My car is a 1991 coupe NA
APEXI POWER INTAKE (connecting to AFM)
2.5'' catback exhaust single exit

Car has a popping sound during idle and I think it's a problem with AFM because when I put my silencer in the exhaust (simple funnel shaped) the popping sound becomes much more subtle, harder to hear. The possible source of this popping sound was alerted to me with several Malfunction (SST) tests which showed only "Ignition coil Trailing side" and "AFM" to be problems.

 

The AFM warning is triggered by the voltage being out of range -- either too low or too high. Check wiring for voltage drop/other issues.

 

Okay, my book says the air flow meter needs special equipment to be read accurately and needs to be taken to a dealer service department. Any chance someone knows the correct voltage for the air flow meter.

 

Perhaps if you were to check it's calibration, then yes, it would require special equipment. However, you should be able to see if it's within acceptable ranges for fully closed/open using the FSM. The ECU threshold for triggering an error code is roughly 0.41 volts if I recall (this going by the ECU code from memory). It should more or less move from ~0.5v to ~4.75v at full open.