GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2002 🢒 Safari - 4WD 🢒 HVAC 🢒 HVAC Systems - Manual 🢒 Heating Performance Diagnostic

Heating Performance Diagnostic Front System

Step

Action

Yes

No

DEFINITION: Heating system performance.

1

Were you sent here from Symptoms or another diagnostic table?

Go to Step 2

Go to

2
  1. Start the engine.
  2. Allow the engine to idle.

Does the engine reach a normal operating temperature?

Go to Step 3

Go to Step 15

3
  1. Allow the engine to idle.
  2. Select the warmest temperature setting.
  3. Select the minimum blower speed.
  4. Select the OFF position for the auxiliary heating system, if equipped.
  5. Feel the temperature of the heater inlet and outlet hoses at the hot water bypass valve.

Does the inlet heater hose feel warmer than the outlet heater hose?

Go to Step 13

Go to Step 4

4

Disconnect the vacuum hose from the hot water bypass valve.

Is vacuum being applied to the valve?

Go to Step 14

Go to Step 5

5

Remove the hot water bypass valve. Refer to .

Inspect the hot water bypass valve for the following conditions:

    • Binding
    • Restrictions
    • Damage

Are any of the listed conditions present?

Go to Step 16

Go to Step 6

6

Is the vehicle equipped with RPO C36?

Go to Step 7

Go to Step 8

7

Inspect the auxiliary coolant bypass valve for the following conditions:

    • Binding
    • Stuck open

Is the auxiliary coolant bypass valve stuck open?

Go to Step 17

Go to Step 8

8

Install the hot water bypass valve. Refer to .

Is the action complete?

Go to Step 9

--

9
  1. Allow the engine to idle.
  2. Select the FLOOR mode.
  3. Select the minimum blower speed.
  4. Select the warmest temperature setting.
  5. 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?

Go to Step 13

Go to Step 10

10
  1. Install a thermometer into the center IP PANEL air outlet.
  2. Secure a thermometer to the heater core outlet hose.
  3. Select the PANEL mode.
  4. Select the maximum blower speed.
  5. Select the warmest temperature setting.
  6. Record the temperature at the following locations:
    7. • The center IP PANEL air outlet
    • The heater core outlet hose
  7. Compare the recorded temperatures.

Are the two temperature readings about equal?

Go to Step 11

Go to Step 12

11
  1. Inspect the following areas of the vehicle for cold air leaks:
    2. • The cowl
    • The recirculation door
    • The HVAC module case
  2. Perform the necessary repairs.

Are the repairs complete?

Go to Step 19

--

12
  1. Inspect the temperature door operation. Refer to .
  2. Perform any necessary repairs.

Is the repair complete?

Go to Step 19

--

13
  1. Turn OFF the engine.
  2. Backflush the heater core.
  3. Start the engine and allow to idle.
  4. Select the FLOOR mode.
  5. Select the minimum blower speed.
  6. Select the warmest temperature setting.
  7. 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?

Go to Step 18

Go to Step 19

14

Repair the vacuum condition. Refer to in HVAC Systems - Manual.

Is the repair complete?

Go to Step 19

--

15

Repair the low engine temperature concern. Refer to in Engine Cooling.

Is the repair complete?

Go to Step 19

--

16

Replace the hot water bypass valve. Refer to .

Is the repair complete?

Go to Step 19

--

17

Replace the auxiliary coolant bypass valve. Refer to .

Is the repair complete?

Go to Step 19

--

18

Replace the heater core. Refer to .

Is the repair complete?

Go to Step 19

--

19

Operate the system in order to verify the repair.

Did you find and correct the condition?

System OK

Go to Step 2

Heating Performance Diagnostic Auxiliary System

Step

Action

Yes

No

1

Were you sent here from Too Cold in Vehicle?

Go to Step 2

Go to Too Cold in Vehicle in HVAC System - Manual

2
  1. Start the engine.
  2. Allow the engine to idle.

Does the engine reach a normal operating temperature?

Go to Step 3

Go to Step 14

3

    Caution: Refer to Moving Parts and Hot Surfaces Caution in the Preface section.

  1. Allow the engine to idle.
  2. Select the maximum auxiliary temperature setting.
  3. Select the minimum auxiliary blower speed.
  4. Feel the temperature of the inlet and outlet hoses to the auxiliary hot water bypass valve.

Does the inlet hose feel warmer than the outlet hose?

Go to Step 4

Go to Step 6

4

Disconnect the vacuum hose from the auxiliary hot water bypass valve.

Is vacuum being applied to the valve?

Go to Step 13

Go to Step 5

5

Remove the auxiliary hot water bypass valve. Refer to Heater Water Bypass Valve Replacement .

Inspect the hot water bypass valve for the following conditions:

    • Binding
    • Restrictions
    • Damage

Is the repair complete?

Go to Step 16

--

6
  1. Allow the engine to idle.
  2. Select the minimum auxiliary blower speed.
  3. Select the maximum auxiliary temperature setting.
  4. Feel the temperature of the inlet and outlet hoses to the auxiliary heater core.

Does the inlet hose feel warmer than the outlet hose?

Go to Step 10

Go to Step 7

7
  1. Install a thermometer into an auxiliary air outlet.
  2. Secure a thermometer on the outlet hose of the auxiliary heater core .
  3. Select the maximum auxiliary blower speed.
  4. Select the maximum auxiliary temperature setting.
  5. Record the temperature at the following locations:
    6. • The auxiliary air outlet
    • The auxiliary heater core outlet hose
  6. Compare the recorded temperatures.

Are the temperatures similar?

Go to Step 8

Go to Step 9

8
  1. Inspect the following areas of the vehicle for cold air leaks:
    2. • The auxiliary HVAC module case
    • The auxiliary air ducts
  2. Perform any necessary repairs.

Is the repair complete?

Go to Step 16

--

9

Inspect the auxiliary HVAC module for correct temperature door operation. Refer to Symptoms - HVAC Systems - Manual .

Does the temperature door operate correctly?

Go to Step 16

Go to Step 15

10
  1. Turn OFF the engine.
  2. Backflush the auxiliary heater core. Refer to Flushing in Engine Cooling.
  3. Start the engine and allow the engine to idle.
  4. Select the minimum auxiliary blower speed.
  5. Select the maximum auxiliary temperature setting.
  6. Feel the temperature of the inlet and outlet hoses in the auxiliary heater core.

Does the inlet hose feel warmer than the outlet hose?

Go to Step 11

Go to Step 16

11

Replace the auxiliary heater core. Refer to Heater Core Replacement .

Is the repair complete?

Go to Step 16

--

12

Replace auxiliary hot water bypass valve. Refer to Heater Water Bypass Valve Replacement .

Is the repair complete?

Go to Step 16

--

13

Repair the vacuum condition. Refer to Vacuum Control System Diagnostic in HVAC Systems Manual.

Is the repair complete?

Go to Step 16

--

14

Repair the low engine temperature concern. Refer to Engine Fails To Reach Normal Operating Temperature in Engine Cooling.

Is the repair complete?

Go to Step 16

--

15

Repair the temperature door operation.

Is the repair complete?

Go to Step 16

--

16

Operate the system in order to verify the repair.

Does the system operate correctly?

System OK

Go to Step 2