when i start my car the Rpm goes from 1000 to about 750rpm then jumps to about 1500 rpm and back down to 750 rpm for about 2 to f4 minns i ahve no idea why it does that

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Weather is changing ~ temps are now warmer than when you drove the car all winter. Could your Z have some sensor which needs to be adjusted for the warmer weather?

Even the very early 240s had a rudimentary seasonal adjustment ~ a simple lever on the aircleaner intake which you pushed from "winter" to "summer" to let more air reach your SUs. Your ZX is far more complicated than those old cars were, but if Datsun made provisions to accommodate weather changes back then, you can be sure they have something fancier on your space rocket.

And why not turn the heater/defroster/Airconditioner completely off next time you start it cold, and see if it does the same thing. The A/C clutch might be cycling on/off, causing a varying load on the engine, which would change rpms for a few moments.

*(Hey, ZX owners ~ are the defroster and heater driven by the A/C compressor, the same way they are on American cars?)

Hi Allen,

Soundss like your air idle control system is either dirty or has failed. Possible the Mass Air is failin as well.

This is common with 300Zx's now because of age.

Try cleaning the throttle body.

i have been meaning to clean the TB might have to do it on my next day off any one got suggestions what type of cleaner i should use and techniques that can make it a bit easier

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Brake clean and an old tooth brush.