GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 1999 🢒 Savana 🢒 Engine 🢒 Engine Cooling 🢒 Thermostat Diagnosis

Thermostat Diagnosis Engine Overheat

Step

Action

Value(s)

Yes

No

The temperature stick is a pencil-like device that has a wax material containing certain chemicals which melt at a given temperature. Use the temperature sticks to determine a thermostat's operating temperature by rubbing 87°C (188°F) and 97°C (206°F) sticks on the thermostat housing. The marks made by the sticks should melt when coolant temperatures reach 87°C (188°F) and 97°C (206°F), respectively. These temperatures are the normal operating range of the thermostat. If the coolant flows as indicated in the table, the thermostat may be worn.

1
  1. Remove the radiator cap. Refer to Pressure Cap Description .
  2. Rub a 97°C (206°F) J 24731 on the thermostat housing.
  3. Warm up the engine at fast idle.
  4. Check for coolant flow before the mark begins to melt.

Is there coolant flow before the mark melts?

--

Go to Step 2

Go to Step 3

2

The problem is caused from something other than the thermostat.

Does the engine still overheat?

--

Go to Engine Overheating .

System OK

3
  1. Replace the thermostat. Refer to Engine Coolant Thermostat Replacement .
  2. Check the system.

Does the engine still overheat?

--

--

System OK

Thermostat Diagnosis Slow Warm-up

Step

Action

Value(s)

Yes

No

The temperature stick is a pencil-like device that has a wax material containing certain chemicals which melt at a given temperature. Use the temperature sticks to determine a thermostat's operating temperature by rubbing 87°C (188°F) and 97°C (206°F) sticks on the thermostat housing. The marks made by the sticks should melt when coolant temperatures reach 87°C (188°F) and 97°C (206°F), respectively. These temperatures are the normal operating range of the thermostat. If the coolant flows as indicated on the table, the thermostat may be worn.

1
  1. Remove the radiator cap. Refer to Pressure Cap Description .
  2. Rub a 87°C (188°F) J 24731 on the thermostat housing.
  3. Warm up the engine at fast idle.
  4. Check for coolant flow before the mark begins to melt.

Is there coolant flow before the mark melts?

--

Go to Step 3

Go to Step 2

2

The problem is caused from something other than the thermostat.

Does the engine warm up properly?

--

System OK

--

3
  1. Replace the thermostat. Refer to Engine Coolant Thermostat Replacement
  2. Check the system.

Does the engine still fail to reach normal operating temperature?

--

--

System OK