Fuel injectors inject fuel. Duh. They are basically very fast-operating valves controlled by your engine’s electronic control unit (ECU). While the injector is open, fuel (pressurized by the fuel pump) flows through it. The longer the ECU holds the injector open (pulse width), the more fuel is injected. Simple! But as always, the devil is in the details...
The ECU has no way of knowing how much fuel is actually being injected by each injector. It only tells ALL the injectors how long to stay open and relies on each injector to do the same, correct thing: inject the correct amount of fuel to achieve complete and clean combustion. The ECU does watch the oxygen sensor to make sure that the average air/fuel mixture is in the ballpark and can make adjustments if it is not. But, it is actually very difficult to tell whether each injector is spraying the right amount of fuel.
The first problem is that the O2 sensing system isn't perfect or foolproof. There are times when the ECU can't count on oxygen (O2) sensor system feedback to correct the mixture, most notably at startup (while the O2 sensor is still warming up) or at full throttle (when you actually want a bit of extra fuel to keep your pistons cool and make more power). These conditions are called “open loop” because the ECU has no feedback data from the O2 sensor to make corrections and “close the loop.” And even when the ECU is getting data from the O2 sensor (running in “closed loop”), it can't react quickly enough to totally correct the fueling. For example, if you're running rich under one particular running condition and lean in another—like when you jump from idle to heavy throttle—the ECU relies on the injectors to behave as they should. This is why properly operating injectors are critically important in keeping an engine running at peak efficiency. The O2 sensor system by itself cannot be relied upon to make this happen.
You, as an astute, mechanically-savvy reader, probably already know the second problem here: there is only one oxygen sensor, but there are four injectors. This is why one single O2 sensor can only give your ECU information on the average performance of all four injectors. For example, if one injector is not injecting enough fuel (too lean), the ECU cannot simply correct that one injector. All it can do is command all four injectors to inject more fuel to make up for the one that isn’t injecting enough. This is why it is so important that all four injectors flow the exact same amount of fuel at any operating condition.
We’ve spent countless hours testing injectors by both measuring individual flow rates on the bench and using a wide-band oxygen sensor during live testing to monitor air/fuel ratio (AFR) on the fly. Apart from the injectors we’ve tested that just plain didn’t work—either the tips leaked, they failed early or often, etc.—we found many that didn’t deliver the necessary flow rates compared to each other (or at all). Some flowed too lean in one condition, too rich in another, or wrong all the time. Even some of the most promising injectors were a letdown, because they were not consistent from one batch to the next.
Long ago, the best available injectors were the original equipment (OE) Bosch-type. Considering the technology available when they were designed, they worked amazingly well. However, compared to modern injectors, their spray pattern wasn’t great, and their flow tolerance (how one injector flows compared to the next) was very wide—but they worked. When new Bosch injectors dried up, we started offering high-quality rebuilt injectors that were just as good. But over time, the used injector pool got worse and worse to the point we were just not getting enough rebuildable injectors back to rebuild.
At that point, we had our own new injectors made as a direct reproduction of the OE injector with a tighter flow tolerance. These were excellent, but eventually the manufacturer simply could not continue to reproduce the same antiquated Bosch design. That is when we decided it was time for these parts to jump into the 21st century.
Our brand-new injectors offer the same performance you’d find in any modern, non-direct fuel injected engine. The manufacturer produces them in batches of 100+, tests each one individually at different pulse widths, then groups them in sets of four such that the flow rates are practically identical from one injector to the next at every pulse width. The actual flow report is provided for each set! This test is much more thorough than common static flow testing, which only measures how much fuel flows through the injector when it’s left wide open.
So, yeah, all a fuel injector does is inject fuel. Seems simple enough at first, but now you know it certainly isn’t!