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Dear CPG,
Hope you're well. The first thing to do, when idle speed varies randomly, is to check the On-Board Diagnostic codes. See the site's FAQs for how to do this and post back with any codes.
The next thing to do, is to inspect and - if necessary - clean the idle air control valve (IACV). If the IACV is dirty - black, sooty deposits inside the tubes, to which the hoses connect - it needs to be cleaned.
The IACV is mounted on a bracket, beside the intake manifold. It has two rubber hoses at the end closest to the front of the engine bay. These hoses are secured with screw-operated clamps; if factory-original a 7mm socket is required. There is an electrical connector at the end closest to the firewall. Press on the release bail and pull gently to remove the connector.
Keep the valve's nose pointed down, so that any solvent used does not get into the motor compartment. If you use cotton-tipped swabs, be sure all fibers are removed. If you want to immerse the valve in solvent, put it in hose nose down, and limit solvent depth so that solvent does not get into the motor compartment.
ECUs are the last thing to replace: they rarely fail. If the air mass meter had failed, it would have set a code.
Your mechanic may be a random parts replacer, rather than an analyst. Such mechanics are a cause of much wasted money.
Hope this helps.
Yours faithfully,
Spook
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