System Performance Check
NOTE:
The following step numbers refer to the numbers in the accompanying flow chart(s).
- 1) This test checks carburetor ability to change air/fuel mixture. Disconnecting M/C solenoid makes carburetor run full rich, reconnecting it with dwell lead grounded makes carburetor run full lean. RPM normally drops 300-1000 RPM as solenoid is reconnected.
- 1A) If plugging PCV, Purge, or Bowl Vent hose causes RPM to drop more than 300 RPM, that hose leads to the source of the problem.
- 2) This test checks for proper control of idle circuit.
- 2A) This indicates a full rich command to the carburetor, caused by: lean engine condition, grounded O2 sensor wire or bad sensor, open wire from ECM terminal "14" to ground, open wire to ECM terminal "22" or open coolant sensor circuits 410 or 452.
- 2B) This indicates an open loop condition that can be caused by: an open O2 sensor circuit or bad sensor, an open coolant sensor circuit or an open wire from ECM terminal "14" to ground.
- 2C) This indicates a full lean command which can be caused by: M/C solenoid wires reversed, leaking Bowl Vent valve, excessive fuel in vapor canister, fuel in crankcase, faulty carburetor calibration or carburetor or silicone contaminated O2 sensor.
- 2D) Indicates "closed loop" operation. On 1987-1988 5.0L (VIN G/H), normal dwell reading is 10-45 degrees but varying (TSB No. 88-445-6E, December,1990). Run engine at 2000 RPM for one minute to ensure O2 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 valve, or a defective valve, may cause air to leak into the exhaust ports at higher RPM only.