mechanical injection didn’t even employ an oxygen sensor.
    The old Bosch units came equiped with an idle mixture adjustment to
    dial in idle quality and help pass the old two speed emissions tests
    that were once in use.  Carburetors have various circuits designed into
    them, one of them being the idle circuit.  A problem with this system
    would only effect idle quality/emissions.

    Newer systems including all OBDII vehicles are in closed loop even at
    idle.  Heated O2 sensors and faster warming TWC converters help make
    this possible.  Furthermore, specific monitoring routines and
    strategies that target idle performance are built into the master set
    of routines in many of the noncontinuous monitors.  Even if an
    emissions problem does exist only at idle, it won’t likely sneak past
    the MIL anymore as in the past (for those older cars that were even
    *equipped* with a MIL)

    Toyota MDT in MO

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