Possible Causes
A. A/C idle control relay and/or related wiring^ switch A/C OFF
^ start engine and let idle
^ remove A/C idle control relay from position 4 of auxiliary fuse/relay panel (located under left side of instrument panel)
If fluctuation stops
^ check A/C idle control relay wiring in auxiliary fuse/relay panel using wiring diagram, repair as necessary
If wiring OK
^ replace relay
If fluctuation continues after removing relay
^ problem is NOT A/C related; continue checking for the problem using the troubleshooting sequence on the following pages
B. Carbon canister solenoid valves
^ start engine and let idle
^ pull off sealing cap (arrow) from charcoal canister
^ there should be little or NO vacuum present
If there is a strong vacuum or hissing sound
^ charcoal canister frequency valve is stuck open, repair or replace as necessary
^ ensure that supply voltage to valve(s) is OK, repair as necessary
^ check wiring using wiring diagram, repair or replace valve(s) as necessary
C. Intake system vacuum leaks from secondary throttle plate
^ diagnose and repair as necessary, see Group 25 of Repair Microfiche
D. Idle switch
^ check opening and closing using multimeter US 1119
^ 0.5 ohms maximum in closed position
^ adjust or replace as necessary, see Group 25 of Repair Microfiche
^ ensure that Campaign GJ has been performed
If NOT
^ perform Campaign GJ now
E. "Y" connector
^ inspect "Y" connector (arrow) for possible cracks or leaking connections, repair or replace with metal "Y" connector
^ Part No. 026 103 693B
CAUTION
The "Y" connector has a restriction in one of its ports. Connect this port to the crankcase ventilation hose. Failure to do so can cause driveability problems.
F. Idle stabilizer valve
^ trial replace idle stabilizer valve, see Group 25 of Repair Microfiche
Note
While checking the idle stabilizer valve it is possible to disturb the vacuum hose connection underneath the valve. Be sure that this connection is OK, and that there are no vacuum leaks.
G. Coolant temperature sensor
^ check and replace coolant temperature sensor if necessary, see Group 25 of Repair Microfiche
H. Cracked distributor cap, high resistance in rotor or spark plug wires
^ replace as necessary, see Group 28 of Repair Microfiche
If all checks up to this point are OK
^ proceed with the following check and adjustment
1. Airflow sensor potentiometer, adjusting to correct fluctuating idle condition
Requirement
^ idle stabilizer valve OK
^ all possible vacuum leaks eliminated
^ disconnect potentiometer harness connector and connect VW 1501 test adaptor between potentiometer and harness connector
^ switch multimeter US 1119 to 20 volt range and connect to terminals 1 and 3 (U, voltage) of VW 1501 adaptor
^ switch ON ignition
^ must be 4.35 to 5.35 volts
If NO
^ check wiring between Motronic control unit and potentiometer using wiring diagram, repair as necessary
If wiring OK
^ replace Motronic control unit
^ switch multimeter US 1119 to 2 volt range and connect to terminals 2 and 3 (U2 voltage) of VW 1501 adaptor
^ switch ON ignition (but do NOT start engine)
^ U2 voltage must fall within shaded area of graph
If NO
^ remove sealing compound covering head of potentiometer trim screw
CAUTION
Do NOT damage the trim screw when scraping away the sealing compound.
^ adjust U2 voltage via trim screw (arrow), while observing multimeter, until required voltage is obtained
^ start engine and observe U2 voltage
^ must fall within shaded area of graph
If the U2 voltage does NOT fall within the graph
^ the U2 reading is being influenced by another fault condition; possibly a vacuum leak that was overlooked
^ potentiometer/airflow sensor plate assembly is faulty
^ control unit or related wiring is faulty
If U2 adjustment NOT possible
^ replace airflow sensor plate/potentiometer assembly
CAUTION
Part numbers are for reference only. Always check with the parts department for the latest information.