[W126 Coupe] O2 sensor - CIS E saga continues.............

[Paul Miller]

Thanks from all for the input so far.....I am learning as I go and starting
to understand the CIS-E system a little

Went to the local dealer today and checked my ECU part numbers.  They have a
listing for components to retrofit O2 sensor ( and CAT ) to ECE 560's.

My ECU part number is the correct number for the conversion kit, which
includes the expected bits and pieces, some brackets, O2 sensor and a vacuum
valve ( not sure why at this stage )  Obviously I do not want a CAT, just
the improved management an O2 sensor will bring. 

Reading in various web sites ( porsche / audi / vw ) the consensus seem to
be that the lambda sensor is an integral part of the CIS-E and NOT just an
evil component for emission controls,  ( although it was necessary to
protect the CAT converter from high HC emissions. ) 

Having said that a well tuned car will run fine without an O2 sensor,  as
mine does, however the issue is can I improve fuel economy with a properly
functioning  O2 sensor. 

The following was ripped from a VW web site discussion about CIS-E
operation. 

( http://www.scirocco.org/list/archives/2003/July/msg01320.html )

Here's the long and short of it:  CIS-E meters fuel by measuring airflow 
> with a plate.  As the plate moves further upward in response to more air 
> travelling past it, it opens 4 valves in the fuel distributor which 
> increase the amount of fuel going to the injectors.  It's a brilliantly 
> simple system.
> 
> Certain situations call for a mixture leaner or richer than the baseline 
> level (somewhere around 'Stoich') [cold starting and running, overheating 
> engine, throttle transitions, full throttle, fuel cutoff, 
> etc].  Furthermore, using feedback from the O2 sensor, it can determine
the 
> *actual* mixture, and make very fast continual adjustments to even more 
> precisely control mixture.  The CIS-E computer sends signals to the DPR, 
> which allows it to make these corrections to the mixture.
> 
> There are two "halves" of the fuel distributor on CIS-E cars.  The lower 
> chamber is fuel pressurized by the fuel pump and sitting there waiting to 
> be fed to the injectors.  The upper chambers are fuel that is on its way
to 
> the injectors.  There are two ways that fuel can pass from the lower 
> chamber to the upper chambers:
>          (1) Through one of the 4 valves that are opened by the airflow 
> plate, and
>          (2) The differential pressure regulator (DPR).
> 
> If your engine is perfectly tuned, you can disconnect all of the 
> electronics from a CIS-E car and it will run fine.  (Mine runs 
> fantastically when it's warm with no CIS-E computer hooked up to it at 
> all).  It will meter fuel on a purely mechanical basis (i.e. through those

> 4 valves controlled by the airflow plate), and you'll lose all the 
> electronic babysitting services that the "E" added to CIS-E:  It'll start 
> hard, since it won't have cold-start enrichment.  It'll run rough when
cold 
> because it won't have cold-running enrichment.  It will lose some power 
> under full-throttle because it can't richen the mixture.  Your injectors 
> won't shut off when coasting under no throttle, causing a slight
backfiring 
> and a reduction of fuel mileage.  Your emissions will be higher because
the 
> computer can't adjust mixture based on O2-sensor feedback.  Some CIS-E 
> systems (like Mercedes CIS-E systems, for example) will richen the mixture

> at high engine temps to cool the motor down, or use an altitude-sensor to 
> compensate for high altitudes...
> 
> The computer does all of these neat little tricks by using the DPR.  The 
> DPR is a connection between the lower chamber of the fuel distributor and 
> the upper chambers of the fuel distributor.  It uses an electro-magnetic 
> membrane that acts as a valve to allow additional fuel to 'sneak' past the

> other 4 valves and supply more fuel to the injectors.
> 
> When no current is supplied to the DPR, the valve is like half closed.
The 
> fuel getting to the injectors is the fuel that passes by the 4 fuel 
> distributor valves and through the DPR.  The more current that is supplied

> to the DPR, the more fuel it allows to pass through it.  (So the mixture 
> will be richer for any given position of the air flow plate).  Likewise, 
> the less current that goes to the DPR, the less fuel that is allowed to 
> pass through it, and correspondingly, less fuel is delivered to the 
> injectors for any given position of the plate.
> 
> If you watch what happens to the DPR after you start the car, you can see 
> that it gets a high current from the computer, richening up the mixture at

