To coin a phrase, you "hit" on the MOST likely cause of low and high idle. It's LESS likely you have MAF issues or Ignitor troubles. The idle speed control (ISC) and BAC have a plunger/piston inside the ISC that is designed to move up or down and adjust air flow minutely. It is sensitive and does not like dirt. Old coolant is the usual suspect. A sticking plunger will absolutely cause erratic idling. What causes the sticking? Usually, it is dirty coolant run far too long that, together with other non-sequitor chems from Oreilly's/Autozone, cause the sensitive anti-friction coating (rosy pink teflon) on the idle piston to rub inside its cylinder. Even changing the coolant after the damage was done will not necessarily fix the symptom. The fix (which I'm sure has been discussed in previous MX6.com years) is to remove throttle body; then, remove BAC and ISC combo; then, separate those two; then, clean clean, clean. Did I mention clean?? DO NOT USE ABRASIVEs on the plunger. DO NOT use Silicone or oils or ANYthing else on it when reassembling. DO TRUST the slippery anti-friction factory coating. Per Mazda factory manual there are no "owner servciceable" parts, but we know better.
. Instead, says the factory, replace the ISc or BAC combo. Right? NO ! Clean its internals. Tips: The one-way screws holding ISC onto BAC must be removed with a vise grip one millimeter at a time. Plan on discardng those several factory screws, thus, before disassembly, buy new metric ones with normal phillips heads. You will also need a of tube gasket sealer. Hylomar Universal Blue (spelling?) works best. Apropos gaskets, be EXTRA CAREFUL when pulling apart the ISC and BAC because there are two gaskets that are difficult to clone if ripped. Note, too, the EXACT order of the plunger, springs, semi-hard round gaskets, etc. Clean, clean clean. Oh yes, I did mention clean, didn't I ?. Clean again. NO sandpaper or steel wool on internal parts. Reassemble everything as if you were the original assembler following Mazda engineer instructions. Just say "no" to clever modification ideas... unless you want to do it all over again and again. Otherwise, follow generally accepted best pactices for installation, ensuring no sensor wires are broken/frayed. Of course, you will want to ensure the coolant is tip top, lest the same malaise afflict your steed once again. Hope this helps. If Engine Code 08 is the true source of all your issues, I humbly apologize for this post. Sajo