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CIS
Air/Fuel & Idle Adjustments |
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Duty Cycle Explained |
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Duty cycle is the pulsed
ground signal sent from the oxygen sensor control unit (Jetronic box under
the dash) to the frequency valve that determines the amount of time the
valve is open, which adjusts the fuel pressure for various conditions
(courtesy of tolusina of VWvortex.com):
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Cold-running
enrichment (forced during warm-up).
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Open-loop (just hangin'
at 50% after cold enrichment is over, waiting for oxygen
sensor activity).
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Closed-loop (oxygen
sensor has finally kicked in).
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Full-throttle
enrichment (driver has floored the accelerator).
The pulsed signal sent from the oxygen sensor control
unit can be measured in duty cycle on a 0-100% scale, or in dwell on a 0-90°
scale; this information is used to adjust the air/fuel mixture on a CIS car.
Here's how the system
works (provided everything is functioning as it should):
If you start the car when
the engine temperature is cold, the oxygen sensor thermoswitch closes and
sends a signal to the control unit. The control unit, in turn, sends a
fixed 72° dwell/80% duty cycle (58.5° dwell/65% duty cycle for 1987-1989)
signal to the frequency valve. That fixed cold-running enrichment
signal (72°/80%) remains in place until the coolant reaches a specific
temperature (77+°F, 104+°F, or 177+°F, depending on the thermoswitch
installed in your car); the thermoswitch then opens, cutting its signal to
the control unit. If the oxygen sensor is not at its operating
temperature when the thermoswitch closes, the system reverts to open-loop,
or "limp-home" mode (45° dwell/50% duty cycle). Open-loop occurs only
between the end of cold-running enrichment and the start of closed-loop with
a (finally) warmed-up and functioning oxygen sensor. Once the oxygen sensor
is heated to its proper temperature, it sends a signal to the control unit
and the system enters closed-loop, or its normal regulating function (45°±7°
dwell/50%±8% duty cycle). Should the full-throttle switch be
activated, closed-loop is overridden for forced full-throttle enrichment
(58.5° dwell/65% duty cycle).
Up through and including the 1986 model
year, cold-running enrichment (72° dwell/80% duty cycle) overrides
full-throttle enrichment (58.5° dwell/65% duty cycle). From the 1987
model year onward, cold-running enrichment does not override full-throttle
enrichment because the two values are the same (58.5° dwell/65% duty cycle).
"Open-loop will also happen when the car has
been parked briefly, the oxygen sensor has cooled off, and the
thermoswitch hasn't closed; upon restarting, it takes a minute or more
before the oxygen sensor heats enough to produce a signal and send the
system into closed-loop. That's why oxygen sensors are now heated; they
start working faster, systems go into closed-loop sooner.
"There isn't always an open-loop stage during warm-up. If the
engine is cold (real cold, like winter in Montana cold), and the oxygen
sensor is in great shape, the oxygen sensor's heater and the exhaust
heat may get the oxygen sensor up to operating temperature before the
thermoswitch warms enough from from the coolant to open. In that sort of
circumstance, the system will go straight from cold-running enrichment
to closed-loop; no open-loop transition at all. Same car, same
cold weather: The oxygen sensor will cool quickly when the car is
parked, while the coolant will retain heat much longer; on restart,
there will be no cold-running enrichment -- the system will be in
open-loop until the oxygen sensor warms up enough to put the system into
closed-loop." ~tolusina of VWvortex.com
Likewise, if the car is in a warm climate (or during
warm seasonal months), the system may revert to open-loop upon initial
start-up, skipping cold-running enrichment due to the coolant already being
at or above the thermoswitch's pre-set temperature.
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| Before
Making Any Adjustments |
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Requirements:
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Car should be otherwise well-tuned (see
below for further information)
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No vacuum leaks
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Distributor advance mechanism(s) should be
operating correctly
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Timing should be correctly set with a closed
throttle plate
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Air/fuel mixture plug removed and mixture
screw free of gunk
Tools needed:
Start with a complete tune up
(if not recently conducted):
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Replace ignition components: spark plugs,
cap, rotor, wires (use Bosch ignition parts and use only W7DC or W7DTC
plugs)
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Replace air and fuel filters
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Replace injector seals and idle screw O-ring
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Tighten the injector holders; replace the
injector holders if needed
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Check for vacuum leaks using carburetor
cleaner (spray around hoses, gaskets, injector O-rings); if idle goes up,
leak found (replace faulty part -- use a bit of RTV when replacing
gaskets)
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Check your K-Jetronic grounds
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Test your oxygen sensor
and replace if need be
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Check your throttle switches and idle boost
valves
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Check your CIS system and control pressures
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Check your distributor's centrifugal and
vacuum advance mechanisms
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Set your timing, if
need be
"Do not try to correct engine trouble by
making simple adjustments to the idle speed or the idle mixture (CO).
