Our 99 miata idle will drop to around 600 and run rough- then start to run smoth like it should around 750 or 800 for a time then do it over again , any idea.
You should have scan done for fault codes
but you could inspect the throttle body for carbon deposits and clean as required first and see how it goes
When you replaced the cap, did you reset the "check engine" light. The problem codes stay in the memory until you reset them. To reset, you can disconnect the battery or remove the fuses that power the computer. The fuses will be marked, with CP or ECH or some similar marking. They may have a number behind it. You can take all these fuses out for about 5 minutes or disconnect the battery for 5 minutes. When you restart watch the light, it should be out. If it comes back on in the first minute or so you still have a problem. Check the code again. Likely the light will stay off once you reset it.
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Gaker
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"Intake manifold carbon buildup" in this model is balony. Unless you've removed your air filter or have serious back-firing, the only things ever in the intake manifold are atomized fuel and air. I fell for this "old wive's tale" and removed my intake manifold. It was clean, with not a hint of carbon or anything else that shouldn't be there. Besides, in hindsight, it would have been possible to blow compressed air into the vacuum port on the intake manifold to clear it of any blockage. I have your year and model and had your problem. The solution was to replace the coils and ignition wires, after which the CEL never again came on. Another lesson learned the hard way.
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