So we are dealing with a JDM B20 with a LS intake manifold. It has fuel, compression, spark, and timing but it barely runs. It idles very low and rough even with the idle screw adjusted all the way unless you unplug the vacuum line for the FPR. We thought maybe fuel pressure was an issue so we swapped to a ITR FPR and still the same thing, unless the vacuum line is unplugged it barely idles, like 600RPM max.

If you crack the throttle it will rev but is very rough and has hesitation then usually falls to an idle so low it stalls. We tried 3 different ECU's, a new distributor, a new fuel pump, new injectors... does the IACV effect throttle input past idle? The only CEL's depending on the ECU are O2 sensors, crank position sensor using a USDM ECU on a JDM motor that lacks that, and VTEC solenoid using a VTEC ECU on a non VTEC motor. The only common CEL/sensor is the primary O2 which would just make it run rich.

So what is left to check or diagnose?

 

is the motor in time? do you have the map and tps plugs in the correct locations?