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re: running rich under boost (long response)
When you mention you are getting a "running rich" smell what do you mean exactly. Are you
smelling raw fuel/gasoline while under boost or are you getting a sulphur smell? I believe
the sulphur smell is actually caused by running the engine/cat convertor slightly lean for
long periods and then under a rich condition (ie going up a hill under heavy load) the sulphur
smell is produced.
With the fuel system in closed loop operation the O2 sensor voltage cycles up and down between
0.1V and 0.9V as the engine computer senses the O2 voltage and then changes the frequency valve
duty cycle. You can watch the frequency valve signal duty cycle change back and forth on an
oscilloscope and if you connect up the O2 voltage signal on channel 2 of the scope you will
see how the high (rich) O2 voltage will cause the frequency valve duty cycle to change in order
to lean out the mixture. Then the O2 voltage responds to this lean condition and goes low and
then the frequency valve duty cycle will change again to richen up the mixture
(only small changes in mixture are occuring). This toggling back and forth rich and lean actually
allows the 3-way catalytic convertor to oxidize Hydrocarbons and CO and to reduce NOx.
The O2 voltage frequency of oscillation is around 1 to 2 Hz (cycles/second) at idle and increases
slightly at higher engine RPM (higher exhaust flow). In other words the engine computer is
tweaking the mixture back and forth at this 1-2 Hz rate.
As mentioned before the O2 output voltage is close to 1V (0.9V) when running rich and down
close to 0.1V when lean, but I caution, the term "rich" could be when the Air/Fuel ratio is only
slightly rich past the 14.64 to 1 stoichometric point the O2 sensor was designed to be used with.
If the mixture is at 14.3 to 1 or way down at 12.0 to 1 and the O2 sensor voltage would be
showing close to 1V output. These sensors only really indicate a switch from rich to lean
across this 14.64 to 1 ratio point.
The CIS K-Jetronic systems with O2 sensor (closed loop feedback system) used in the MC Turbo
engines normally goes to a fixed duty cycle signal controlling the frequency valve which
regulates the pressure in the fuel distributor. The system is no longer under closed loop air/fuel
operation under full throttle acceleration. This richens up the mixture way past the 14.64 to 1
ratio. I have done some testing with an expensive ($5000) portible 4 gas exhaust analyzer with
air/fuel ratio indication that I borrowed to use on my 86 5000 CS Turbo. Under sustained
full throttle the Air/Fuel ratio goes down to around 12.6 to 1 and the O2 sensor voltage is
pegged high close to 1V. This test was done before and after I did some modifications to raise
my boost up to 1.8 bar.
The O2 sensor found in most modern vehicles has a linear output "only" at or near the 14.64 to 1
Air/Fuel ratio and I don't believe it is a good Air/Fuel ratio indicator sensor. There are other
O2 sensors I have read about in several SAE articles called Universal Exhaust Gas Oxygen
(UEGO) that were designed to work over a broader Air/Fuel range from 11.5 to 1 up to 18 to 1
I question the ability of some of these low cost Air/Fuel ratio indicators that use the vehicles
existing O2 sensors and have LED readouts to indicate Air/Fuel ratio. From all of the SAE
technical papers I have read on O2 sensors, I really don't think these standard O2 sensors can
indicate a wide range of Air/Fuel ratio info. These LED indicators could be useful for giving a
rough indication on whether the engine is very lean under full throttle acceleration
if a higher than 14.64 to 1 air/fuel ratio occurs.
Just my $0.02
Here are some O2 sensor SAE articles if anyone is interested.
SAE article # 930352 "Static Characteristics of ZrO2 Exhaust Gas Oxygen Sensors"
Ford Motor Co.
SAE # 920234 Universal Air-Fuel Ratio Heated Exhaust Gas Oxygen Sensor (UEGO) and further
applications. NGK Spark Plug Co., Ltd.
SAE #930388 "A Feedback A/F Control System for Low Emission Vehicles"
M.J. Anderson. Ford Motor Co.
SAE # 830986 "An Engine Dynamometer Study of the A/F Compatibility between a Three-Way
Catalyst and an Exhaust Gas Oxygen Sensor"