Q: car shaking when idle, bad exhaust, low engine temperature

If the muffler is loud because it is leaking exhaust gases (as opposed to just having broken internal baffles) the car is not safe to drive because exhaust gases contain carbon monoxide. The cost to repair the loud exhaust depends on where the fault lies, for example in pipes versus the muffler versus the catalytic converter and so forth. The fastest, and cheapest, way to get a resolution though is just request a loud exhaust diagnostic. The responding certified mechanic will perform a thorough inspection, right at your location, and let you know exactly what must be repaired, or replaced, in order to make the car safe and reliable. The vehicle engine shaking at idle is probably due to an engine management fault (fuel, ignition, air intake, sensors) and the mechanic who diagnoses and repairs the exhaust system can deal with that. Shaking on braking is typically due to problems with the brake rotors/pads and/or loose or worn suspension components. In order of priority, where the objective is to ensure YOUR safety, the exhaust noise/leak problem should be fixed first. Once the car is safe to drive, then the engine fault should be diagnosed and repaired and finally the brake issue should be resolved. If you have further questions or concerns, do not hesitate to re-contact YourMechanic because we want you to make the most of your repair dollars and help you to get the best possible results.