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.
Toyota MDT in MO
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
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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