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Old 03-20-2024, 05:20 AM   #1
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Join Date: Jul 2009
Location: Boston
Posts: 3
Exclamation 99 986 - idle air control valve buzzes but does not open

I am having an issue with my 986 2.5 running rough, seemingly to be very rich when below 2500 rpm or so. I believe the issue is with the idle air control valve not actually opening when it should be. My understanding is that if the ECM believes the IAC to be open and allowing additional air into the engine, but it actually is not, that would result in the air / fuel ratio to be too rich with fuel, which would explain how the engine is running poorly.

Twice now I have bought a new (although generic) IACV. My original (bosch), and the 2 new generic ones all do the same thing. They buzz loudly when the key is in the ACC position, but the engine is not running. I tried testing them with the durametric test, and they buzz very loudly, and then buzz quieter, back and forth every few seconds. I tried performing that test both while the engine was idling, and the idle speed did not change as the test was running. I somewhat expected that if the IACV valve opened during the testing, that it would change the air / fuel ratio, and cause a change in RPM, but that did not happen.

IACV video of it buzzing with key in ACC position, but not opening:

IACV video of the sounds it makes when having my durametric perform the IACV test:

It seems highly unlikely that I would have 3 bad IACV valves, so I am leaning towards the ECM sending a bad signal to them somehow. Anyone have any idea where to go from here? Much thanks.

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