Miata Mailing List: June 1993, Message #9
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   From: (none)
Subject: Idle Speed Control
   Date: (none)


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.

So, I started running through the shop manual, and decided to
concentrate on the air valve, the idle dashpot, and the idle-speed
control (ISC).

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?