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My Toyota Vitz 2007 idles roughly when AC is on. If AC is turned off, the idle becomes much smoother.

This vibration is more visible if I suddenly stop in traffic. As soon as I stop, several rough vibrations occur and then it becomes smoother.

I got the vehicle serviced: cleaned the fuel filter, replaced Air and AC filters, replaced engine oil. But the problem hasn't got resolved.

What could be the reason for ths?

  • How many miles on the vehicle? – Pᴀᴜʟsᴛᴇʀ2 Dec 13 '15 at 13:37
  • @Paulster2, 86000 km. – Softcode Dec 13 '15 at 13:41
  • There is a thing which causes the engine to idle up when the AC is turned on ... it seems this isn't working correctly. Your engine gets dragged down a little bit and causes the rough idle. I don't know what this is called exactly. Maybe someone else will have an exact answer for you. – Pᴀᴜʟsᴛᴇʀ2 Dec 13 '15 at 19:39
  • Did the issue come about suddenly? – Zaid Dec 13 '15 at 21:42
  • @Zaid The car displayed rough idling for a long time and became more visible gradually over time. But it kind of got sever lately. – Softcode Dec 14 '15 at 1:00
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The symptoms are consistent with deposits in the throttle body. There are other potential causes. These deposits accrue in the intake manifold over time. They build faster with short trips and low quality fuels. Inspection and cleaning of the throttle body would be first thing to look at for this symptom. This engine is equipped with a computer controlled throttle system. It is designed to adjust its opening to keep the idle speed close to 700 rpm regardless of engine load. If to much dirt collects around the throttle blade the system can get beyond the limits of the systems ability to adjust idle speed with the extra load of the air conditioning.

To clean the throttle use a soft cleaning cloth soaked in a solvent. Open the throttle blade carefully. Clean the throttle blade and the surrounding bore. Do not scrub, let the solvent do the work, no scratches are allowed on the polished bore.

After cleaning start the engine and let it idle for at least 20 minutes while the system learns it's new throttle position. This vehicle should relearn on its own. It may need to be revved up and put in gear several times during the learing time. Note that other designs require tools and procedures to perform the throttle system relearn.

Cautions;

  • These geared blades are very strong and can damage body parts if they activate during cleaning, Make sure the vehicle key is away from the vehicle. Open them slowly and carefully. We use shotgun bore cleaning swabs so we do not have to insert fingers into the blade area
  • Do not spray solvents into the bore and intake, just soak the cloth.
  • Do not use metal tools for cleaning, it is easy to cause damage and these units are expensive.

If indicated, have cleaning done by an experienced technician if at all unsure since it is possible to get injured or damage a very expensive part.

  • Thanks for the answer. I will clean the throttle and see whether it resolves the problem. – Softcode Dec 14 '15 at 0:55
  • @fredwilson Nice answer again. – DucatiKiller Dec 14 '15 at 1:00
  • @fredwilson Can you please tell me whether it is necessary to trigger the re-learn in some way after cleaning the throttle body? I have asked a separate question for it. mechanics.stackexchange.com/q/23407/11037 – Softcode Dec 15 '15 at 1:25
  • Cleaned the throttle body and disconnected the battery for some time. After engine re-learned, vibration has reduced significantly. – Softcode Jan 19 '16 at 2:41
  • I'm glad you got it worked out. In addition to the dirty throttle body, an idle control air valve could cause this issue. The idle air control valve adjusts how much air is bypassing the throttle plate, increasing air flow when the AC compressor clutch actuates to help the engine stay at idle speed when under the extra load. – Dacid Salin Feb 2 at 5:47
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Had the same issue on my Toyota Matrix turned out I had several vacuum leaks replaced the pcv valve and hose; and both the intake manifold gasket, as well cleaned the throttle body and put a new gasket and solved the issue. Hope this helps

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