91 Miata Engine trouble...

[SCCA MX5]

91 Miata 1.6L (Short nose, good keyway)

Will not keep a steady rev under constant load. In the Video, the throttle is being held at around 10%, and you can see the cable is not moving- but the engine dips and revs by itself. Does this cold or hot.

- Idles fine
- Runs to Redline fine
- Not down on power
- No engine codes


What we have done-
-New Coil Pack (tested good)
-New Timing belt +pulleys (double checked)
-New AFM (MAF) (Tried 2 others from running cars as well)
-New Plugs/wires
-Checked for Vacuum leaks (cant find anything)
-New Fuel Pressure Regulator
-New Fuel Filter
-Ran Injector cleaner

What could it be?

Ecu? CAS? Something else?

http://www.youtube.com/watch?feature...&v=XXtHwdUbba4

[n33d4sp33d13]

i had sorta this same problem in the colt and unfortunatly never really figured it out till the engine was rebuilt. i know my car has a idle air control valve (probably called something different on a miata) that i have tinkered with that has fixed several issues i have had with idle and misfires. i always adjusted my idle with my cable but turns out i was supposed to adjust the timing then adjust the IAC then tighten the cable. if your car has a IAC or something like that it could be that. i know the only similarities a miata and a colt have r they are both japanese but you never know. did you check the timing?

[hklvette]

sounds like the frequency of the surges is increasing. Have you sprayed for vacuum leaks w/ carb cleaner or similar? Also, does it exhibit the same behavior at any rpm?

[SCCA MX5]

It was a faulty Throttle position Sensor. FIXED!