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When the maniflod gaskets go bad you will usually see coolant leaking somewhere, either to the outside of the engine, where it is easily seen or internally where it mixes with oil and the oil turns milky. On rare occasions you may see some coolant coming out the exhaust. Poor performance can be an indicator of this as well. As for your idle problem check for vacuum leaks and then clean the Idle air control motor on the side of the throttle body. Probably very dirty and causing the idle issue.