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.
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!
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).
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.
What are the squiggles on every other drop, you ask? Those are the injectors closest to the test port and what you’re seeing is the ringing in the fuel rail caused by the pressure changes. If these squiggles were on drops that were significantly different than those without the squiggles, I’d be more suspicious but as it is, I’ll ignore them.
This test took no more time than conventional testing would. In fact, it took less time since I didn’t have to go through the effort of priming the rail and firing each injector, then doing the math. With just a few connections of my already up and running scope, I performed a baseline test that gives me the direction I need to reach a quick diagnosis.
A Look Inside – Without Going Inside
Pressure sensors opened up a whole new arena of diagnostic possibilities. One early use of pressure sensors (most are piezo-electric designs that create a voltage output in reaction to the pulses generated by pressure change) was another relative compression test that used exhaust pulses exiting the tailpipe. These same sensors were also used to monitor changes in engine vacuum. Together, they can be used to identify valve train and other mechanical issues and there are even software programs designed to help with misfire diagnosis based, in part, on the inputs from these sensors.
But these sensors couldn’t actually measure the pressure, only react to it. Pressure transducers are devices that can measure pressure and today, they are being used to measure all kinds of pressures. I shared the fuel pressure example, and many technicians are monitoring transmission line pressures to help with the diagnosis of today’s complex automatics. But the single (in my opinion) biggest breakthrough in diagnostics was the use of pressure transducers in place of a cylinder’s spark plug – a total revisit of the old school “running compression” test.
To perform this test with your scope, you’ll need a pressure transducer capable of measuring peak cylinder pressures of 200 psi or so. The transducer I use is capable of measuring vacuum and pressure, both fluid and gas, so the one tool covers all the bases for me. There are other aftermarket offerings, so check around to see what will match up with your scope the best. Remove on spark plug and install the transducer in its place. For best diagnostic results, you’ll want to perform a cranking test, a running test and a snap throttle test.
I’m not going to get into a lot of detail here. We are fortunate at Motor Age to have the guru of running cylinder pressure testing, Automotive Test Solution’s Bernie Thompson, as a contributor and he has authored several article for us on this very topic. His latest was just published in our September issue. But here are some of the basics and I think you’ll agree with me that this single test can provide you with so much useful information you’ll wonder why you haven’t started using this method! Reference my example of a running pressure test (Figure 2), if you please. (Note: The vertical colored divisions are references to the crankshaft position in 180° intervals.)
A – This point in the pattern is Top Dead Center (TDC) on the compression stroke. The vertical line added as an annotation splits the peak into what should be (for most engines) a symmetrical pattern. If the peak appears to slope to the right, odds are there is a compression loss in the cylinder.
B – This is the point of Bottom Dead Center (BDC) on the power stroke. The curved area just before is referred to as the “exhaust pocket” and you’ll notice that the annotation (in red) bisects the upward slope of the pocket near the center. This is an indication of exhaust cam timing. The intersection will normally fall in the area indicated by “G”. If the intersection is off to either side, suspect a cam timing issue. You can also use this pattern to test and monitor cam phasing on engines equipped with variable timing on the exhaust cam.
C – The light blue annotation here represents TDC on the exhaust stroke. The plateau between points B and C is representative of the pressure in the exhaust system as it enters the now open combustion chamber via the exhaust valve. It can be used to identify exhaust restrictions during a snap throttle test. Since the intake valve and exhaust valve are both open at TDC, the ramp you saw at “B” is slightly offset. The window annotated “H” is the area where typically the center of the intake ramp will occur. As with “G”, it can be used to identify cam timing issues caused by a skipped belt or by a VVT system that isn’t functioning correctly.
D – The light green vertical line is BDC of the intake stroke. The measured value here is absolute manifold pressure, and should read very close to the Manifold Absolute Pressure sensor data (if equipped). On a normal engine, this area of the pattern should be close to even with the base of the exhaust pocket as shown by the horizontal line “F”. If the exhaust pocket extends below this line, there is a loss of compression in the tested cylinder.
E – And that brings us back to the starting point, TDC compression.
This testing method is as close to viewing the internal workings of a running engine as we can get, and the more you understand about the factors that influence this pattern the faster you’ll be able to isolate and diagnose a variety of engine mechanical conditions like sticking valves, retarded or advanced cam timing, clogged exhausts, and much more.
I hope these few examples of the diagnostic uses of the DSO will inspire you to blow the dust off of that unit you have in your shop, and if you don’t have one I hope you’ll make the investment in a basic scope and dive in. The DSO, with a little training and experience, can be a powerful tool in the hands of a creative user. And who knows? You may be the one that develops the next revolutionary testing technique to the benefit of us all!
