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when i stop the car and go to neutral the revs remain at about 1800rpm for 10 to 15 seconds before dropping to 800rpm. This occurs when the engine is at mormal operating temperature - Is this normal ? Chris

I had the same problem
it turned out to be the clutch switch ( i had disconected it and used it for the cruise control)
but it could also be the neutral switch

See Juffa's post just over half way down the following page where he found a fix for what Russ describes: viewtopic.php?f=29&t=39507

But you might first want to try resetting your ECU to see if this fixes the problem:
disconnect the battery, then depress the brake pedal a few times over a period of about 10 seconds. This resets the ECU & clears fault codes, if any. Then drive the car to check idle - if it's still a problem, check the fault code/s as they will be the most recent & therefore relevant: http://www.miata.net/garage/faultcodes.html

Btw, was any work done on the car just before this started happening?

thanks for the help - re have I had any work done on the car recently, yes just had the timing belt changed.

Chris

OK - the idle may require resetting. Try resetting ECU first, & checking & noting fault codes. If that doesn't work, then try resetting idle as per instructions in the following: http://www.miata.net/garage/KnowYourCar/S7_Idle.html

Idle speed resetting, along with ignition issues, are also covered in the following:
http://www.miata.net/garage/ignition.html

thanks - will try tomorrow

success ! The MX5 drives much better when the revs fall correctly between gear changes. Thanks for the good advice

Chris