A pop tester is a tool that hooks up to an individual fuel injector, on the bench. It has a hand lever that pumps fuel to the injector, one stroke at a time, and a gauge that measures the pressure.
This gives you a way to measure several things that the injector must do.
First, it must not dribble fuel with low pressure going into it.
Second, when it does begin to spray, that is called it's "pop" point. It should hold without leaking and then pop open at a designed pressure.
Third it should have a very defined spray pattern with excellent atomization.
You can look up the pop point of specific mechanical injectors and check to make sure they do not dribble. And then see the pattern they make.
This is a tool used on mechanical injection systems, not common rail.
By pumping the handle, you can repeatedly hit the pop point and repeatedly see the spray pattern.
That is why I said the hydraulic porta-power pump is not suitable for this operation. You don't slowly pump it up, like a jack and then release with a valve. You keep firing it over and over to see how it responds. With a hydraulic jack type pump, as soon as the pop point is reached, the pressure is immediately gone. With the pop tester you can initiate the spray and keep it going long enough to see it well while maintaining a high pressure.
It's a much closer simulation to what happens with the engine's injection pump.
All of this testing reveals the condition of the injector and tests it after rebuilding it. Old ones might begin to get a bad pattern or poor atomization. Water in the system can cause corrosion and dribbling, or a broken tip, as water does not go through fuel injectors very well.
If a tip gets blown off it will send in a unatomized stream that will cause puffing black smoke, hard starting and can burn a hole in the piston. With excellent atomization, you probably won't see continuous black smoke until about 75% of the rated horsepower. At medium power levels you might get some light brown smoke showing good combustion.