Changing these settings without the proper measuring equipment will only be
a hindrance when trying to locate the real fault."
To adjust the idle
without a DVOM, click here.
Once you've made sure all the basic stuff is
working as designed, should you still find improper idle conditions and/or
drivability problems, post your findings/troubles on the Cabriolet forums
listed on the Links Page so that it can be discussed to a conclusion, or
more likely, several conclusions. |
| Measuring
the Duty Cycle & Adjusting
the Air/Fuel Mixture and Idle |
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Step |
Picture |
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1. Warm up the car to operating temp (80°C or
higher),
cooling fan should cycle on/off at least once. Be sure all electrical
accessories are off (A/C, lights, cooling fan, etc.). |
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2. Connect the red (+) DVOM wire to the
blue/white wire's female terminal in the two-pin test connector by the
cold-start valve. Connect the black (-) DVOM wire to any convenient
ground*. If using the dwell setting, set meter to "4 cyl";
if using the duty cycle setting, set meter to "frequency %" (read
your meter's instructions; some meters require an additional step, such
as pressing a button or two).
*If you
have installed spade terminals in place of the OEM connector, simply
attach the red (+) probe’s alligator clip to the blue/white wire
terminal and the black (-) probe’s alligator clip to the brown wires’
terminal. |
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3. Your target reading is
45° ±7° dwell / 50% ±8% duty cycle (if it is not within this range, complete the next
steps).
45° dwell = 50% duty cycle
- Higher than 45°/50% = running lean
- Lower than 45°/50% = running rich
The picture at right shows an initial dwell
reading bouncing between 17.1° and 20.9° (this Cabriolet is running
rather rich). If the dwell bounce is higher than a range of 10 (a
bounce between 35 and 50, for example), there are other issues that need
resolving before continuing. |
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4. Get out your long 3mm Allen wrench to
adjust the air/fuel mixture (next to the fuel distributor on top of the
air filter housing). The weight of the wrench will affect the
mixture! Turn the wrench a little, just enough to feel it move, then
lift the wrench out of the adjustment screw. Repeat this process until
45° ±7° dwell / 50% ±8% duty cycle is achieved.
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5. Have your 7mm open-end wrench handy; you
may have to tweak the idle bypass screw (backside of the throttle body)
if the idle changes too much. You need to stay below about 900 RPM
while adjusting the idle. Use the tach on the dash and your ears, you
know what sounds right and normal. If you go over 900 RPM, the
centrifugal advance starts to kick in, changing the timing and confusing
all your readings. Set the idle RPM at 900± but above the threshold of
the idle boost valve. |
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6. If the idle boost valve kicks in while you
are adjusting, you've dropped too low on idle RPM. You can unplug the
idle boost valve electrical connector, or better, pinch off either of
the hoses going to it. When the idle boost valve (or hose) is
reconnected, idle should not change if you've adjusted the idle
correctly. |
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7. Switch on the A/C (if installed) to make sure that the A/C
idle boost valve works; idle should stay about the same with the A/C
compressor running. |
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Should you find questionable dwell meter operation while setting
frequency valve duty cycle (ex., the reading never changes), connect
your dwell test lead to the Hall generator green/white wire and test
your meter. It should read a very steady 36-37° on the 4 cylinder
scale, 40.5-40.6% duty cycle. If the meter proves to be good, conduct
the tests in the following table. |
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all idle adjustment info
above supplied by "tolusina" of VWvortex (edited by Cabby-Info.com); all
pictures supplied by KamzKreationz |
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| Table
for Oxygen Sensor System Testing |
If you have out-of-the-ordinary readings
during the mixture adjustment: After verifying that your meter is good, print the
following chart, conduct the tests listed (warm-up and operating temp are
the same measurements you took initially; they're included in the table for
convenience), write your readings in the "Your
Reading" column and compare them to what the readings should be to help
determine what component could be faulty.
