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Continuation: Dust off that scope

Practical and everyday tests for fun and profit
Tuesday, December 1, 2015 - 08:00
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In this continuation to the article “Dust off that scope,” I continue to show you tests you can perform with your scan tool of choice that can help hone your skills and offer enhanced service for your customers. After reviewing tests you can perform on your scan tool to evaluate battery/starting/charging systems, starter current and fuel delivery, now we move on to pressure testing.  

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What About Pressure?
Ever deal with a leaking or restricted fuel injector? Some scan tools allow for injector balance testing and if that isn’t possible, there is always the old way of firing the injectors manually while recording the fuel pressure change. But that’s time consuming and we’re working on flat rate! Is there another way using my scope?

Scope users are creative in thinking up new ways to use their scopes. Early on, someone got the bright idea of tapping into the fuel pressure vacuum regulator using a pressure sensor to test injector pressure drop. The theory was that with each pressure drop, a pulse would be generated in the rail that would cause the vacuum regulator’s diaphragm to move, exhaling if you will through the vacuum nipple. And we could measure that!

Scopes can be used for troubleshooting a variety of systems. You're only limit is your own imagination.

It worked pretty well, but wasn’t as accurate as hoped. Position of the regulator in relation to the injectors was just one hurdle to overcome. Then came pressure transducers – devices that didn’t just react to pressure change, they could actually measure it! As with the low amp clamp, this is an investment that will pay off in time savings and open up all kinds of diagnostic possibilities for you.

In Figure 1, you’ll see I have three channels running at the same time. The red trace is the fuel pump current pattern we just discussed, the blue trace is a fuel injector voltage pattern (taken by placing the positive channel lead to the ground side of the injector and the negative channel lead to the battery ground post) and is being used as a reference for the third channel (the green trace).

Figure 1

This channel is attached to a pressure transducer, which in turn is connected to the same test port I would normally connect my manual fuel pressure test gauge. Now I have a true pressure reading to accompany my current pattern, making comparisons even easier. I’m reading an in spec pressure of 50 psi, and so much more. Notice how there is a drop every so often. If you count them, there are six drops in between each pair of injector events. That makes sense since this is a six cylinder engine. Those are the pressure drops across the injectors as each opens. It is very easy to see that they are all uniform, and that is a solid indication that none are restricted.

  Why does the pattern repeat every 8 peaks? Because there are 8 commutator segments on the pump motor.
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