DIRECTION FOR THERMOSTAT REPLACEMENT FOR A 1998 MONTANA

If engine is a 3.4(3400) thermostat is located behind exhaust pipe that connect both cylinder heads you can change it without removing pipe both bolts are 13mm you have to access one with a wrench and other with socket and extension

The exact location of the thermostat varies depending on the particular engine model on your Pontiac Montana. Although 2.0L turbo and non-turbo engines locate the thermostat on the upper section of the engine, other models may follow the lower radiator hose to a lower position on the engine, requiring the removal of one or more components for replacement. see below

• Park your car in a safe place and open the hood.
  
• 2Locate the thermostat housing by following the upper or lower radiator hose from the radiator towards the engine. The end of the hose should be connected to an aluminum or metal housing attached to the engine by two mounting bolts.
  
• 3Remove the air cleaner duct assembly and the crossover exhaust pipe, if necessary, using a wrench and ratchet and socket.
  
• 4Place a drain pan underneath the radiator drain valve. Open the valve and drain some of the coolant to empty the radiator hose; then close the valve.
  
• 5Remove the radiator hose from the thermostat housing by disconnecting the clamp using a pair of rib joint pliers, Phillips screwdriver or ratchet and socket, depending on type of clamp used on your particular model; then remove the hose.
  
• 6Remove the two thermostat housing bolts and lift the housing off the engine. Before removing the thermostat off the engine, pay careful attention to the position of the relief valve on the thermostat in relation to the mounting base on the engine, looking for any special marks. The new thermostat has to be mounted in this same exact position. Also, note that the thermostat spring has to point towards the engine once it is seated on the engine; then remove the thermostat and any gasket material if necessary.
  

• 1Clean the thermostat housing and engine mounting surfaces from gasket material, if necessary, using a plastic scraper; also, clean the bolt threads of the thermostat housing bolts of sealer, if necessary, using a wire brush.
  
• 2Apply a light coat of Room Temperature Vulcanizing (RTV) sealer to the new paper gasket if your particular model requires one; or install a new O-ring to the center of the thermostat; also, apply a light coat of RTV sealer to the threads of the thermostat housing mounting bolts.
  
• 3Install the new thermostat on the engine mounting base in the same exact position as the old one; install the paper gasket if required and set the thermostat housing in place. Start the housing mounting bolts by hand and then tighten the bolts.
  
• 4Connect the radiator hose to the thermostat housing; then install the crossover exhaust pipe and air cleaner and duct assembly, if necessary. Fill the radiator with new coolant, start the engine and let it reach operating temperature. Check for leaks and add more coolant to the reservoir to bring the level between the Add and Full Hot marks and turn off the engine.
  
• 5Bleed the cooling system if the thermostat housing is provided with a valve screw on it. Once the engine reaches operating temperature, loosen the valve screw until coolant begins to leak through the screw; then tighten the screw again