Computers and Control Systems: Adjustments

Air/fuel mixture and idle speed is always controlled by the Digifant control unit. The purpose of the idle mixture adjustment is to fine-tune the mechanical adjustment of the air flow sensor and match it to the electronic controls. The CO. adjustment screw does not affect the actual air/fuel mixture, nor does the idle air bypass screw affect the actual idle speed.


Making these checks and adjustments together is very important to both driveablity and emissions control. If the equipment necessary to accurately perform this work is not available, we suggest turning the job over to a local repair shop that has the needed test equipment. In a properly equipped shop, these checks and adjustments can be made quickly, accurately, and at resonable cost.

NOTE: The first step in any Digifant I system troubleshooting must be to check the control unit fault memory, find and correct any faults that may be indicated, then erase the fault memory before continuing with testing procedures. For more information refef to COMPUTERS AND CONTROL SYSTEMS.

TEST AND ADJUSTMENT CONDITIONS

1. Engine oil temperature must be at least 176° F (80° C) and radiator cooling fan has operated at least once.
2. All electrical consumers switched OFF (radiator cooling fan must not be running during checking or adjusting).
3. A/C switched OFF.
4. Throttle valve potentiometer is functioning properly. For more information refer to COMPUTERS AND CONTROL SYSTEMS.
5. Fault memory must be clear of stored faults. If any are found, they must be corrected and then fault memory erased before proceeding. For more information refer to COMPUTERS AND CONTROL SYSTEMS.
6. The exhaust system must be free of leaks.
7. There must be no engine vacuum leaks.
8. The oxygen sensor must be connected and operating properly. For more information refer to COMPUTERS AND CONTROL SYSTEMS.
9. Throttle cable adjustment must be OK.
10. Idle stabilizer valve must vibrate and hum with the ignition switch turned ON.

TEST AND ADJUSTMENT SET-UP

1. Put vehicle in PARK or NEUTRAL set parking brake.
2. Turn ignition switch OFF.

Tester VAG 1367 Hookup:

3. Connect VAG 1367 engine tester using VAG 1367/8 inductive pickup, or a suitable strobe timing light and tachometer may be used.
4. Remove the cap from the CO tap tube that comes from the exhaust manifold and connect an EPA approved CO tester to the CO tap tube using VW special tool #6006-0019 high temperature silicone connecting hose, (or equivalent).

NOTE: Non-approved connecting hoses may dramatically alter your readings. Adaptor must be connected tightly to CO tap tube to prevent measuring errors.

TEST AND ADJUSTMENT PROCEDURE

Crankcase Control Valve And Heater:

1. Disconnect crankcase ventilation hose from emission control valve (arrow #1) and route so that only fresh air is drawn in.
2. START engine and let idle. Engine will run 1 to 4 minutes at increased engine rpm.

Coolant Temperature Sensor Location:

3. Once idle speed is reduced, disconnect harness connector from the blue coolant temperature sensor.

NOTE: Do not start engine with coolant temperature sensor disconnected. The control unit will go into open loop making accurate adjustment impossible.

4. Raise engine speed to a level between 2000 and 2500 rpm and check ignition timing. Timing should be:

4° to 8° BTDC at 2250 ± 50 rpm

5. Adjust ignition timing if needed.
6. Loosen distributor and rotate until specification is correct, adjusting value is:

6° ± 1° BTDC at 2250 ± 50 rpm

7. Tighten distributor bolts to specification.

IDLE SPEED AND CO CONTENT

1. Check CO content and idle speed. Record readings before adjusting, idle speed and CO content must be:

750 to 850 rpm
0.3% to 1.1% CO content

2. If readings are not within specifications, remove tamper proof cap from CO potentiometer.

CO And Idle Air Bypass Adjusting Screws:

3. Adjust by alternately turning the CO adjustment screw (#2) and the idle air bypass screw (#1) until both specifications are obtained.
4. Reconnect blue coolant temperature sensor harness connector.
5. Reconnect crankcase ventilation hose to emission control valve.
6. Recall fault memory and erase, (disconnecting the coolant temperature sensor while engine is running will register as a fault). The fault memory must be recalled and then erased for proper system operation. Refer to COMPUTERS AND CONTROL SYSTEMS for procedure.
7. Raise engine speed 3 times to 3000 rpm then let engine idle.
8. Check CO content and idle speed, idle speed and CO content must be:

750 to 850 rpm
0.3% to 1.1% CO content

9. If readings are still not within specifications, check the oxygen sensor and the control unit, refer to COMPUTERS AND CONTROL SYSTEMS.
10. After repairs or adjustments have been completed, install new plug over the CO adjustment screw.