oxygen sensor and missfire
10-29-2008, 08:09 AM
#2
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If it's a single cylinder misfire, it may come
from cylinder imbalance (like injector delivery).
Take the case where you have one injector
short-shooting fuel by 20%. The closed loop
fueling tries to get the -average- exhaust
oxygen content as seen by the O2 sensor,
to the point that indicates 14.7:1 (gas).
If it does this with 7 100% and one 90%
injector, what happens it you end up with
something like 7 delivering 101% of ideal
and one at 91% - still way lean (16:1) in
one hole, prone to random misfire if not
knock under load.
This is not the fault of the O2 sensor per
se, just that the loop in which it plays a
role, has a simpleminded goal and can't fix
all possible impairments. Injector balance
is assumed ideal and nobody checks it
until things go way wrong (because there
are no good tools in the hobbyist price
range, and few good ones at all).
from cylinder imbalance (like injector delivery).
Take the case where you have one injector
short-shooting fuel by 20%. The closed loop
fueling tries to get the -average- exhaust
oxygen content as seen by the O2 sensor,
to the point that indicates 14.7:1 (gas).
If it does this with 7 100% and one 90%
injector, what happens it you end up with
something like 7 delivering 101% of ideal
and one at 91% - still way lean (16:1) in
one hole, prone to random misfire if not
knock under load.
This is not the fault of the O2 sensor per
se, just that the loop in which it plays a
role, has a simpleminded goal and can't fix
all possible impairments. Injector balance
is assumed ideal and nobody checks it
until things go way wrong (because there
are no good tools in the hobbyist price
range, and few good ones at all).
