GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2003 🢒 Malibu 🢒 Engine 🢒 Engine Cooling 🢒 Thermostat Diagnosis

Step

Action

Value(s)

Yes

No

1

Pressure check the cooling system and the cap for leaks. Repair any leaks before proceeding with the diagnosis. Six cylinder engines use a 91°C (195°F) thermostat.

Ambient temperatures should be between 13-38°C (55-100°F). Place the HVAC controls in the OFF position. Run a cold engine at idle (68-70°F) for 15-20 minutes before checking the engine temperature.

Is the repair complete?

--

Go to Step 2

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2

Verify the engine coolant temperature with a Scan Tool at idle.

Is the engine temperature below 85°C (185°F)?

--

Go to Step 5

Go to Step 3

3

Verify the engine coolant temperature with a Scan Tool at idle.

Is the engine coolant temperature between 85-96°C (185-205°F)?

--

System OK

Go to Step 4

4

Verify the engine coolant temperature with a Scan Tool at idle.

Is the engine coolant temperature over 96°C (205°F)?

--

Go to Step 7

--

5

Test the thermostat for the following conditions:

  1. The thermostat is opening early.
  2. The thermostat seal is leaking.

Are either of these conditions present?

--

Go to Step 6

System OK

6

Replace the thermostat. Refer to Engine Coolant Thermostat Replacement .

Is the repair complete?

--

System OK

--

7

Is the radiator inlet hot?

--

Go to Step 8

Go to Step 14

8
  1. Turn on the heater.
  2. Check for hot air at the heater outlets.

Is the air hot?

--

Go to Step 9

Go to Step 12

9

Inspect for blockage in the radiator.

Did you find any blockage?

--

Go to Step 10

Go to Step 11

10

Repair the blockage. Re-inspect the radiator with the Scan Tool .

Is the repair complete?

--

System OK

--

11
  1. Replace thermostat. Refer to Engine Coolant Thermostat Replacement in Engine Cooling.
  2. Re-inspect the radiator with the Scan Tool .

Is the repair complete?

--

System OK

--

12
  1. Rev the engine several times to remove any air.
  2. Inspect for blockage in the heater circuit.
  3. Inspect for pinched or buckled hoses.

Did you find any blockage?

--

Go to Step 13

Go to Heater Core Replacement in HVAC Systems with AC - Manual

13
  1. Repair the blockage.
  2. Re-inspect the radiator with the Scan Tool .

Is the repair complete?

--

System OK

--

14

Air may be in the system.

  1. Add coolant to surge tank if low.
  2. Re-inspect the radiator inlet hose.

Is the hose hot?

--

Go to Step 8

Go to Step 15

15

Inspect for blockage in the following locations:

    • The cylinder head
    • The radiator
    • The radiator hoses
    • The radiator nipples

Did you find any blockage?

--

Go to Step 16

Go to Step 17

16
  1. Repair the blockage.
  2. Re-inspect the radiator with the Scan Tool .

Is the repair complete?

--

System OK

--

17
  1. Replace the thermostat. Refer to Engine Coolant Thermostat Replacement .
  2. Re-inspect the radiator with the Scan Tool .

Is the repair complete?

--

System OK

--