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I have a 1990 manual Honda Civic with ~200k miles. When I'm stopped at a red light, the engine revs up to about 2k-3k rpm and then down to 1k rpm. This cycle repeats about once every two seconds. My foot is off the gas, and it happens regardless of whether the clutch is in and I'm in gear or the clutch is out and I'm in neutral. It doesn't happen consistently; sometimes it behaves normally. So far, it hasn't caused any problems, it's just annoying. What could be causing this problem?

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    Is your A/C running when this happens? If so it could be your A/C idle speed compensator, aka idle-up valve. – BillDOe Sep 20 '17 at 18:03
  • @BillOertell Nope, it happens without the A/C on. – jmcampbell Sep 20 '17 at 20:47
  • Is there a short powering that valve when it shouldnt? Can you run it with that valve disconnected to see if it still does it? – Solar Mike Sep 20 '17 at 20:50
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There is an air leak into the intake manifold. What happens is that air mass increases so the PCM responds by added fuel to match, thus increasing rpm. Next the idle rpm limit is reached so the PCM cuts off fuel to lower the rpm.

The fix requires that the air leak is found and repaired. Common causes are leaks through the PCV valve or idle air control valve. Other leaks are possible. Test by blocking off possible leaks until the idle returns to normal.

  • Does an air leak in this case mean that the engine is pulling in unfiltered air? – GdD Sep 21 '17 at 7:53
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    @GdD It depends on what is leaking. The PCV and idle control, for example, are leaks of filtered air but that bypass the throttle plate. – Fred Wilson Sep 21 '17 at 19:38

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