1. Verify Correct Gasket and Installation:
* Correct Gasket: Ensure you used the correct thermostat gasket designed specifically for your 1999 Oldsmobile Alero's engine. A wrong gasket won't seal properly. Double-check the part number against your vehicle's specifications.
* Proper Installation: The gasket must be clean and properly seated. Any debris, old gasket material, or uneven surfaces on the thermostat housing or thermostat itself will prevent a good seal. Carefully clean both mating surfaces with a wire brush and degreaser, then rinse thoroughly and allow to dry completely before installing the new gasket and thermostat. Make sure the gasket isn't twisted or pinched during installation.
2. Thermostat Housing Issues:
* Warped Housing: The thermostat housing itself might be warped or cracked, preventing a good seal even with a new gasket. Carefully inspect the housing for any imperfections. If warped, it will likely need to be replaced.
* Cracked or Damaged Housing: Look closely for cracks, especially around the bolt holes. Even small cracks can lead to leaks. If you find cracks, replacement is necessary.
* Loose Bolts: Ensure all the bolts securing the thermostat housing are tightened to the manufacturer's specified torque. Over-tightening can also warp the housing. Using a torque wrench is highly recommended.
3. Thermostat Issues:
* Faulty Thermostat: Though less likely if you've already replaced the thermostat, a defective thermostat could contribute to the problem. If the thermostat is sticking or not opening and closing properly, it could cause pressure buildup leading to gasket failure.
* Incorrect Thermostat: Verify you installed the correct thermostat for your engine. An incorrect thermostat can cause unusual pressure that forces the gasket to leak.
4. Other Potential Causes (Less Likely):
* Cooling System Pressure: Excessive pressure in the cooling system can overcome even a good gasket. Check for other potential leaks in the system (radiator, hoses, etc.). A pressure test of the cooling system is recommended.
* Head Gasket Issues: (Least likely but important to consider) While less probable, a failing head gasket can sometimes manifest as a thermostat gasket leak. This would require further investigation, such as a compression test.
Steps to Take:
1. Re-inspect the Housing and Gasket: Carefully remove the thermostat and housing again. Thoroughly clean all surfaces, paying close attention for cracks or warpage.
2. Replace the Housing (if necessary): If the housing is damaged, replace it with a new one.
3. Double-check the Thermostat: Ensure it's the correct part and functions properly (if you haven't already checked it).
4. Reinstall everything carefully: Use a new gasket. Pay close attention to proper seating and torque specifications for the bolts.
5. Pressure Test the Cooling System: This is crucial to rule out other system leaks causing excessive pressure.
If the leak persists after addressing these points, you might need professional help from a mechanic. A more serious problem with the cooling system may be the culprit. Remember safety first – allow the engine to cool completely before working on it.
