Bad thermostat
All summer I noticed my car will knock on acceleration after it sat for a while. The knock will eventually go away. As it got cold I noticed that the heat was warm not hot. The temperature gauge showed an engine that got colder the more I drove in cold weather. I finally guessed I had a stuck thermostat. I replaced it this evening in my driveway. The car quickly got to operating temperature. I had heat once again. And that pesky knock is gone. The car idles 300 rpm lower. I am sure to see improved fuel mileage.
If you experience any of these symptoms have your mechanic check the thermostat. If you are handy like me do it as preventive maintenance.
The gasket and thermostat cost less than $12.00. It took me less than the 15 minutes.
Good luck and happy motoring.
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