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My car (93 miata) has been suffering from low idle so I decided to raise it a little bit but I have no idea where to put the new speed at or at what temperature should I make this adjustment, so here are few questions I have about idle speed:

  1. Should the idle speed be adjusted when the engine is hot (like after a good highway drive) or when the engine is warm. I imagine the former because that is where the car will be spending most of its time?
  2. Which rpm range should I pick for my engine or how can I tell which range to choose from for my new idle speed?
  3. Does having a high idle (ex: 1.1/2k) signify anything but more fuel consumption? would it increase wear? isn't the engine spending a small amount of time in idle that it is not that significant, and really the engine is already operating at a much higher speed most of the time.
  4. I heard that engines are not running really efficiently at idle, does this statement has any truth to it and if so why?
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    Your vehicle is fuel injected. You shouldn't be messing with it unless you are doing it via a computer connection. Usually, if a fuel injected vehicle isn't idling correctly, it's because somethings wrong with a sensor like the idle air control valve or some other part. Look for whats wrong as turning up the idle will only cause issues. – Pᴀᴜʟsᴛᴇʀ2 Nov 13 '16 at 3:26
  • @Pᴀᴜʟsᴛᴇʀ2 the car has a plate in the throttle body that you just screw clock or counter clock wise to adjust the speed, does that still apply? – method Nov 13 '16 at 3:38
  • It actually has an IAC which should control the idle speed. This is all done by computer. On most FI cars, once you start messing with the screw, it never runs the same again. Just saying. – Pᴀᴜʟsᴛᴇʀ2 Nov 13 '16 at 3:53
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    What does the engine idle at? Spec is 850 RPM with the engine up to temp, no load on engine. Base timing should be 10° at idle RPM. When you adjust the idle on that year miata you need to put the ECM in self test mode. – Ben Nov 13 '16 at 4:27
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    yes the ten and gnd terminals will work. Then adjust at the screw. But like paulster2 said you shouldn't mess with it and find the root cause. – Ben Nov 13 '16 at 4:43
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Here is the method I used on some vehicles to set the minimum idle speed. The cars I did this with did not have a MAF, so I am not sure how that will effect things.
Get the engine warmed up. Just warm is fine. You don't want the ECU in high idle mode.
Pull a vacuum line. This will cause the idle to go higher than expected, so the Idle Air Control Valve (IACV) will close to try and lower the idle.
Disconnect the IACV so it remains closed.
Reconnect the vacuum line. At this point the idle will drop, possibly far enough to stall.
Set the idle.
Reconnect the IACV. It may take some time for the IACV to relearn the idle. You might want to disconnect the battery for ~5 minutes to reset the ECU.

Side note: Be sure the IACV port in the throttle body is clean. This can effect your idle.

Stick with stock spec. The ECU will have final control over the idle speed, this just sets the minimum. Lower should be fine as the IACV can compensate, but higher can reduce engine braking.

Engines are inherently inefficient at idle.
The engine is not under load. There is some load on a car with an automatic in park.
Your typically idle when the car is not moving. 0 speed = 0 mpg.
The RPM is low, so the air speed in the intake is slow (below optimal). This leads to the fuel not being optimally atomized. Direct injection gives better atomization.

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