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Is it possible to diagnose an IAC as needing cleaning without actually taking it apart?

For example, I recently was having some idle issues with my 99 Nissan Almera 1.6L which were solved by adjusting the idle air screw. The person who suggested that solution felt that maybe the TB valve was worn and closing too much, but I wasn't convinced that it wasn't a dirty IAC.

Another case is on my 98 Mazda 626 2L. Unlike the Almera, which has three devices for adjusting intake air flow, the 626 has only a single IAC.

At warm idle, at 650 rpm, the IAC is around 46% open which would make me think that maybe I need to open the idle air screw a bit. However, turning the AC on full blast only increases IAC open to about 54% which isn't that much extra and makes me think plenty of air is getting through the idle air screw and that the IAC is open so much because it's partially clogged.

Anyways, how would one go about diagnosing an IAC dirty enough to be partially restricted and in need of cleaning?

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Interesting question, assuming by 'taking it apart' you mean removing the IAC. In that case the only option that I can think of is a back-to-back comparison with another car of identical spec.

Assuming that all components on both vehicles were identical spec and condition, and that the operating conditions (temp, engine speed etc) were identical, you could read IAC position for both engines and compare.

This answer is probably just academic though, since I'm guessing you won't have another identical car to compare against.

Also, in the absence of any symptoms of a fault I'd personally assume that it is 46% by design, and leave it alone.

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