Mechanic advice, missfire cause?

[SpeedFreak_Josh]

I've posted before of this problem but have recently noticed something.I have a 90 300zx tt with a miss that gives off no code. I've checked the ptu, coil packs/injectors(visually), and replaced the spark plugs and ran the self diagnostic check. NOTHING.I noticed while coasting at 20mph up to around 45mph or so that the tach jumps around at idle. 750-1150rpm give-or-take. It smooths right up when I come to a slow speed or stop. This makes me think that I have a problem reading the air intake.What do you guys think?Thanks

[izmyttz]

could be the MAF...

[SpeedFreak_Josh]

Wouldn't that give me a code 12 (air flow meter circuit)??

O well, guess it's off to Nissan for investigation. I just hate taking my car to another mechanic who isn't going to thoroughly investigate and pay him to tell me he found nothing wrong with my car. He'll probly just take it out for a (tire)spin and wipe oil all over my clean interior.

[Canyoncarver]

You probably have a aftermarket cone air filter and/or some of your stock plastic sheilds are missing from the front of the car. What you are experiencing when idling at speed is a common problem. It is caused by air turbulence across the maf hot wire.

Make sure all shields are in place and try searching for "duct tape trick" or Ash's "flower pot mod" on tt.net or 300zxclub.com.

On the missfire, it could be anything from a small vacuum leak to burnt/worn valves. Search the same forums above for a wealth of info on "missfire"