On Thu, 31 May 2007, vanagonvw wrote:
> On Thu
>> Another cause is an engine running lean - look over your air filter
>> housing, any gaskets or rubber boots between the air-flow meter and
>> the heads, etc.
>
> Been over it, but intend to do it again, because I am sure wanting it
> to be something simple :)

Don't be afraid to really poke and prod at the soft parts.  When you're
coasting with your foot off the pedal, the intake manifold vacuum is
quite high and it can "suck open" a small slice or hole that won't be
very obvious when you're looking at it with the engine off.

> at 15mph:
> HC  ppm 122  vs max allowed of 398
> CO % 1.04  vs max allowed of 3.08
> NO ppm is 1848 vs max allowed of 2723
>
> at 25mph:
> HC  ppm 125  vs max allowed of 343
> CO % 0.86  vs max allowed of 2.98
> NO ppm is 1696 vs max allowed of 2523

I don't have too much experience interpreting smog numbers, but those
look reasonably OK - the NOx might be a little high.

>> Twiddling every adjustment there is every 5 miles will make your
>> engine unhappy, but so will not touching any of the adjustments for
>> 50,000 miles.  :)
>
> Thanks Matt. I appreciate the assist. Unless I miss my guess, there is
> precious little to adjust, beyond cap, rotor, and plug replacement.

The main thing I see missing from this list is timing.  It is _probably_
not the main source of a backfiring problem, but could contribute to it.

You might also go through the checks in the Bentley for the fuel
injection system sensors.  You could have a lean backfire from too much
air, or a lean backfire from not enough fuel.  Again, this is probably
not the main problem, and given that you passed smog there isn't
anything too horribly wrong, but it could contribute.

Matt Roberds