On cold days the engine vibrates excessively when starting, then stops when the engine is warm. Why would the vibrations disappear once the engine is warm?

There are a number of possible causes for this cold-only vibration, including faulty fuel injectors or spark plugs. If either or both of these are compromised, the result could be less than optimal combustion, which in turn could be causing the vibration. While a faulty engine mount could cause excessive vibration, the vibration would be likely to continue after the engine had warmed up. In any case, we would advise having your vehicle checked by a qualified technician to ascertain the exact cause of the vibration.