Low rough idle.

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Roland Smith - 19 Jun 2006 18:05 GMT

My '90 (short nose crank) miata suddenly developed a low stumbling idle
(200 to 500 rpms) with intermittant surges to 1,500 rpms and then back to
stumbling. It runs smoothly at anything above 1,500 rpms and pulls hard
to redline through the gears.

The end of the crankshaft seems to still be firmly attached, the timing is
steady and the crankshaft pulley bolt is still tight.

The car has 77,000 miles on it and has been supercharged for the last
50,000 miles. There are no obvious vacumn leaks and the symptom developed
over the course of a 60 mile drive. Spark plugs and wires are about
15,000 miles old.

Any ideas?

Roland Smith
Wrightwood, Ca

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Christopher Muto - 19 Jun 2006 19:18 GMT

when was the last time you replaced the pvc valve and gromet?
http://www.finishlineperformance.com/cgi-bin/shopper?preadd=action&key=M1015
http://www.finishlineperformance.com/cgi-bin/shopper?preadd=action&key=M1003

> My '90 (short nose crank) miata suddenly developed a low stumbling idle
> (200 to 500 rpms) with intermittant surges to 1,500 rpms and then back to
[quoted text clipped - 13 lines]
> Roland Smith
> Wrightwood, Ca

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KWS - 24 Jun 2006 18:33 GMT

There is a vacuum plug on the intake manifold near the firewall; the loss
of this plug can cause the symptoms you describe. It's a rubber item that
can become cracked or lost over time. You have to look for it; it's not
highly visible. There are various plugs available that slip over this
unused metal tube (about 5mm diameter?) that you can buy from your local
hardware store.

Rule this out before you start changing a lot of stuff.

Ken

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