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Test |
Oxygen Sensor |
Full-throttle Switch |
Oxygen Sensor
Thermoswitch |
Running Condition/Stage |
Dwell Reading
Should Be... |
Duty Cycle Reading
Should Be... |
Your Dwell or Duty Cycle Reading |
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Warm-up |
Connected |
Connected |
Connected |
cold-running
enrichment |
72° ±2°
1987+:
58.5° ±2° |
80% ±2%
1987+:
65% ±2% |
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Operating
Temp |
Connected |
Connected |
Connected |
closed-loop |
45° ±7° |
50% ±8% |
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1 |
Connected
or
Disconnected |
Connected |
Disconnected |
open-loop |
45° ±2° |
50% ±2% |
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2 |
Connected |
Connected
&
actuated |
Disconnected |
full-throttle
enrichment |
58.5°
±2° |
65%
±2% |
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3 |
Connected |
Connected |
Disconnected, leads bridged w/
jumper wire |
cold-running
enrichment |
72° ±2° |
80% ±2% |
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3a
1987-1989 |
Connected |
Connected |
Disconnected,
brown/red wire grounded |
cold-running
enrichment |
58.5° ±2° |
65% ±2% |
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3b
1987-1989 |
Disconnected |
Connected |
Disconnected,
leads bridged w/
jumper wire |
cold-running
enrichment |
58.5° ±2°
(45°: check
vacuum switch) |
65% ±2%
(50%: check
vacuum switch) |
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4 |
Disconnected,
ECU lead grounded |
Connected |
Disconnected |
lean condition, rich correction (closed
loop) |
78.3° and increasing |
87% and increasing |
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5 |
Disconnected,
+1.5V applied to ECU lead, -1.5V
grounded |
Connected |
Disconnected |
rich condition, lean correction (closed
loop) |
18° and decreasing |
20% and decreasing |
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Notes |
Tests 4 & 5 require a D-cell 1.5V battery |
Front part of throttle body |
Underside of coolant flange on head |
Tests 1-5 are condition simulation
tests; being at operating temp is unnecessary |
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Negative slope %; if readings are
positive, subtract from 100 |
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If the meter, thermoswitch, frequency valve, and oxygen sensor control
unit all prove to be working correctly, but the readings are still
whacky (as shown here), the test port should be suspected
of being faulty.
Solution: Cut off the stupid OEM plug and install spade terminals
(one
on the blue/white wire, one on the two brown ground wires);
use alligator clip adapters on your meter's probes and attach them
to the new spade terminals
as seen here. Your readings should now be within
specs outlined in the table above; if they are, resume your air/fuel
and/or idle adjustments. |
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| Testing
the
Oxygen Sensor
Control Unit & Oxygen Sensor |
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Tools needed:
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Step |
Picture |
Testing the cold-start enrichment
function:
- Disconnect the leads from the oxygen sensor thermoswitch
(underside of the cylinder head coolant flange) and bridge the
terminals using a jumper wire with spade terminals.
- Run the engine.
- Duty cycle should be a constant
80% ±2% (or 72° ±2°
dwell).
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Testing the open-loop (limp-home)
function:
- Disconnect the leads from the oxygen sensor thermoswitch (do not
bridge them).
- Run the engine.
- Duty cycle should be a constant
50% ±2% (or 45° ±2°
dwell).
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Testing the rich correction function:
- Disconnect the leads from the oxygen sensor thermoswitch (do not
bridge them).
- Disconnect the oxygen sensor wire (black) from the oxygen sensor
control unit wire (green).
- Run the engine.
- Ground the oxygen sensor control unit wire's spade terminal on
bare metal (extension used in the picture at right due the wire
being too short to reach a good ground).
- Duty cycle should be 87% and higher (dwell should be 78° and
higher)
.
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Testing the lean correction function:
- Disconnect the leads from the oxygen sensor thermoswitch (do not
bridge them).
- Disconnect the oxygen sensor wire (black) from the oxygen sensor
control unit wire (green).
- Run the engine.
- Ground the negative end of the D-cell battery while touching the
oxygen sensor control unit wire's spade terminal to the positive end
of the D-cell battery.
- Duty cycle should be 20% and lower (dwell should be 18° and
lower). Engine may stall; if the reading is within spec,
stalling is normal.
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Testing the oxygen sensor, method 1:
- Start the engine and let it run until it reaches operating
temperature (80°C or higher); if it's already warm, let it run for
two minutes.
- Clamp off the hose from the idle speed boost valve (white valve,
if your car has A/C).
- Disconnect the crankcase ventilation hose.
- Make note of the duty cycle reading.
- Plug the crankcase ventilation hose. The duty cycle should
drop, then rise and fluctuate.
- If the duty cycle does not drop, then rise and fluctuate, the
oxygen sensor is faulty.
