dollars to donuts the issue is either a vaccum leak or the MAP (Manifold Absolute Pressure) sensor has taken a dump. was there a check engine light? a check engine light will come up on many jap cars because of them tossing on larger intake systems because the MAF (Mass Air Flow) sensor is reading a larger volume of air then it knows what to do with. the MAP sensor will aslo do this if it is not registering a reading. it will read a static number (for instance on a 2000 impala with the 5v wire broken, it will read a static 31.96 inHg). I am really leaning towards something as simple as a vaccum leak though.