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After the PCM has received the air flow mass, it calculates the amount of fuel required. It then turns on the injectors for just long enough to spray this amount of fuel. The amount of time the injectors are "turned on" is a small fraction of a second, and hence the "on time" or "pulse width" needs to be measured in "milliseconds", which are 1/1000ths of a second. The ScanMaster tells you what pulse width the PCM has calculated. Note that it is NOT measuring how long the injector actually stayed open, or if it opened at all. It is only showing what the PCM thinks is the amount of time required to spray the required amount of fuel. If your injectors have locked closed due to excessive pressure, or they are stuck open because they are dirty, the computer will still show the calculated pulse width.

The injectors are rated at how may pounds an hour of fuel they would flow, if the fuel pressure is 43.5 psi (3 bar) and they are wide open all the time. This is why you can't change the injectors without telling the PCM, and correcting the values used for pulse width calculation. And, if you change the pressure in the fuel system, there is no way for the PCM to directly correct the calculation. It has to wait until the extra flow (in the case of a higher fuel pressure) shows up in the exhaust as "rich", and then change the long term fuel corrections to reduce the amount of pulse width.

At idle, you will see pulse widths of 1.4-1.6mS. At WOT, in a stock engine, you will see pulse widths of about 15.0mS. And, the left and right values should be approximately the same. If they are significantly different, you have a malfunction that is only affecting one side of your engine.

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