Expert: Mp2 - 8/21/2009

Question
I have a 90 miata with 62,000 miles. New ac system installed, The problem is when the AC is on, the idle will shoot up to 1500-2000 then drop down to eight hundred then back up then go into stall. When You put the ac on in the beginning it revs at 11000 then after driving around it will have the above problem. The mechanics took out the Idle control valve and cleaned all the carbon on it, but they say this is the problem. It only happens with the ac on, is this the problem. Thanks.

Answer
The idle control solenoid (ICS) is an electro-mechanical device it also uses coolant to help control idle during warm up (in unison with the air valve)

Being electro-mechanical it gets a signal from the ECU when ever the AC, the power steering pump or the regulator (in the alternator) demand a load from the engine.  I meters the bypass air around the throttle plates to compensate idle for these demands.

Typically when the solenoid within fails the engine will not idle up to compensate for these demands, most notably the AC since it draws the most HP from the engine.  If you switch on the AC (be sure to observe the AC clutch to physically engage) and the RPM's fall the device is not functioning.  A quick test of the device is, engine off, key on disconnect the plug from it and listen for the relay to click as you re-plug it.

In all likelihood you have more than one issue presenting itself.  It is common at this age (years not miles) for the ICS to fail, it is a rather expensive part but, replaced Mazda new it will remain in good use for another decade plus.  If the ICS passes the test and is functioning, cleaning can help it to operate as blow by residue and varnish from the throttle plates cause an effect with time.  I would check the base idle and timing with the ECU shunted, commonly technicians increase the throttle stop to compensate idle without shunting the ECU in situations like this which forces the ECU to counter the adjustment so the idle ends up higher than it should.  It's the type of fine tuning best left up to people who know Miatas.  Lastly, if the AC system is overcharged the compressors head pressure will cause the compressors high pressure safety to cut power to the AC clutch, a built in safe guard, the rest of the system remains active but no compressor so the result is the engine revs excessively high.

A couple of scenarios for you, the best thing for a Miata, check the basics first.

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