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Old 05-03-2011, 07:51 PM   #1 (permalink)
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Default NA miata. high idle. help!

hey guys. i recently got a 1990 miata with 167k miles. it has a 1.6l motor. took the beast to smog it (in california) and it failed. needed a new cat and high idle. i got the cat part taken care of but i cant seem to figure out why the idle is high. originally it was around 1700 rpm and then i adjusted the throttle cable all the way down and it went down to 1200 rpm. the guy who i took my car to says that it should be around 900 rpm at idle.

i have already
1. checked the coolant
2. changed the pcv
3. visually checked the vacuum lines and replaced a cracked hose
4. reset the ecu

the car does have an aftermarket air filter on it as well. i can try to find an oem box but the car passed smog as is just this last nov.

any help will be much appreciated.
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Old 05-03-2011, 08:17 PM   #2 (permalink)
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Default Re: NA miata. high idle. help!

Did you check your ISCV. It is attached to the bottom of the throttle body.
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Old 05-03-2011, 08:22 PM   #3 (permalink)
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Default Re: NA miata. high idle. help!

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Originally Posted by emartoo View Post
Did you check your ISCV. It is attached to the bottom of the throttle body.
+1
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Old 05-03-2011, 11:03 PM   #4 (permalink)
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Default Re: NA miata. high idle. help!

Quote:
Originally Posted by emartoo View Post
Did you check your ISCV. It is attached to the bottom of the throttle body.
Sorry but how does one check the ISCV (I assume you are talking about the Idle Air control valve)?

I do agree its probably the culprit though. What I really think is going on is that your IAC valve was set incorrectly, you can change it using the set screw.
First, you have to short TEN and GND on diagnostic port (its right above the driver's side wheel well. A gray box/connector cover thing.
Once thats done, find the Idle set screw and turn it down.
http://www.miata.net/garage/ignition.html (scroll down or search for idle speed adjustment, btw the jumper they are talking about is the short between GND and TEN. Instructions for that are above the Idle speed adjustment section.)

Also, you need to have a slight bit of slack in the throttle cable. This ensures the TB flap is completely closed. You can google Mazda's spec. for it.

If that doesn't solve it, then I would start searching on google!

Last edited by RedemPhex; 05-04-2011 at 08:32 AM.
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Old 05-04-2011, 12:28 AM   #5 (permalink)
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Default Re: NA miata. high idle. help!

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Originally Posted by RedemPhex View Post
Sorry but how does one check the ISCV (I assume you are talking about the Idle Air control valve)?

I do agree its probably the culprit though. What I really think is going on is that your IAC valve was set incorrectly, you can change it using the set screw.
First, you have to short TEN and GND on diagnostic port (its right above the driver's side wheel well. A gray box/connector cover thing.
Once thats done, find the Idle set screw and turn it down.
http://www.miata.net/garage/ignition.html (scroll down or search for idle speed adjustment, btw the jumper they are talking about is the short between GND and TEN. Instructions for that are above the Idle speed adjustment section.)

Also, you need to have a slight bit of slack in the idle cable. This ensures the TB flap is completely closed. You can google Mazda's spec. for it.

If that doesn't solve it, then I would start searching on google!
awesome! i will give it a shot in the morning.
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Old 05-04-2011, 08:32 AM   #6 (permalink)
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Default Re: NA miata. high idle. help!

btw I had an error, instead of idle cable (lol?), I meant throttle cable.
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Old 05-04-2011, 01:49 PM   #7 (permalink)
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Default Re: NA miata. high idle. help!

i tried to adjust the idle using the method mentioned above with no luck . i looked under the throttle body and see a crap load of rtv silicone on what i think is the idle speed control valve. after a quick search both on here and google, i see that they cost around $500. does anyone know if these are rebuildable or if there is a source to get quality used parts? even a mazda part number will help. i did find that on mazda-parts.com they list an idle air control valve for only $178.04. anyone know if this can somehow be the part i need?
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Old 05-04-2011, 02:19 PM   #8 (permalink)
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Default Re: NA miata. high idle. help!

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Originally Posted by gurt View Post
i tried to adjust the idle using the method mentioned above with no luck . i looked under the throttle body and see a crap load of rtv silicone on what i think is the idle speed control valve. after a quick search both on here and google, i see that they cost around $500. does anyone know if these are rebuildable or if there is a source to get quality used parts? even a mazda part number will help. i did find that on mazda-parts.com they list an idle air control valve for only $178.04. anyone know if this can somehow be the part i need?
the silicon under the TB is a coolant line to prevent the TB from getting freezing over. The IAC is a small metal hunk connected directly to the bottom of the TB (Its purpose is to get air around the TB when idling). You can remove it if you like and yes they are quiet expensive. Try to get a used one from mazmart, parts group, or one of those used parts sellers (its not exactly a performance part and getting it new or used has no effect on its functionality).

I would search google for causes of miata high idle. it could be something entirely different. My last guess is that some people have stated that if you clean the TB with anything other than TB cleaner, then it will remove a protective coating that helps the butterfly and the TB itself seal which can cause higher idle.

Lastly, I suggest you try to reset the idle again. perhaps you skipped a step or something? Good luck, you can try to post about this on m.net they have a lot more expertise than me on this stuff.
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Old 05-04-2011, 04:13 PM   #9 (permalink)
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Default Re: NA miata. high idle. help!

