I have a 2005 Toyota Vitz with a 1KR-FE engine. When idling, there is a rough idle from time to time when the car is in Park, Neutral or when stopping in traffic, with vibrations shaking the car. The problem worsens when the A/C is switched on sometimes. What could be the problem? Car has been regularly serviced and fuel is 98RON. the issue has been going on since I bought the car second hand. No check engine lights. Thanks.
Let's assume you have already checked for small vacuum leaks in the rubber tubing on the intake manifold and replaced the PCV valve (if applicable) . . .
I had the same problem on three different cars (two late-model Mercedes-Benz). The problem were due to dirty fuel injectors, spark plugs, intake valve stems, idle air control valve and flow ports, and other deposits. Cars are particularly susceptible to deposits in cold climates, like Ireland.