Dwell Fixed Greater Than 50 Degrees

This indicates a full lean command from the ECM to the carburetor caused by one of the following:

• M/C solenoid wires reversed.
• Leaking bowl vent valve, excessive fuel in vapor canister or fuel in crankcase.
• Faulty carburetor calibration or carburetor.
• Silicone-contaminated O2 sensor.
  1. Run engine at fast idle for at least 2 minutes to ensure vehicle has reached normal operating temperature. Allow engine to idle. Monitor mixture control solenoid dwell while creating a large vacuum leak. This determines if problem is related to engine or electronics. Normal response is dwell decrease. If dwell decreases, this indicates O2 sensor, harness and ECM are okay. Problem is a rich engine condition. If engine is very rich, a large air leak may be required to lean mixture. When mixture is lean enough, engine will begin to run rough and dwell will drop at least 20 degrees.
  2. If dwell drops at least 20 degrees, check evaporative canister for being loaded with fuel. Check related purge and vent valves. If plugging PCV or bowl vent vacuum hose causes dwell to decrease, that hose leads to source of problem. Also check for fuel in crankcase. If no problems are found, service carburetor, including checking float level.
  3. If dwell did not drop at least 20 degrees when vacuum leak was created, disconnect oxygen sensor harness connector and ground ECM side of harness (not sensor). This checks ECM response to a "lean" O2 signal. Normal response is drop in dwell. No dwell change indicates a defective ECM. This test also eliminates the possibility of an open sensor wire. An open wire would cause "open loop" operation and may set Code 13.
  4. If dwell drops to less than 10 degrees when ECM side of oxygen sensor harness is grounded, turn ignition off. Turn ignition on with engine off. Remove ground from ECM side of oxygen sensor harness. Using a DVOM, check voltage on ECM side of harness. If reading is less than .55 volt, replace oxygen sensor. If reading is .55 volt or greater, check for short to voltage on harness. If short to voltage is not present, replace ECM.