GM Lumina/Silhouette/Transport APVs 1990-1999 Repair Guide

Thermostat

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A pellet-type thermostat is used in the coolant outlet passage to control the flow of engine coolant, to provide fast engine warm-up and to regulate coolant temperatures. A wax pellet element in the thermostat expands when heated and contracts when cooled. The pellet element is connected through a piston to a valve. When the pellet element is heated, pressure is exerted against a rubber diaphragm, which forces the valve to open. As the pellet element is cooled, the contraction allows a spring to close the valve. Thus, the valve remains closed while the coolant is cold, preventing circulation of coolant through the radiator. At this point, coolant is allowed to circulate only throughout the engine to warm it quickly and evenly.

See Figure 1

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Fig. Fig. 1: View of the pellet that sits in the engine. This is a 195° thermostat; note the designation on the thermostat

As the engine warms, the pellet element expands and the thermostat valve opens, permitting coolant to flow through the radiator, where heat is dissipated through the radiator walls. This opening and closing of the thermostat permits enough coolant to enter the radiator to keep the engine within operating limits.

REMOVAL AND INSTALLATION



3.1L Engine
  1. Disconnect the negative battery cable.
  2.  
  3. Partially drain the cooling system into a suitable container, to a level below the thermostat.
  4.  
  5. Disconnect the radiator hose from the thermostat housing.
  6.  
  7. Unfasten the thermostat housing bolts.
  8.  
  9. Remove the housing. Note the installed position of the thermostat, then remove the thermostat and gasket.
  10.  
  11. Thoroughly clean the gasket mating surfaces.
  12.  

See Figure 2

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Fig. Fig. 2: Exploded view of the thermostat and housing assembly

To install:
  1. Position the new thermostat in the manifold.
  2.  

The pellet MUST go towards the engine.

  1. Install a new gasket, using sealer as necessary.
  2.  
  3. Install the housing and secure with the retaining bolts. Tighten the bolts to 25 Nm (18 ft. lbs.)
  4.  
  5. Reconnect the radiator hose to the housing.
  6.  
  7. Replace the coolant. Check for leaks.
  8.  
  9. Connect the negative battery cable.
  10.  

3.4L Engine
  1. Disconnect the negative battery cable.
  2.  
  3. Remove the air cleaner and duct assembly.
  4.  
  5. Partially drain the cooling system into a suitable container, to a level below the thermostat.
  6.  
  7. Disconnect the radiator hose from the coolant outlet.
  8.  
  9. Unfasten the coolant outlet retaining bolts.
  10.  

See Figure 3

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Fig. Fig. 3: Unfasten the bolts and remove the coolant outlet

  1. Remove the outlet, then remove the thermostat.
  2.  

See Figure 4

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Fig. Fig. 4: Once the outlet is removed, you can remove the thermostat

  1. Thoroughly clean the gasket mating surfaces.
  2.  

See Figure 5

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Fig. Fig. 5: Location of thermostat housing and outlet on engine.

To install:
  1. Install the new thermostat with the pellet towards the engine.
  2.  
  3. Install the outlet to the intake manifold. Apply RTV sealant to the housing bolts and tighten to 18 ft. lbs. (25 Nm)
  4.  
  5. Reconnect the radiator hose to the thermostat coolant outlet.
  6.  
  7. Install the air cleaner and duct assembly.
  8.  
  9. Refill the coolant and check for leaks.
  10.  

When adding coolant, it is important that you use DEX-COOL® (orange colored, silicate-free) coolant. If silicate coolant is added to the system, premature engine, heater core or radiator corrosion may result. In addition, the engine coolant will require change sooner.

  1. Connect the negative battery cable.
  2.  

3.8L Engine
  1. Disconnect the negative battery cable.
  2.  
  3. Partially drain the cooling system into a suitable container, to a level below the thermostat.
  4.  
  5. Disconnect the radiator hose from the thermostat housing.
  6.  
  7. Unfasten the water outlet bolts, then remove the water outlet and thermostat
  8.  
  9. Thoroughly clean the gasket mating surfaces.
  10.  

See Figures 6 through 10

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Fig. Fig. 6: View of the thermostat seated in the engine with the water outlet removed



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Fig. Fig. 7: While pulling the thermostat out of the engine, notice how the pellet sits in the engine



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Fig. Fig. 8: If damaged, remove the rubber seal from around the thermostat.



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Fig. Fig. 9: Carefully scrape the old gasket material from the thermostat water outlet



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Fig. Fig. 10: Exploded view of the thermostat and water outlet assembly-3.8L engine

To install:

To perform properly, the thermostat must be installed with the pellet towards the engine.

  1. Install the thermostat into the housing.
  2.  
  3. Position a new gasket on the manifold, using sealer as necessary.
  4.  

See Figure 11

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Fig. Fig. 11: Position a new gasket, then install the water outlet and secure with the retaining bolts

  1. Place the water outlet in position. Coat the retaining bolts with RTV, then install and tighten to 20 ft. lbs. (27 Nm).
  2.  
  3. Connect the radiator hose to the water outlet.
  4.  
  5. Refill the coolant and check for leaks.
  6.  
  7. Connect the negative battery cable.
  8.  

 
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