System Performance Check

Fig. 022 - Chart - System Performance Check.:

Fig. 024 - Procedure for System Performance Check. VIN 6, VIN 9, VIN A, VIN G, VIN H, VIN X:

SYSTEM PERFORMANCE CHECK
CARBURETED


Test Description:

Step numbers refer to step numbers on a diagnostic chart.
1. Checks for the ability of the carburetor main metering system to change the Air/Fuel mixture. Disconnecting the M/C solenoid makes the carburetor operate full-rich (0% Duty Cycle) and reconnecting it with the dwell lead grounded makes it operate full-lean (100% Duty Cycle). Normal response is for rpm to drop, as M/C solenoid is reconnected, due to enleanment. Normal drop should be within a 300 to 1000 rpm range.
1a. If plugging the PCV, Purge, or Bowl Vent vacuum hose causes rpm to drop over 300 rpm, that hose leads to the source of the problem. If rpm increases as M/C solenoid is connected, it indicates the system is running extremely rich. This can sometimes be caused by incorrect valve timing.
2. Checks for proper control of idle circuit.
2a. This is a full-rich command to the carburetor and can be caused by:
1. Lean engine condition.
2. Grounded oxygen sensor CKT 412 to ECM terminal "9", or bad sensor.
3. Open CKT 413, from ECM terminal "14" to ground.
4. Open CKT 452 to ECM terminal "22".
5. Open in coolant sensor CKTs 410 or 452.
2b. There is an "Open Loop" condition. It can be caused by:
1. An open oxygen sensor circuit or bad sensor.
2. An open in coolant sensor circuit.
3. An open CKT 413, from ECM terminal "14" to ground.
2c. This is a full lean command which can be caused by:
1. Rich engine condition caused by:
a. M/C Solenoid wire connections reversed.
b. Leaking Bowl Vent valve, excessive fuel in vapor canister, fuel in crankcase, faulty carburetor calibration or carburetor, or silicon contaminated oxygen sensor.
2d. Normal reading - operates in "Closed Loop" dwell is between 10°-50°, but varying. Run engine for 1 minute at fast idle to make sure the oxygen sensor is warm.
3. Checks for proper control of main metering system. RPM must be at least 3000 to get into the main metering system operation.
3a. A missing O-ring, between the switching valve solenoid and the valve, or a faulty valve, may cause air to leak to the exhaust ports at higher rpm only.