Air/Fuel Mixture: Adjustments

Requirements:
Engine at normal operating temperature, 80°C (176°F).
All electrical consumers (A/C, rear window defroster, etc.).
Radiator fan must not be running while checking or adjusting.
No fuel preassure gauges connected to the fuel system.
Throttle valve in the proper idle position.
Throttle valve switch must be adjusted properly and working.
Exhaust system must be tight and free from leaks.
Oxygen sensor system in good working order.
NOTE; If the injection lines were loosened or replaced, run the engine up to at least 3000 RPM several times to clear the system of trapped air before making adjustments.

1. Initial setup
a. Disconnect the harness connectors from both charcoal canister solenoid valves.
b. Remove the cap from the elbow on top of the intake air boot. (The elbow contains a calibrated orifice and must remain in the intake air boot.
c. Remove the crankcase breather between the intake air boot and the valve cover.
d. Plug the hose.
e. Connect a test harness (VW 1315 A/2 or equivalent) between the idle control valve and the harness plug.
f. Connect a multi-meter to the test harness, and set it to the 2 amp range.
g. Connect an exhaust analyzer to the exhaust sampling port.

2. Check idle speed
a. Start the engine and let it idle.
Idle control current should be 430 mA
b. If the current reading is not within specification, adjust the idle by turning the screw in the throttle housing.
c. After adjustment, idle speed should be 720 +/- 50 RPM.

3. Check CO
a. Connect a duty cycle meter to the two terminal test connector located neat the fuel filter.
b. With the engine idling;
The duty cycle should be 25% to 65% and fluctuating.
The CO value should be 0.3 - 1.2% with oxygen sensor connected.
NOTE; If the CO is above 1.2%, and the duty cycle is between 25% and 65%, check the following:
Ignition system (check the primary and secondary systems).
Exhaust system (look for leaks, and repair as necessary).
Fuel distributor (check for sticking piston, unequal injector volumes, etc.).
c. If the duty cycle is less than 25% or greater than 65%, adjust the CO using the adjustment screw in the air flow sensor.

4. Adjust the CO
a. Switch the engine OFF.
b. Remove the intake air boot from the air flow sensor.
c. Drill a 3 mm (3/32 in.) hole in the concealment plug (located between the fuel distributor and the air boot).
d. Clean away any metal shavings, then extract the plug with a sheet metal screw and a pair of pliers.
e. Re-install the air boot.
f. Start the engine and allow it to idle.
g. Using a 3 mm allen wrench, adjust the duty cycle using the CO adjusting screw.
h. Turn the adjusting screw:
Clockwise (richer) to decrease the duty cycle
Counter-clockwise (leaner) to increase the duty cycle.
The duty cycle should be 50 +/- 8% and fluctuating.
CAUTION:
Do not push the adjustment wrench downward while adjusting. This will cause the engine to run lean and possibly backfire, causing damage to the air flow meter.
Do not accelerate the engine with the adjustment tool in place, as it will bind in the adjustment hole, possibly damaging the air flow meter, the adjustment tool, or both.
h. Re-check the idle speed. The idle speed should be 720 +/- 50 RPM
i. Adjust idle speed if necessary.

NOTE; After adjustments are completed, re-connect the crankcase breather hoses. If the idle control current and the CO change, this is not due to improper adjustment, but rather to oil contamination. If the car is used only for short trips, fuel vapors can accumulate in the oil and dilute it. A long distance drive will return the oil to normal again. If a ling drive is not possible, an oil change will cure the problem temporarily.