> first.  This value then drops to the "baseline" value, which is usually 
> around 10mA.  (Don't quote me on some of these numbers, every system is 
> different and I never remember which is which -- Mercedes, for example, 
> uses Volts instead of Amps as a measurement for the DPR).  This value is 
> the limp-home mode.... it's the default value that the computer picks if
it 
> can't figure out what else to do.
> 
> If you blip the throttle, you'll see the current jump up, and then drop 
> down to a negative value for a split second, and then return.  What's 
> happening here is that the potentiometer I mentioned earlier is seeing a 
> quick change in air flow, and signaling the computer to fatten (=richen)
up 
> the mixture temporarily.  Then, the computer sees the idle switch being 
> activated, which means your foot is completely off the gas.  So it turns 
> the injectors off completely by closing the DPR's valve as much as it 
> can.  Once it returns to like 1300rpm, the fuel is switched back on, and 
> the value returns to "normal".
> 
> Once the O2 sensor begins to provide feedback, the DPR current will start 
> to fluctuate.  You'll see it rise and sink, rise and sink.  This is the 
> computer continually (over)compensating for the mixture.  It'll go a
little 
> too lean, then a little too rich, then a little too lean, and so on.  Any 
> of the other factors we mentioned before (like fuel cutoff or cold-engine 
> enrichment) will vary that current by a certain amount.  So, for example, 
> the DPR current (calculated based on O2-sensor feedback) will be reduced
by 
> a few mA to richen the mixture when the car is still a little cold.
> 
> At full throttle, the computer completely ignores the O2 sensor and goes 
> into a fixed enrichment mode... I can't remember what it is, but say 8mA.
> 
> Remember, this current reading isn't the "mixture" reading, it's a 
> *modifier.*  The real mixture is determined by the air-flow plate and its 
> subsequent opening of those 4 valves.  The DPR current tells us how much 
> the computer is MODIFYING the metering that the air-flow plate is giving
us.
> 
> Thus a reading of 0Ma shows is that the airflow plate is giving the engine

> too much fuel, and that the computer is telling the DPR to close its valve

> to help lean out the mixture.  Likewise, a DPR current of 35mA would mean 
> that the mechanical metering was giving the engine far too little fuel,
and 
> the computer was trying to compensate for that by letting lots of fuel
flow 
> through the DPR to richen the mixture.
> 
> So when we look at the DPR current reading on a healthy system, we should 
> see a value of something like 5mA.  (Don't quote me on the numbers, 
> remember).  This number corresponds to the point where the 
> mechanically-metered mixture is as close to "dead on" as it can get, and 
> allows the DPR enough room to compensate in both directions (i.e. it can 
> sufficiently LEAN or RICHEN the mixture).  If you set the mixture too lean

> with the screw (mechanically), you risk having the DPR be out of range:
It 
> can't provide enough fuel to compensate, and even doing all it can, you'll

> still be too lean.
> 
> The CIS-E box has a bunch of inputs that it relies on to determine what
the 
> mixture compensation (DPR current) should be.  I don't remember them all 
> off the top of my head, but some of them are:  The idle switch, the 
> full-throttle switch, the coolant temperature sensor, the thermo-time 
> switch, the Oxygen sensor, the airflow plate potentiometer, and some kind 
> of RPM sensor.  If the computer is getting an invalid reading (or no 
> reading at all) from one or more of the sensors, it will go into limp-home

> mode.  Obviously, the O2 sensor is one of the most important sensors -- 
> without it, the mixture compensation can't happen at all -- only the 
> "special" cases - like cold running, full-throttle, etc, are dealt with.
>


I will explore further the costs of retrofitting O2 sensor  ( now I know it
is possible ) and if realistic cost will consider doing as experiment.  (
extra 1-2 mpg would be very nice !! ).  I have a twin down pipes and a twin
non connected system  split exhaust so will need to either read one side
only, install 2 o2 sensors and combine outputs to average or install a
balance tube between left and right pipes and install o2 sensor there. 

I will continue to research and see what the local dealer comes up with.


Paul

1992 560 SEC
1994 E300D

PS Hi Talbir, I am in Oxford.  I have followed your coupes in various
articles and would love to have a look at them sometime and to meet you.