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My '99 VW Jetta is intermentently overheating. I have changed out the coolant cap, but that didn't solve the issue. Could it be the thermostat? My Jetta is from the first half of '99.


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Sounds like you could have a bad thermostat. A thermostat is a spring loaded seal. When the temperature gets to a set point (usually around 110 degrees) the thermostat will open allowing coolant to flow through the system.

The thermostat has a high failure rate.

There could be a bigger problem than the thermostat, you should check the coolant level anytime an overheat is present.

1. Check the actual temperature of the radiator and compare it to the temperature of the cylinder head.

2. If the radiator remains significantly cooler than the cylinder head, check the water pump impeller for damage, missing fins or spinning on the water pump shaft. The water pump impeller is plastic and can crack or loosen, allowing the water pump shaft to freely spin without moving the impeller. Insufficient water flow through the cooling system due to the faulty impeller will cause an overheat condition.

3. The cooling fans will not come on due to low coolant flow because the fan thermo switch is located in the lower left of the radiator and will not get hot enough for fan operation before the engine overheats.

Since the problem is intermittent the thermostat is the probable answer. (Given the coolant isn't low)

Here is how to replace the thermostat.

Thermostat

 

Print

REMOVAL & INSTALLATION



4 Cyl. Models



Click image to see an enlarged view

Fig. Remove the housing bolts




Click image to see an enlarged view

Fig. Once all bolts have been remove, pull the thermostat housing away from the bottom of the water pump unit




Click image to see an enlarged view

Fig. Note the position of the thermostat and gasket



NOTE
On some models the thermostat is in the bottom of the water pump housing.



  1. Place a large drain pan under the vehicle to collect the coolant.
  2.  
  3. Remove the radiator cap and drain the cooling system.
  4.  
  5. Loosen one bolt at a time until coolant begins to flow.
  6.  
  7. Remove both bolts and remove the thermostat.

  8.  


To install:

  1. Install the new O-ring.
  2.  
  3. Torque the housing the bolts to 7 ft. lbs. (10 Nm).

  4.  


VR-6



Click image to see an enlarged view

Fig. Exploded view of a VR6 thermostat housing

On 6 cylinder models, the thermostat is mounted in the housing. The thermostat housing is located on the rear of the cylinder head.

  1. Drain the engine coolant.
  2.  
  3. Remove the mounting bolts from the flange.
  4.  
  5. Remove the flange and the thermostat.
  6.  


To install:

  1. Install the new thermostat and O-ring.
  2.  
  3. Install the thermostat housing.
  4.  
  5. Install and tighten the fasteners.

  6.  
  7. Fill the cooling system with approved antifreeze.
  8.  


Fox

  1. The thermostat is in the bottom of the water pump housing and the lower radiator hose connects to the thermostat housing. Place a large drain pan under the thermostat housing. On some models, it will be necessary to position the power steering pump aside.

  2.  





Click image to see an enlarged view

Fig. If necessary, remove the power steering pump




Click image to see an enlarged view

Fig. Hang the pump with a wire and position it aside

  1. Remove the radiator cap and drain the cooling system.
  2.  
  3. Loosen one bolt at a time until coolant begins to flow. Allow this residual coolant to drain into a pan.

  4.  





Click image to see an enlarged view

Fig. The thermostat housing is secured by two bolts




Click image to see an enlarged view

Fig. Slowly pull the housing away from the engine. Stand back and allow any residual coolant to drain into a pan

  1. Remove both bolts, then remove the thermostat. It's not necessary to disconnect the radiator hose.
  2.  





Click image to see an enlarged view

Fig. Pull the thermostat out of the water pump housing. Once again, stand back to allow any left over coolant to drain

  1. Discard the O-ring on the thermostat housing.
  2.  





Click image to see an enlarged view

Fig. Always replace the O-ring on the thermostat housing

To install:

  1. Place the thermostat into the water pump housing, then install the new O-ring. Install the housing and torque the bolts to 7 ft. lbs. (10 Nm).

  2.  
  3. Fill the cooling system. Start the engine and check for leaks.
  4.  


 

check the thermostat.

check the thermostat. coolant temp sensor. they are next to each other. also check your thermo fans. connectiions, fuses, etc.


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