The throttle cable's are supposed to have about 2mm of play.
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Old 05-04-2011, 04:24 PM   #10 (permalink)
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Default Re: NA miata. high idle. help!

thanks guys for all of your responses. can someone tell me if this throttle body has the iscv attached to it? i read that it is very common to see throttle bodies sold with it already attached. thanks

http://cgi.ebay.com/ebaymotors/OEM-9...Q5fAccessories
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Old 05-04-2011, 04:44 PM   #11 (permalink)
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Default Re: NA miata. high idle. help!

yes it does.

its the portion of the TB assembly to the right of the throttle body inlet in that picture.

a caveat about the ISCV, there is a gland valve between it and the throttle body. usually once the two come apart, it can no longer be used in most cases. the dealer and larger online parts dealers (like finishline) carry the gasket. also, the screws used to attach the ISCV are quite soft and round out easily. i use a small pair of vise grips AND a correclty-sized phillips screw driver together when trying to remove them.

also, when you remove the throttle body from the intake manifold, it usually destroys the throttle body gasket. they are fiber material. i usually smear a little vaseline or silicone grease on both sides of the new gasket so it comes off clean and isn't destroyed when I need to separate the two again.
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Old 05-04-2011, 05:53 PM   #12 (permalink)
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Default Re: NA miata. high idle. help!

thank you kung fu! do you know what that gaskit that goes in between the tb and the iscv is called? i am pretty happy that i found it for the price i did. lets just hope that it works.
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Old 05-05-2011, 11:59 AM   #13 (permalink)
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Default Re: NA miata. high idle. help!

http://www.finishlineperformance.com...cat=317&page=2

the one pictured is for the 1.8, but the 1.6 is similar. you only need that if you separate the ISCV from the TB. that unit on ebay is complete, so I wouldn't separate the two unless you absolutely need to.

here is teh TB to intake manifold gasket:

http://www.finishlineperformance.com...cat=317&page=4
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Old 05-05-2011, 01:47 PM   #14 (permalink)
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Default Re: NA miata. high idle. help!

I separated my Idle speed control valve from my TB (for cleaning) and it has worked fine so far. Not sure what problems or symptoms would be expected though...

I will say if you are buying a complete TB+ISCV package, then might as well put it in and see how it works first. You will probably experience Idle droop right off the bat, so I would personally take the ISCV off to clean it up and then manually reset the idle just to nip that at the bud.
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Old 05-05-2011, 01:53 PM   #15 (permalink)
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Default Re: NA miata. high idle. help!

Definitely good advice. I am hesitant to say that because the ISCV is pretty sensitive. Since my first ome went bad, i have swapped it out with probably 2 or 3 used ones. The ISCV is something i grab when the opportunity arises at a junkyard, swap meet or FS thread. I am curios about that new one for $178. The one in my car now sticks, so i get an idle at 1200-1500, but it steps down to 1000 where I set it soon after.
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Old 05-05-2011, 02:50 PM   #16 (permalink)
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Default Re: NA miata. high idle. help!

thanks again guys.
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Old 05-05-2011, 02:55 PM   #17 (permalink)
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Default Re: NA miata. high idle. help!

I agree with kung fu, junkyard hunting is a great way to save money on these parts. Especially when you aren't going to a miata specific yard (I think there is one in S.C. and probably one in Cali since there is so many miatas out there) where they don't know how much an item is really worth to us, so they sell it to you for cheap. Then again, if you go to our online retailers, you can get some nice stuff for cheap.

For example, The Parts group sold me a gauge cluster for $20 since it had some physical damage (as compared to 100-150 normally) which I got to use for some gauge modifications and tests.

One of the things I really like about the miata community is the easy access to used parts. Plus we got a lot of very helpful people around!

Let us know when you get this fixed gurt. I am curious to know if the ISCV is the problem.
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Old 05-06-2011, 10:04 AM   #18 (permalink)
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Default Re: NA miata. high idle. help!

FYI on the junkyard hunting note...ISCV for other B-engined cars (FWD) will work, but the tubes run the wrong way. The units looked the exact same except the tube orientation. you would need to run different lengths of hoses to connect them properly. helpful if you're in a pinch, probably. I had one left over from my Escort GTs I had many years ago and compared it up to another spare ISCV I had for my B6 miata. I wondered if the tubing can be rotated to work without damaging them, but never tried it.
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Old 05-11-2011, 05:20 PM   #19 (permalink)
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Default Re: NA miata. high idle. help!

hey guys, so i got the parts and swapped them in today. god i love how easy these cars are to work on. way easier then my other car for sure! i got the entire tb/iscv swapped over in 30 min or so and it did the trick.the car drives so much better now. and it passed smog just fine.again thank you guys for all of the help.
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Old 05-11-2011, 05:22 PM   #20 (permalink)
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Default Re: NA miata. high idle. help!

Awesome! Im glad it worked.
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Old 05-11-2011, 05:30 PM   #21 (permalink)
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Default Re: NA miata. high idle. help!

Woo!! Glad it worked for ya.
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Old 06-28-2012, 10:09 PM   #22 (permalink)
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Default Re: NA miata. high idle. help!

Damn gonna try this mines at 1100
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