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Once my 97 Mazda 323 get's up to normal operating temperature, around say 90 C, the RPM's will occasionally nosedive after taking it out of gear, releasing the clutch and letting off the gas. What I mean is that they'll dive down to around 200, nearly stalling, and either bounce for a bit between 200 to 750, or rev back up to around 1500 and settle back to normal 750 idle. A little more rarely it will result in an actual stall. Rarely in this case meaning maybe once a day, in about 50 kilometers of city driving.

EDIT

Something I noticed is that if I don't release the clutch, the RPM's will stay stable. I've also noticed that as the ambient air temp increases the frequency of the problem increases as well.

So I took off the throttle body and this is what it looked like:

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The IAC passage didn't look particularly dirty, but I sprayed some carb cleaner into it from both the manifold side and the intake pipe without taking it off the throttle body. I also cleaned the throttle plate and sprayed some cleaner into the very nasty, dirty manifold for good measure. I also reset the ECU after doing this and it seems that the problem has been solved.

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    It sounds more like you are having an issue with the idle air control (IAC) valve. You should try to clean it and the upper intake tract. – Pᴀᴜʟsᴛᴇʀ2 Mar 3 '15 at 23:13
  • Throttle body was replaced by previous owner about a year ago and is very clean - I would assume that they cleaned the IAC at the same time, or would that be too much of an assumption to make? – Robert S. Barnes Mar 4 '15 at 3:47
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    @Paulster2 Convert to an answer and I'll accept it - cleaned the IAC and throttle body, plus reset the ECU today and after driving about 20 km city it looks like the problem has went away. – Robert S. Barnes Mar 11 '15 at 17:41
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This sounds like a problem with the Idle Air Control (IAC) valve, or something in that general vicinity. You should try to clean it and the upper intake tract.

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