PS: I'll do the O2 vs misfire "false lean" twisting up the feedback loop in a subsequent reply. This one is already too long.
Before diving down the misfire/false lean rabbit hole, I will first make the following statement:
If you have a misfire and your fuel trims & O2 sensor graph data are off, you have to take the
resulting fuel trims & O2 graphs with a grain of salt. Not saying to discount it entirely, but be
aware that the engineers who set up the Closed Loop would be the first to tell you that a misfire
is an illegal condition, and that the feedback loop can't correct for this...and might actually make
a bad situation worse.
It can be doubly hard to troubleshoot a twisted up A/F feedback loop on top of a misfire. If possible,
divide and conquer by getting the engine to run misfire-free Open Loop first, and only then reestablish the
Closed Loop and continue troubleshooting if needed/as necessary.
NOTE: When these trucks were new/nearly new these problems compounding each other was a rare thing.
24-36 years later, it's not out of the realm to have a marginal feedback loop (ie: tired O2 sensors) being
tormented by one or more misfiring cylinders.
****
Our GMT400s implement the ability to fine-tune the A/F ratios to stay very close to a 14.7:1 Air/Fuel
ratio despite changes in elevation, fuel formulation, rpm, load, aging injectors, etc. At the time, the
biggest bang for the buck was to use the narrowband O2 sensors described in the previous reply.
During normal operation, the feedback loop constantly hop-scotches back and forth endlessly across the
450 millivolt ideal. The healthier the feedback loop, the faster/tighter that the O2 sensor(s) on one end
and ECM/PCM/VCM on the other end can do this A/F ratio dance.
Let's assume that the #2 spark plug wire burns against the adjacent header tube, resulting in a loss of
spark and a misfire. Instead of the oxygen in the intake charge for the #2 cylinder being consumed by
a successful combustion, it instead gets pumped out the open exhaust valve and past the waiting O2
sensor. Of course the O2 sensor, seeing oxygen on both sides of the zircon membrane, generates
a matching lower (lean) voltage and sends that up to the computer.
In response, the computer attempts to 'correct' the too lean reading by steadily enriching ALL cylinders
on that bank. (Since there's only 1 upstream sensor per bank, they are all treated the same, with the
implied assumption that all cylinders *are* the same. So, thanks to the burnt spark plug wire on the
#2 cylinder, cylinders 4,6, & 8 are all being fed a too-rich mixture.
Depending upon just how sideways everything gets, once we cross the +150% emissions threshold,
the SES light will come on. And if the issue is bad enough that the engineers were concerned that the
cat will be damaged, the SES light actually starts flashing, alerting the driver to a serious condition
requiring immediate attention.
****
Years after the GMT400s were manufactured, more modern systems would identify a misfiring cylinder
that could damage the cat with unburnt fuel, so the computer would protect the cat by disabling the
fuel injector to that misfiring cylinder. Pretty cool. But not on our old trucks.
Misfires are bad juju. If Bank #1 has pretty upstream sensor graphs and reasonable fuel trims, but
at the same time the Bank #2 upstream sensor is not crossing the 450 millivolt threshold and the
fuel trims are stuck against the stops, then even if all you are getting is the P0300 misfire, you can
still tell that the problem is on the right side of your engine. ('96+ Vortec owners. The OBD1 TBI
system doesn't identify which cylinder isn't pulling it's weight -- you have to troubleshoot them the
old-school way. :0)
Hopefully I was able to explain this in such a way that you can use the 'goofed' live data to help
you narrow down where to look for the root cause. Maybe not pinpoint the problem, but still better
than nothing.
Food for A/F and misfire thought.
Cheers --
Last edited: