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CISe system q's [Answers - long]
Thanks guys. :)
I'll try to make some time to re-write it and clean it up a bit (and verify
numbers and stuff) and then I can resend it and we can add it to the tech
articles...
Jason
At 10:53 AM 7/25/2003, Matt Coates wrote:
>GREAT post. That is the best explanation I've ever seen for CIS-E. That
>one's
>going in the save file, the garage wall, the glove compartment etc ;)
>
>If we included the correct MA readings for the DPR I think it would be a great
>addition to the tech section of scirocco.org and the FAQ on vwvortex.com.
>
>If I get around to looking it up in the Bentley I'll resend it w/ the correct
>MA numbers, a must have reference!
>
>Thanks Jason!!!
>
>Matt Coates
>
>--- Jason <jason@scirocco.org> wrote:
> > At 12:58 PM 7/24/2003, Ryan H wrote:
> > >Well, if your car was running limp-mode, you'd really notice it, cause
> > >you'd have NO power. None.
> >
> > She actually was -- I saw the 10mA reading.
> > Whether the car will actually run like shizzynit is going to be a
> factor of
> > where the mixture is set. I'll explain in a minute.
> >
> >
> >
> > >The connector on the back of the fuel block is what you measured (DPR)
> > >that read 10mA. Not exactly sure how this unit works, though. A reading
> > >that is stuck usually means a dead O2 sensor, but I was under the
> > >impression by others that the reading would be 20mA in such a case,
> not 10.
> >
> > Actually, I've seen it read 0 mA with a dead O2 sensor, too. As O2
> sensors
> > age, they put less and less voltage out, which causes the computer to
> think
> > the car is becoming increasingly lean. It'll compensate for it by
> > richening the mixture (reducing the DPR current, eventually to 0). At
> some
> > point, the sensor will be totally dead, and then the computer will go into
> > limp-home mode. (More below).
> >
> >
> > > And the sensor on the front really has nothing to do with the CIS-E
> > > loop - it's an airflow sensor that is only used when you first start the
> > > car - it supplies additional fuel when you hit the throttle for the
> first
> > > 30 seconds or so after starting it. If you were to unplug it then blip
> > > the throttle right after starting, the car would bog nicely because of
> > > the missing signal. HTH.
> >
> > It actually works all the time -- not just the first 30 seconds. It's a
> > potentiometer that measures rate of change of the airflow meter. It will
> > respond by richening the mixture under any quick throttle
> > transitions. Unplug it, and you'll feel no big difference under
> > steady-state cruising. But any time you make a sudden downward
> movement of
> > that pedal, the engine will bog for a second until the mixture catches
> > up. There are a lot of reasons why you want a richer mixture under
> > transitional throttle, but one of the big reasons this device is used
> is to
> > compensate for the time it takes for the air flow plate to move and to
> > actually supply extra fuel to the injectors when the throttle is suddenly
> > opened.
> >
> > Julie, I recommend getting the Bosch Injection book. I'm going to look to
> > see if I have it on CD-ROM (or at least parts of it) for you to
> > download. It's a great resource and explains everything in great
> > detail. It's very technical, but I know you'll have no problem
> > understanding it.
> >
> > 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.
> >
> > If your DPR current never moves or changes, there's something causing your
> > computer to just give up. You need to check that all of the input signals
> > are actually getting to the computer... and that all of the things the
> > computer controls (DPR, aux air bypass valve, etc) are functioning
> > properly. If I remember correctly, the computer will throw in the
> towel if
> > the aux air bypass valve is disconnected, for example. (My Mercedes CIS-E
> > did...)
> >
> > And that's it. :)
> >
> > Good luck and HTH!
> > Jason
> >
> > PS: If you know how the aux air bypass valve works, (or the Idle
> > Stabilizer Valve, ISV, as it's been called), you'll recognize that the DPR
> > works in a similar way. The only 2 ways for air to enter the engine
> > through a closed throttle are (a) through the idle bypass, which is
> > adjusted by that screw, and (b) through the ISV. If the screw is set
> > properly, the ISV will be at the mid-point of its duty cycle, which means
> > that it's half open. This way it has maximum flexibility to raise idle
> (by
> > opening all the way), or reduce idle (by closing the rest of the way).
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> > > >
> > > > From: "Julie Macfarlane" <juliemac57@hotmail.com>
> > > > Date: 2003/07/24 Thu AM 07:10:59 EDT
> > > > To: scirocco-l@scirocco.org
> >
>=== message truncated ===
>
>
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