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Testing the oxygen sensor, method 2:
- Start the engine and let it run until it reaches operating
temperature (80°C or higher); if it's already warm, let it run for
two minutes.
- Disconnect the oxygen sensor wire (black) from the oxygen sensor
control unit wire (green).
- If you have two DVOMs, you can connect the second one as shown
in the "Measuring Duty Cycle" table above to read the duty
cycle/dwell at the same time.
- Connect the black DVOM lead to any convenient ground and connect
the red DVOM lead to the oxygen sensor wire (black).
- Set the DVOM dial to 2 DCV (or 4 DCV, if that's the lowest
setting on your meter).
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Ground the oxygen sensor control unit lead wire to bare metal to
simulate a lean condition
(an extension adapter is being used in
the picture at right due to the control unit wire not being long
enough to reach a ground). The oxygen
sensor's voltage should go high, 0.8 to 1.0 DCV
(duty cycle/dwell should go high, 85%/75°
or higher).
Connect the 1.5 volt battery's positive end to the oxygen control
unit lead wire and ground the negative end of the battery against
bare metal; this simulates a very rich condition. The oxygen sensor's voltage
should go low, 0.15 DCV or lower
(duty cycle/dwell should go very
low, 11%/25°). Note: The system may go so lean
that the engine stalls; this is normal.
A sluggish oxygen sensor may cause a failed smog inspection while
exhibiting absolutely no other drivability issues.
Should you see an operating range at the oxygen sensor ranging from
-0.5 DCV to +0.5 DCV (instead of the normal +0.1 DCV to +0.9 DCV), your
sensor has been permanently damaged by chemical contamination and needs
replacing.
Cabriolets built from
July 1987 through 1989 have heated (3-wire) oxygen
sensors. Should you see 12 DCV (or charging voltage) at the oxygen sensor
wire, replace the oxygen sensor immediately -- the heater has
shorted to the sensor.
Should the oxygen sensor control unit happen to be faulty, it will fail to
compensate for the simulated lean/rich conditions described above. The
oxygen sensor control unit rarely goes bad; therefore, before condemning the
control unit as being faulty, verify that the control unit is receiving
power and that all ground wires/connections are good. Additionally, if the
duty cycle stays at 65% or 80%, disconnect the cold running enrichment
switch (if installed) and the full-throttle switch, one at a time, then both
together if need be.
above test method supplied by
"Thomas_covenant" of Yahoo! Groups and "tolusina" of VWvortex (edited by
CabbyInfo.com) |
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| DIY
Timing |
The proper method for
setting your car's timing requires the use of a timing light. If you're
without a timing light, use the
Bad Habit Rabbit's
method until you can set the timing properly:
"All of your VWs have marks on the rotating
parts to denote 0° TDC. The cam has a dot that lines up with the valve
cover. The flywheel has a notch that lines up with the center of the hole on
top of the bell housing. The distributor has a slash that lines up with the
rotor. Line all of these up and your car is at 0. No advance. No retard.
In a pinch,
I've put the car in 5th gear (with the car off!) loosened the distributor
and pulled the car forward until the advance mark on the flywheel appeared
in the little timing hole. I then turned the distributor until the hash mark
lined up with the rotor and tightened her down. If your flywheel notch is at
3° or 5° or whatever, now your distributor is at 3° or 5°. It's probably
good for keeps or at least until you can get your hands on a timing light,
but it DOES work accurately!" |
| Removing
the Anti-tamper Plug |
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It’s very
possible that your air/fuel mixture adjustment hole still has its
anti-tamper plug installed. If so, it will need to be removed, or tampered
with, in order to adjust the air/fuel mixture as described in the idle
adjustment section.
The
following tampering instructions are courtesy of “tolusina” of VWvortex.com:
“The easiest way to [remove the anti-tamper plug] is to flip the complete
sensor plate assembly over and punch the plug out with a long thin punch.
The other tamper method requires drilling a centered hole in the plug,
screwing in a sheet metal screw and pulling the screw up. You've got to be
sure in advance that the drill bit used is small enough that the sheet metal
screw you choose will have plenty to grip. Also, grind the point off the tip
of the sheet metal screw so it doesn't bottom out.
Oh, yeah; you can't drill all the way through the aluminum plug [because]
there's a floating steel disc in the bottom of the plug that will just spin
when the drill bit hits it. The steel disc is to prevent drill chips from
dropping inside. The steel disc is also what a pointed sheet metal screw
will bottom out on before the rest of the screw gets a good grip.
If you booger things up with a drill and sheet metal screw, no worries; flip
the assembly and punch it out, bottom-up.” |
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