Testing Injector Flow With a Scope

December 10, 2009

By Peter Meier. Fuel flow and injector faults can result in drivability problems. Some OEM scan tools, as well as some high end aftermarket tools, allow you to perform power balance and/or injector balance tests on those OEs that support such a test. Injector balance can also be done using a tool that fires the injectors individually while you measure the drop with your fuel pressure tester, but this can be a time consuming task. There is another way to check this flow, and it uses a DSO and a special pressure transducer.

I’ve used the SenX FirstLook transducer for many tests involving low pressure or vacuum pulses, and have shared those tests in the past. Today, I’d like to share another product from SenX…the ES300 Fuel Pressure transducer.

This tool connects to the fuel rail at the supplied Schrader valve. It also comes with an adaptor that allows it to be connected to vehicles without a Schrader, using the quick disconnect coupling common to many professional fuel injection pressure testers. With the ES300 connected, the next step is to bleed any air out of the sensor much like you would your fuel pressure gauge, and then connect the sensor to the scope and running the engine. (The tool will work with any DSO, and comes with both BNC and banana fittings).

Here’s what a raw pattern might look like. This one is taken from a 2003 Dodge Dakota with a 4.7 liter V8 with over 90,000 miles on the clock and no drivability complaints.injector flow 1

This pattern is created by the changes in pressure in the fuel rail. The theory is if all injectors are functioning normally and flowing correctly, the pattern should be repetitive and uniform. On this capture, the blue trace is taken from the #1 injector, and the firing order is 1-8-4-3-6-5-7-2. Using that information, I saved the screen capture and used Microsoft “Paint” to mark of the individual injector firing times. I also color coded the events to show the cylinders on bank one (cylinders 1-3-5-7) and bank two (cylinders 2-4-6-8). The connection point for the transducer is directly to the fuel rail Schrader valve, located on the right bank fuel rail between cylinders 4 and 6.

Injector flow 2

The ES300 creates a positive voltage during a DROP in pressure, so the peaks you see here are just that…the pressure drop across each injector as it fires. In this capture, the peaks are all reaching about the same level, indicating the injector flow rate is pretty even. However, notice the small variances in each wave. Some have two peaks, some don’t. The ones with the distinct “double peaks” are on the same side of the engine, and this may be caused by the proximity of the sensor…it’s closer, so the signals are dampened less than those on the left bank. It may also indicate that the injectors are dirty. I haven’t had a chance yet to capture before and after patterns.

What happens when we remove an injector?
Injector flow 3

Can you tell which one I unplugged? Remember, positive peaks indicate a drop in pressure.

That’s right…it’s #5. Notice that there is no drop in pressure on this cylinder. Because the injector did not open, fuel rail pressure rose slightly higher than normal and caused a higher pressure drop when the next injector in line fired, then the pattern returns a more normal level. If diagnosing a misfire, I would instantly know where to narrow my focus. Just keep in mind, that low flow or no flow can be caused by both a mechanical fault and an electrical one. Ideally, I could monitor injector current, voltage and pressure change…all on one screen. You can also test the injectors under different conditions when looking for a fuel flow issue, comparing idle to a power braking 1500 rpm pattern.