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When a clutch is replaced the transmision has to be removed from the back of the engine. Sometimes the engine pipe of the exaust system has to be disconected in order to remove the transmision if it is not replaced propperly you will have an exaust leak. Other times the exaust is just left on, and the engine weight just hangs on it, this put strain on the exaust system, because there is nothing hold the rear of the engine in place anymore. Front wheel drive vehicles use flex pipes in the exaust system to compensate for engine torque -movment. That is where the engine pipe will usualy break.
In other words when the clutc was replaced either they didn't put the exaust system back propperly or it broke.
As far as the high idle again we go back to the hanging engine it is posible that some vacuum lines got pulled of the engine or the throttle cable is routed wrong and binding.
Have the shop check these things out if you have any other questions please post back.