Does Your Miata Idle Dip?

"mXxxxXm24 /O/ /O/" (mxxxxxm24)
06/26/2015 at 10:06 • Filed to: Arisu, PSA, How To, Miata

Read this (now with illustrations).

I own the rights to all the pictures in this article .

Disclaimer: Setting the idle control valve correctly will not fix other underlying issues! Be sure there isn’t something else like a vacuum leak, affecting you idle down stream before setting.

For the (almost) year I’ve owned my Miata I’ve had a consistent problem with my idle randomly dropping. It would happen sometimes at start up... It would happen when I would turn the windows down... It would happen when I turn my hazards on... It would happen when I’ve come to a complete stop... It would... Well now that this was beginning to turn into a Dr. Seuss rhyme game, I’ll stop.

I had no idea what the issue was. People told me I had faulty electrics and other things that mislead me. Today I learned that while the idle on the Miata is controlled by ECU, there is also an often overlooked mechanical or ambient idle, if you will. I’m sure this will help at least one person, so I thought it necessary to write up this fix, found on Miata.net.

Tools: phillips head screw driver (needs be really short if you have a 1.8) and paper clip.

First thing to note is that this works on 1.6 and 1.8 engines and is almost identical, aside from the orientation of the ambient idle control.

Second is to be sure to warm up your car to operating temperature and note the position of the idle rpm reading (you’ll want to to know this exactly). Make sure everything electrically driven is off (radio, lights, wipers, air, etc.) On the 1.8 (manual transmission) it should be just above the line below the 1000rpm line.

Thirdly, you find the little black diagnostics box in the engine compartment on the drivers side. Find the labels for TEN and GND. Insert paper clip each end into the TEN and GND. This will ground your “temporary engine” turning off the ECU regulated idle which is in control most of the time. If your idle drops upon inserting the paper clip, like mine did, then read on!

Lastly the ambient idle control screw will need to be adjusted with your screw driver. This is located on the passenger side on top of the throttle body. It will be pretty obvious when you see it as it protrudes from the throttle body and is sometimes covered by a plug (mine wasn’t).

Now SLOWLY adjust the screw while looking back to your rpm reading, checking results as you go. Counter clockwise raises rpms. You will need to match the mechanical idle with the ECU idle. While doing this make sure that the paper clip is all the way in and that if the fan kicks in that you wait until it stops before adjusting further (it will affect idle settings).

Once you’ve matched the mechanical idle to the ECU idle, you are done! Now you should have a consistent idle... If not and your car runs roughly upon start ups, then you might want to consider a new ignition coil as another possible fix (especially if you have a ‘99). Another issue could be your injectors.

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Also check out this for information on timing:

http://www.miata.net/garage/ignitio…

Thank you so much for reading! I hope this helped some of you!

DISCUSSION (12)

1111111111111111111111 > mXxxxXm24 /O/ /O/
06/16/2015 at 23:20

I’ve had this happen on and off at stoplights. All of the suden it almost stalls with me just sitting with the clutch in.

mXxxxXm24 /O/ /O/ > 1111111111111111111111
06/16/2015 at 23:28

Well hopefully this helps you!

MR2_FTW - Group J's resident Stig > mXxxxXm24 /O/ /O/
06/16/2015 at 23:45

That’s a pretty normal way to adjust idle on an older EFI car from the 80’s/early 90’s. Most of them have an idle air adjustment on the throttle body that allows more/less air past the throttle plate itself.

mXxxxXm24 /O/ /O/ > MR2_FTW - Group J's resident Stig
06/16/2015 at 23:50

Good to know!

Needmoargarage > mXxxxXm24 /O/ /O/
06/17/2015 at 00:42

Also important that you know your ignition timing before messing with the idle control screw. Some Miata guys like to bump up timing to 14 degrees from the stock 12 or so and this will change idle characteristics.

mXxxxXm24 /O/ /O/ > Needmoargarage
06/17/2015 at 07:47

I thought this was the opposite. You set the idle... Then you can set the timing.

http://www.miata.net/garage/ignitio…

Needmoargarage > mXxxxXm24 /O/ /O/
06/17/2015 at 09:05

Hmm...I know that when I mess with timing it will change the idle around 250RPM or so. I would think that messing with the idle screw is the last step of a chain of settings to get the car to behave appropriately. At least it was like that with carbureted setups, but I could be wrong.

mXxxxXm24 /O/ /O/ > Needmoargarage
06/17/2015 at 09:07

I guess so could I... I’m just going by what people more experienced then I, have written.

Roadster939 > mXxxxXm24 /O/ /O/
06/26/2015 at 11:21

If you’re going after this problem for the first time, it pays to check all the systems that provide input and can affect idle. Vacuum leaks, Air Flow Meter, Idle Control Valve and Air Valve (and the cooling system hoses that tie them together). Otherwise, you can set a steady idle sitting in your garage and still get weird fluctuations when out on the road.

mXxxxXm24 /O/ /O/ > Roadster939
06/26/2015 at 11:36

Very true. Thanks for pointing this out!

popups > mXxxxXm24 /O/ /O/
06/14/2019 at 19:44

Wow this worked perfectly! My 93' Miata’s idle would dip every time pushed in the clutch and came to a stop sign. This did the trick! Thanks!

mXxxxXm24 /O/ /O/ > popups
06/14/2019 at 21:31

Glad I could help! New Miata?