Heater Performance Test (Front System)

WARNING: This page is about a different car, the 2002 GMC Safari and 2002 Chevrolet Astro. However, it is still accessible from the selected car via links, so may be relevant.

WARNING: Avoid contact with moving parts and hot surfaces while working around a running engine in order to prevent physical injury.

NOTE: Definition: Heating system performance.

Were you sent here from SYMPTOM DIAGNOSIS or another diagnostic table? If so, go to next step. If not, go to SYMPTOM DIAGNOSIS under TROUBLE SHOOTING.
Start the engine. Allow the engine to idle. Does the engine reach a normal operating temperature? If so, go to next step. If not, go to step 15.
Allow the engine to idle. Select the warmest temperature setting. Select the minimum blower speed. Select the OFF position for the auxiliary heating system, if equipped. Feel the temperature of the heater inlet and outlet hoses at the coolant bypass valve. Does the inlet heater hose feel warmer than the outlet heater hose? If so, go to step 13.
Disconnect the vacuum hose from the coolant bypass valve. Is vacuum being applied to the valve? If so, go to step 14. If not, go to next step.
Remove the coolant bypass valve. See COOLANT BYPASS VALVE under REMOVAL & INSTALLATION. Inspect the coolant bypass valve for the following conditions: Binding, restrictions or damage. Are any of the listed conditions present? If so, go to step 16. If not, go to next step.
Is the vehicle equipped with rear heater? If so, go to next step. If not, go to step 8.
Inspect the coolant bypass valve for the following conditions: Binding or stuck open. Is the coolant bypass valve stuck open? If so, go to step 17. If not, go to next step.
Install the coolant bypass valve. See COOLANT BYPASS VALVE under REMOVAL & INSTALLATION. After repairs, go to next step.
Allow the engine to idle. Select the FLOOR mode. Select the minimum blower speed. Select the warmest temperature setting. Feel the temperature of the inlet and outlet heater hoses to the heater core. Does the inlet heater hose feel warmer than the outlet heater hose? If so, go to step 13. If not, go to next step.
Install a thermometer into the center instrument panel air outlet. Secure a thermometer to the heater core outlet hose. Select the PANEL mode. Select the maximum blower speed. Select the warmest temperature setting. Record the temperature at the following locations: The center instrument panel air outlet and heater core outlet hose. Compare the recorded temperatures. Are the two temperature readings about equal? If so, go to next step. If not, go to step 12.
Inspect the following areas of the vehicle for cold air leaks: The cowl, recirculation door and the HVAC module case. Perform the necessary repairs. After repairs are complete, go to step 19.
Inspect the temperature door operation. See TEMPERATURE DOOR ACTUATOR under REMOVAL & INSTALLATION. After repairs are complete, go to step 19.
Turn ignition off. Back-flush the heater core. Start the engine and allow to idle. Select the FLOOR mode. Select the minimum blower speed. Select the warmest temperature setting. Feel the temperature of the inlet and outlet heater hoses at the heater core. Does the inlet heater hose feel warmer than the outlet heater hose? If so, go to step 18. If not, go to step 19.
Repair the vacuum condition. See VACUUM DIAGRAMS . After repairs are complete, go to step 19.
Repair the low engine temperature concern. After repairs are complete, go to step 19.
Replace the coolant bypass valve. See COOLANT BYPASS VALVE under REMOVAL & INSTALLATION. After repairs are complete, go to step 19.
Replace the auxiliary coolant bypass valve. See COOLANT BYPASS VALVE under REMOVAL & INSTALLATION. After repairs are complete, go to step 19.
Replace the heater core. See HEATER CORE (FRONT) under REMOVAL & INSTALLATION. After repairs, go to next step.
Operate the system in order to verify the repair. Did you find and correct the condition? If so, system is okay. If not, go to step 2.