Idle Air Controller IAC a definitive test?

[94 clubman perth]

Hi all, having some idle problems with a recently aquired 94 NA8 clubman and believe the IAC is to blame, this is the all in one IAC that sits under the throttle body, not the two part system like on 1600's.

Has started to idle at about 1600 to 1800 hot or cold. Ocasionally will drop to 1200, but not often. After a squirt of carby cleaner down the IAC intake the idle dropped right down for 50kms, and is now back up, maybe a plunger is gummed up, or another coincidence?

This problem occured after I disconnected the battery, dont know if it was just a coincidence, havn't had the car long enough to know if problem is intermitent or just started.
In review of info everywhere on MX5's there is much writen on problems with IAC's, but I cant find a definitive test. e.g what voltage should be at idle motor with ignition on and at what tempersture? what resistance should the idle motor have, can the wax pellet IAC be tested and is it prone to gumming up? can it be cleaned?

I am aware the IAC has 2 components, 1 being the wax pellet that changes consistency with heat, affecting a plunger and changing airflow (for cold idle) and the other being an electric motor that controls bypass air depending on electrical load and controlled by ECU. The electric motor bypass is only supposed to be able to increase idle to 1200rpm, given it idles higher I am leaning towards an intermitent problem with the wax pellet.

There seem to be no airleaks on the intake manifold and the car runs great off idle.

Would love a test procedure. Thanks.

[manga_blue]

What you describing could just as easily be a failure of one or two of the switches that the ECU uses to determine that you want it to idle. These are the 1) the throttle closed sensor in the TPS on the throttle body, 2) the clutch switch on top of the clutch pedal and 3) the neutral switch on top of the main gearbox body. Basically the engine will idle normally when the TPS says it's closed and either the clutch pedal is depressed or the gearbox is in neutral. If the TPS switch is wrong (usually wrongly adjusted) or both the other switches have failed then it will idle at about 1600rpm, falling back to normal from time to time as one switch happens to make contact. Try a multimeter across each to see they work (the TPS switch operates the bottom 2 pins of the 4 pin plug).

[94 clubman perth]

Thanks for the advice, I'll test all 3 and let you know how I go.

[93_Clubman]

If this problem occurred after disconnecting your battery, & you or anyone else didn't do any else to the vehicle, then you might find resetting idle helps. Why? Well disconnecting the battery should reset the ECU in terms of error codes, & so resetting the idle might be useful. While you're at it it's worth checking the timing & adjusting if necessary. This should help:
http://www.miata.net/garage/KnowYourCar/S7_Idle.html

These might also be useful:
http://www.miata.net/garage/faultcodes.html
http://www.miata.net/garage/ignition.html

Another worth while trick in terms of idle fixes is to remove & clean the idle screw & its recess in the throttle body, but be careful what you use on the throttle body as it can damage the ability to seal of the brown throttle plate sealant.

[94 clubman perth]

Thanks to Manga and 93 clubman (good links).

I believe I have cured the idle problem (fingers crossed).

It seems some one had played with the throttle stop screw (not the idle screw but the external screw and locknut setup that all the books say not to touch).

Outwardly it appeared the throttle plate was closing all the way, but backing this nut off showed it was being held open, and hey presto, the idle is back at 850ish. The idle screw is in all the way, probably because the latex/paint is missing from within the throttle body so air is getting around the throttle plate and making a defacto idle circuit. But now when cold it starts at 1400, drops to 850 within a minute or two and kicks up to 1100 with a/c on. Perfect!

I cant explain why it went from idling normally to idling fast intermitently (apparently after I disconnecting the battery?), maybe the explanation in the link below re ecu idle speed vs throttle idle speed may explain it? Only had the car for 500kms and continue to sort out the issues, but very happy now

[manga_blue]

Good to hear it's sorted easily. Idle problems can linger for months.

[93_Clubman]

Likewise good to hear it's fixed - as Manga says this sort of thing can sometimes be difficult to pinpoint, but also can occur after we've done, or had something done, to our cars.

[bensale]

Glad to hear you've got it fixed! Such a PITA of a problem to diagnose. Your solution makes sense.