Help...weird problem

[AB7]

Hi Guys,

I need your help. While I was driving back from work yesterday, I noticed that my car idle weirdly. As my car stop, the rev idle up and down about 3 times from 850 to 650 RPM then back to normal. This happened consistently thoughout the 40 min trip, it was mainly stop and go situation due to heavy traffic.

I did check all the vacumm hoses and all seems fine.

Any clue?
BTW my car is 2001 NB8B

Thanks.

[JBT]

Have you done anything at all to the car recently?

From miata.net:

\"Maybe a little of explanation of how things work will help you solve your problem. The throttlebody purpose is to control the amount of air being sucked into the engine. At idle the butterfly is very close to being completely closed, chokeing air from entering the engine. Thru use, this butterfly area builds up black crud, that further decreases the amount of air the engine can suck in. The factory adjusts, via a set screw with lock nut, the correct amount of air needed for the engine to idle at about 825-875 rpm as a ruff adjustment. There's also an air bleed screw used for a fine adjustment, that by-passes air around the butterfly, it too, gets fouled by this black crud, again decreaseing the amount of air being sucked in. On the bottom of the throttlebody is an electric selonoid, that the computer controls, to allow the engine to suck in more or less air depending on parasistic load, ie, powersteering, heater, air conditioning, etc., it too is subject to building up this black crud. Removing and disassembling the throttlebody for a through cleaning of all these passages, with throttlebody cleaner, not carb cleaner, because of a plastic coating that would be removed if carb cleaner was used, is necessary, depending on how dirty and fouled the throttlebody is. I would recommend a 30k interval for this cleaning. After everything is clean and reinstalled, gently close the air bleed screw, jumper ten and ground to remove the computer control of the iac selonoid, and adjust the butterfly, for a warm engine idle of 825-875 rpm with the butterfly adjustment. Use the air bleed screw for further fine adjustment, by opening it, to adjust first cold start up to 1500 rpm and opening or closeing it, if you experience idle dip or idle rpm hang up of more than 50 rpm when the engine is warm. By adjusting the throttlebody idle this way, you will be centering the air/fuel mixture to what the computer can adjust for, enabling the computer to more quickly and easier to adjust correctly for prevailing conditions. Hopefully this helps you solve your problem.\"

[Garry]

What JBT and the guy on miatanet wrote.

It's quite a common MX5 problem. Easy to fix one everything is squeaky clean and adjusted correctly.

[AB7]

Thanks for the feedback guys.
I'll have a look at it on the weekend.