The ECU can tell the difference between "too rich" and a failing sensor by
the way the sensor works. When cold (or completely dead) the sensor appears
as an open circuit, and the .45 volt bias from the ECU is seen on the sensor
line as a good indication the O2 sensor has gone bye-bye. When the sensor is
working and the mixture is too lean, the output is around 0.1 volt, and when
the mixture is too rich it is around 0.9 volt DC. The ECU uses that
information to dither the mixture rapidly, slightly rich to slightly lean
and back again, hopefully 7 transitions per second or more. The sensors
don't usually die altogether, they just get weak and sluggish so the sensor
is slow to respond.
The ECU could produce the code by being insane, but that would be a pretty
unusual failure. ECU failures are rare enough, and the ones that aren't
caused by the ECU getting wet tend to be the power handling areas going bad.
If the code is "real" the ECU is saying that it is trying to keep the
mixture under control by slaving to the O2 sensor transitions, but the "ded
reckoning" part of the ECU is saying there should be more fuel. This is
often called being "out of trim."
This brings us to Mike's observation that the fuel pressure being high can
cause the code. The ECU calls for a certain amount of fuel to be injected,
but the high pressure forces more fuel through than expected... too rich.
The rest of the possibilities seemed to me to be backward - causing the O2
sensor to think the mixture was too lean. But I get confused about things
like that lately.
But I do wonder if an injector could be leaking into the manifold or slow to
close after a while. I am reminded of the mistake I made in buying
aftermarket injectors for our 85 765T. The first set was the wrong ones
(they were for a non-turbo) and the second set also failed to make the
engine run decently at all... barely idled when cold, perhaps 20 hp max. I
eventually took it to the dealer where it was run through the tests and
parts swaps. New AMM didn't help - the mixture was still way rich. The
injectors flow tested right on the money, but when Volvo injectors were
installed the AMM adjusted right up. Something wrong with them dynamically,
I guess.
Mike
