Dwell Fixed Less Than 10 Degrees

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

• Severe engine misfire.
• False lean O2 sensor signal caused by grounded or open O2 sensor circuit.
• High TPS signal voltage caused by open in ECM ground circuit No. 452 or sticking TPS.
• False cold engine signal caused by open in coolant temperature sensor circuit.
  1. Ensure vehicle is at operating temperature. With dwell meter still connected to Green dwell lead, increase engine speed to 2000 RPM. Choke engine and note dwell (allow one minute for maximum change). This determines if problem is computer or engine related. Dwell should start increasing as soon as engine is choked and increase until it is greater than 50 degrees.
  2. If dwell responds, problem is a lean signal from the oxygen sensor. Check for air or vacuum leaks, including air injection system and deceleration valve (if equipped). Check for exhaust leaks near oxygen sensor. Check for proper vacuum hose routing. Check for proper EGR valve operation.
  3. If dwell does not respond when choking engine at 2000 RPM, disconnect oxygen sensor. Connect a DVOM (minimum 10-megohm input impedance) set on the 20-volt scale between the Purple oxygen sensor lead to the ECM and the positive post of the battery. This checks ECM response to input to O2 sensor circuit. The voltmeter is used to put a voltage signal on the O2 sensor circuit to simulate a rich condition. Dwell should increase (lean command) if ECM and harness are good. If dwell increases, check for on open oxygen sensor ground circuit (terminal No. 14 of ECM) or replace faulty oxygen sensor.
  4. If dwell does not increase, check coolant temperature using a "Scan" tester. This checks for a normal coolant sensor circuit condition. Temperature on a hot engine should be greater than 167°F (75°C). If reading is less than specified, check for an open in the coolant temperature sensor circuit or an out-of-calibration sensor. See SYSTEM/COMPONENT TESTS article for testing of the coolant temperature sensor.
  5. If coolant temperature scans 167°F (75°C) or greater, check for grounded or open oxygen sensor circuits. If circuits are okay, scan throttle position sensor voltage. If TPS is greater than 1.1 volts, adjust or replace TPS as necessary. If TPS scans less than 1.1 volts, check for faulty connection at ECM terminal No. 14. If connection is okay, replace ECM.