Dean Swanson wrote:
tried to make an exhaust tailpipe sniffer (to help with adjusting carbs)
out of a four wire O2 sensor and an AutoMeter Air/Fuel ratio gauge.
Dean, The purpose of the heater circuit is to get the car to go into
closed loop operation as soon as possible. Once the sensor is at
operating temperature it will start to produce between 0 and 900
millivolts depending on the air / fuel ratio it detects. "Operating
temperature" is quite high and I don't think the sensor can achieve it by
the heater alone. IOW, the exhaust makes a significant contribution to
the heating of the sensor. Since the purpose of the muffler (silencer)
is to let the hot exhaust gasses cool and expand, the exhaust temperature
in the tailpipe is MUCH lower than at the engine. So first, I don't
think the sensor is getting hot enough. Secondly, especially in a four
cylinder engine, the exhaust pulses are, even at the tailpipe, quite
distinct. As the pulse of gas hits the atmosphere a similar wave is
"reflected" back up the pipe. (Google "extractor exhaust theory" - my
info comes from a very old magazine article from the 60s) It is possible
that this wave is adding air to the mixture seen by the sensor in the
tailpipe. I've been taught that a leak anywhere in the exhaust, but
especially near the sensor, will fool it into a false reading.
Chris
AN5L35578
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