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Q: my car is going into engine fail mode while driving on 1999 Jaguar XK8

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This happens quite often during decelleration if the throttle body is dirty. A carbon ring will form around the top of the throttle body restricting the airflow when the throttle body is closed. This creates an incorrect fuel to air ratio and forces the engine into fail safe mode. Try cleaning the mass airflow sensor, and then remove the plastic air intake duct to expose the throttle body. Clean around the throttle body with a good throttle body cleaner. Spray it on a rag and clean it. Do not spray it directly into the throttle body. Reassemble and try it. It worked great for me.
Same thing happening to me on 2000 Jag xk8. Also sometimes the engine light symbol also comes on. Am told it is a faulty electronic throttle control assembly that has several sensors with high failure rates, or it is the connector pins (if silver, need to be gold plated). New assembly costs $2100 plus labor, so someone needs to confirm for sure before you go through with it. Usually fault codes P0121/0507.
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