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I have a 1991 Miata that I just bought from a dude in Florida. However, now that I’m back home in Georgia, my RPMs are around 500-200 at idle and when the AC is turned on she dies! I have 2 ideas of what’s causing it. Either the throttle body position sensor or my MAF need to be replaced / cleaned. Just seeing if anyone has any other ideas, tips or tricks to help me out. Thank you!
 

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My first thought is the idle speed control valve (under the throttle body) might be stuck, or its solenoid might have failed. I'm familiar with cleaning the 1.8 version of the valve with carb cleaner spray (just spray cleaner into the air inlet under the main throttle inlet and let it drain back out) but not familiar with the original 1.6, though I doubt it's much different. It's moderately unusual for the solenoid to fail but if you have a multimeter you can unplug the 2-pin plug and measure the coil's resistance, it ought to be something low like around 12 ohms. If it's megohms then the coil has failed and the valve needs replacing.

There's also the bypass screw which you use to set the base idle. (There's a narrow air passage cast into the throttle body which lets air bypass the closed throttle, and a screw controls and restricts that airflow) Even if the ISC valve wasn't working you should still be able to set a reasonable idle speed with that screw (under a rubber cap, on top of the throttle). They do clog up with grime over time and it's usual to have to back the screw out a little from time to time to reset the base idle speed. I was impressed on my first 1.8 when I eventually decided to clean the air passage wth carb cleaner spray; the idle went sky high and I had to wind the screw almost all the way in for the first time in years just to get the idle down to normal.

I hope some of that helps
 

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Read up on procedures for resetting the base idle on the 1.6 before you do it. You need to bridge two pins in the diagnostic connector (I'm thinking GND and TEN, but check) otherwise the idle control circuit will try to compensate your screw adjustment
 
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