Cold weather = cold air = more dense air = =less fuel/air mix = lean mixture = rough engine.
Normally the colder air doesn't bother performance if the mixture were correct in the first place. Whenever I hear about a car that performed normally in warm weather, but is acting up as the weather turns colder, I always suspect a vacuum leak which is causing an even leaner mixture. There are many possible sources of vacuum leaks such as loose or cracked vacuum hoses or possibly a loose carburetor, where it bolts onto the intake manifold. Failed seals around an injector can also cause a severe vacuum leak - especially to one cylinder. In addition I look for a sticky EGR Valve - I remove the vacuum hose on the EGR and drive it for a while to see if the symptoms go away. An absolute check of the EGR Valve is to remove it completely and block the hole in the intake manifold with a metal plate with a gasket between. A visual inspetion of the EGR can determine if it is closed when removed. I also check for a sticking or faulty PCV valve. If that is open at low speeds or at idle it can play havoc with performance - again, a lean mixture. Basically, check anything that is connected to the intake manifold that uses vacuum from the engine for any reason.
Other possible causes for poor cold weather performance are the diverter valve, a faulty thermostat which causes the engine to operate unheated and sends "cold" information to the computer, a faulty coolant temperature sensor which sends erroneous "hot" information to the computer and a faulty heat riser valve in cars that have them. A heat riser valve is the device that causes hot exhaust gas to be circulated under the intake manifold to improve the vaporization of fuel, especially in cold weather conditions.
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