GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2009 🢒 TopKick C-Series (Conventional) C6/C7/C8 🢒 HVAC 🢒 Heating, Ventilation, and Air Conditioning 🢒 Heating Performance Diagnostic

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 Symptoms - HVAC Systems - 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 15

3

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

  1. Allow the engine to idle.
  2. Select the warmest temperature setting.
  3. Select the minimum blower speed.
  4. 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 11

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 Heater Water Shutoff Valve Replacement.

Inspect the hot water bypass valve for the following conditions:

    • Binding
    • Restrictions
    • Damage

Are any of the listed conditions present?

Go to Step 13

Go to Step 6

6

Install the hot water bypass valve. Refer to Heater Water Shutoff Valve Replacement.

Is the action complete?

Go to Step 7

--

7
  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 11

Go to Step 8

8
  1. Install a thermometer into the center I/P 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 I/P PANEL air outlet
    • The heater core outlet hose
  7. Compare the recorded temperatures.

Are the two temperature readings about equal?

Go to Step 9

Go to Step 10

9
  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 16

--

10
  1. Inspect the temperature door operation. Refer to Air Delivery Improper.
  2. Perform any necessary repairs.

Is the repair complete?

Go to Step 16

--

11
  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 12

Go to Step 16

12

Replace the heater core. Refer to Heater Core Replacement.

Is the repair complete?

Go to Step 16

--

13

Replace the hot water bypass valve. Refer to Heater Water Shutoff Valve Replacement.

Is the repair complete?

Go to Step 16

--

14

Repair the vacuum condition. Refer to Vacuum Control System Diagnostic.

Is the repair complete?

Go to Step 16

--

15

Repair the low engine temperature concern. Refer to Symptoms - Engine Cooling.

Is the repair complete?

Go to Step 16

 

16

Operate the system in order to verify the repair.

Did you find and correct the condition?

System OK

Go to Step 2