I agree with Ryan to start by testing or replacing the pressure cap. Be sure it is the proper cap. I believe that car calls for 140 kilopascals. After that a pressure check is in order. There are two important points to remember when doing a pressure test. Don't go over the rating of the cap or you run the risk of rupturing the heater core. Secondly you can only do pressure check on a cold engine. If you do the test on a hot engine you will get mixed results.

Good luck,
Peter

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