I just did a change oil and then change spark plugs (from normal copper to platinum if that matters) and high tension wires for my Protégé 2000 Mazda Automatic Transmission.

Prior change my idling RPM is at 1000 and when I turn on the AC, it drops at 900 with slight vibration. After the changes, both idling (with or without AC) are at 800~900 RPM but both have slight vibrations now.

Is this ok or normal? Do I need to have the mechanic adjust the idling slightly up (is this even possible)? Or what is your opinion regarding this? I'd like to remove the slight vibration on both idling if that is favorable. Or are there any other thing I need to have check?

  • This is very subjective in that what do you mean by rough? It could mean two different things between you and I. It seems the idle is a bit high at 8-900rpm (1000 definitely being high in my opinion). If it's not changing RPM at idle when you engage the A/C, I'm thinking it's not much of a problem. Look to see how it accelerates off idle, as well. Does it do it smoothly? Jun 29, 2015 at 11:10
  • @Paulster2 what do you mean by rough - Not really rough as the tag suggest, as I've said there is minimal vibration at idle. It seems the idle is a bit high at 8-900rpm - True, when I read your comment I recheck the meter and I read it wrong. It is around 600~700. As for accelerating out of idle, it is very smooth. What I'm trying to figure is, is that slight vibration ok at idle?
    – L42
    Jun 29, 2015 at 22:36
  • I don't remember the Protege having any sort of a rough idle (I sold Mazdas back in the late '90s). Considering, though, that it stays at the 600-700 rpm range with/without AC, plus having smooth acceleration (I assume no hesitation when coming off of a stop), I don't think you have anything to worry about. Sounds like it's running good. If any of this changes, then I might be worried about it. Jun 30, 2015 at 0:31
  • 2
    It is possible your engine vibrates more at 800-900 rpm than 1000, and now that the w/o A/C idle is at that range, you get that same vibration more. One cause might be a stuck A/C compressor clutch, or something in the engine management system changed to cause a different A/C idle speed.
    – kmarsh
    Jan 7, 2016 at 17:28
  • 2
    It could have slipped a tooth. Please put that as an answer so this question can be finalized. I'm glad it's running smooth again! Jan 28, 2016 at 11:10

1 Answer 1


As posted in the comments, the issue was resolved after replacing the worn out timing belt.
The camshafts were centered and adjustments on the timing was also done.

Slight vibrations maybe a good indicator if your timing belt is due for replacement.
This will prevent you from expending a lot of money due to broken timing belt resulting to mis-aligned cam's.

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