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.
Add automatic transmission fluid (ATF) to the gas tank at a rate of eight ounces per ten gallons of fuel and drive the car normally for a quarter tank. Then add acetone at a rate of six ounces for every ten gallons and run the engine immediately in low gear at high RPM (3,500 RPM) for 15 minutes and floor it on the highway six times. Repeat this procedure for three tankfuls of fuel (or about 1,000 miles). You can also combine isopropyl alcohol to the mix, also at a rate of six ounces for every ten gallons of gas. The more heat and pressure you apply to the engine, the better, either at high speed or low speed.
After two tankfuls of this mix, clean the intake manifold with carburetor cleaner (or perhaps Liqui-Moly Intake cleaner). Be sure to clean the passages of the IAC valve. Cleaning the IAC will especially resolve the vibration when the A/C is activated.
If you still have vibration, replace the MAF sensor. To avoid vibration in the future, regularly add ATF to the gas tank at a rate of four ounces per ten gallons of fuel. I have been doing so for 25 years and my engines are clean and smooth as silk!