Miata Mailing List: June 1993, Message #23
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   From: (none)
Subject: Re: Idle Speed Control
   Date: (none)

mgr@iti.org writes:
> My '91 Miata base model occasionally drops down to 500 rpm (brief
> stumbling) when decelerating with the headlights on.  I recently
> increased the idle speed, and got an uneven RPM deceleration curve.
> The tach needle would pause and drop in a jerky fashion after
> releasing the throttle.
>
> (text deleted)
>
> One test for the ISC is to unplug it during idle.  The ISC is supposed
> to "click" and rpm's are supposed to jump up to around 1200.  Well, I
> did this, heard the click, and the idle increased as it should.
> However, upon re-connecting the ISC, my idle problems completely
> disappeared.  It's been a few days now, with no recurrance.

> This is probably not a long-term fix, but in some cases it might help.
> Here's what MIGHT be happening.  It seems that the ISC's internal
> moving parts get "sticky."  Unhooking the ISC appears to open the
> internal valve all the way - forcing the moving parts through a larger
> range of motion, which helps alleviate the stickiness.

> Not much science here.  But for those of you with similar idle
> problems, you might try this approach.  Has anyone had problems with
> the ISC - sticky or otherwise?  Has anyone had their ISC replaced by
> the dealer?  If so, what were the circumstances?

I haven't had anything like this happen to my Miata (yet), but it does
sound similar to an experience I had with my SAAB 9000 turbo.  It was
bothered with a sometimes-unstable idle and stalling under deceleration
when the engine was cold.  After pricing a new ISC, and hearing how a
similar problem on a Mustang was cured by cleaning the throttle body, I
bought a can of spray carburettor cleaner and got out the wrenches.  You
didn't mention if your Miata is a high mileage car or if it has been run
in dirty conditions, but my guess is that your "sticky" theory is
correct.  The SAAB had a nice layer of black goo inside the throttle
body near the throttle plate, and a smaller quantity of this goo inside
the air passages of the ISC.  After washing this out and putting
everything back together, the car seemed to idle much better, except for
some bizarre idle speed changes for the first couple of minutes while
the ISC cleared itself of the carb cleaner left inside.  Afterwards, I
still noticed some occasional sticking open or closed of the ISC, but
the "hunting" of the idle speed never returned. (Maybe the lubricant got
washed out along with the goo, or it was just too far gone to save.)
I bought a new ISC while on a trip to Germany for 2/3 what the local
dealer was asking, so I never tried to oil the old ISC or make
further repair attempts.  If you're desperate, you might try a similar
cleaning exercise.  Otherwise, if your car is still under warranty, you
may want to disconnect the air feed pipe from the throttle body and look
inside for dirt so you can point your mechanic in the right direction.

Good Luck.
                              Milton Horst
                              Seattle, WA