In Front of the Flywheel - April - 2018

Pressure and Vacuum Diagnosis, Part I: The Tooling

In the October/November 2017 issue of GEARS, the column In Front of the Flywheel addressed an intermittent compression issue on a 2010 Ford F150. The article referred to vacuum and pressure transducer testing that would be covered in the future.

Well, we’re back to visit this very subject. Over the next few issues, we’ll cover the information you’ll need to apply using a transducer for engine mechanical diagnosis.

I usually follow a very structured path when performing engine mechanical testing. First, I’ll perform a relative compression test (detailed in Jan/Feb 2017’s Engine Mechanical: Go/No-Go) to decide if a mechanical issue might be present.

Second, if I suspect a mechanical issue, I’ll add a vacuum transducer to test engine-cranking vacuum.

Third, if necessary, I’ll perform in-cylinder compression testing to verify the fault. All three of these steps require some special tooling and that’s where we’ll start our series.


There are basically two types of pressure/vacuum transducers used for automotive diagnosis. The first is a piezo-electric device that allows you to look at small pressure changes very rapidly but doesn’t measure the actual pressure.

The second is a true pressure transducer, which will also allow you to view pressure changes very rapidly as well as provide an actual pressure or vacuum reading.

Pressure transducers can also handle much higher pressures than piezo-electric sensors. There are pros and cons to each tool, so we’ll look at them separately.


Piezo-electric crystals are manmade crystals that have electrical properties. They can be used for a variety of applications, such as knock sensors or fuel injectors.

When you apply a voltage to the crystal, it expands; when you reverse polarity it contracts. This is the property used in applications such as fuel injectors.

When movement or exposure to pressure changes flexes the crystal, it generates a voltage. The amount of voltage generated depends on the amount and the rate of change. This property can be used in impact sensors for air bag systems, and is the property you’ll use for pressure diagnostics.

Advantages of the piezo-electric tool are reasonable cost and the ability to view extremely small and fast pressure changes. Another advantage is, since the sensor generates its own voltage, the tool doesn’t require batteries.

A disadvantage of the tool, since it only generates a voltage based on change, is its inability to report an actual pressure or vacuum value.

The most common piezo-electric tool on the market is the FirstLook™ sensor manufactured by SenX (figure 1). It’s available from a variety of vendors for around $400. There are other manufacturers of similar devices and, with some research, you should be able to fabricate a piezo-electric sensor with items commonly available from many electronic parts suppliers.

Automotive diagnostic applications for a piezo-electric sensor vary. Actual pressure measurements, such as compression, fuel pressure, transmission line pressure, or any test associated with a gauge and specification, aren’t practical. But they do lend themselves well to some newer engine diagnostic techniques such as cranking vacuum testing, crankcase pressure fluctuation testing, and exhaust pressure pulse testing.

One of the most valuable uses for a piezo-electric sensor is a cranking vacuum test. Here’s how it works:

  • Connect the sensor to a manifold vacuum source.
  • Disable the fuel system by disconnecting the fuel pump or fuel injectors.
  • Crank the engine and observe the waveform from the piezo-electric sensor.

The waveform provides a good indicator of engine mechanical condition. You can observe each individual intake stroke as it generates vacuum. The intake strokes appear as a dip in the waveform. If all the cylinders are performing equally, each intake stroke should have an equal dip, or vacuum pull (figure 2).

If a valve is sticking closed, the waveform will be erratic (figure 3). Using some additional techniques that we’ll discuss in the next issue, you’ll be able to identify the valve and fault precisely.

Pressure Transducers

A pressure transducer “sees” the actual pressure and converts it to a voltage that you can display on your oscilloscope. You can use these transducers for just about any pressure measurement as long as the transducer is capable of handling the pressure range. Tests done with pressure transducers include compression, vacuum, fuel pressure, transmission pressures, oil pressure, and more.

The advantage of a true pressure transducer is their versatility; a disadvantage is their cost. Some transducers, like the Fluke PV350, are available for a reasonable $400. The current “Cadillac” of transducers is the pico Technology WPS500X, which goes for around $1500.

As you can see, prices range quite a bit. In this case, the adage “you get what you pay for” appears to be true. It’s something you’ll want to consider when shopping for a transducer.

An additional disadvantage of a pressure transducer, although it’s a small one, is that they require batteries, which always seem to fail right when you need the tool most. The less expensive transducers usually have replaceable batteries, while more expensive transducers, like the WPS500X, have internal rechargeable batteries.

When using a pressure transducer to perform cranking vacuum testing, the procedure and results are the same as the ones for the piezo-electric sensor. But pressure transducers allow you to add tests that the piezo-electric sensor won’t. One of these is compression testing. Yes, you can perform compression testing with a conventional gauge, but the transducer provides greater detail.

The increased amount of information allows you to observe details such as ignition timing, valve timing, and more. And a pressure transducer will help you identify some issues, especially intermittent faults, that an analog gauge won’t provide.

You perform cranking compression testing with a pressure transducer the same as you would using a conventional compression gauge:

  • Remove a spark plug and install the gauge or transducer into the cylinder head.
  • Disable the fuel system.
  • Crank the engine to obtain results.

One of the advantages of using a pressure transducer is that it allows you to see the pressure generated by each compression stroke individually. An analog gauge, on the other hand, only displays the peak compression pressure of all of the compression strokes that completed while you cranked the engine. This can make a difference while diagnosing an issue correctly or cause you to miss a potential fault.

Here’s an example from a Ford engine that has an intermittent misfire on one cylinder. The first capture is the compression in one of the functioning cylinders on the engine (figure 5). As you might expect, all of the compression stokes generate the same cranking pressure.

Next, we move on to a compression capture from the intermittently misfiring cylinder (figure 6). Notice that the compression pressures vary from stroke to stroke. This shouldn’t be occurring and is a very strong indicator that there’s a valve sealing issue in this cylinder.

An analog compression gauge would have missed this small detail and potentially resulted in a misdiagnosis. The gauge would have displayed the compression number for the best compression stroke, while the lower pressure strokes would have slipped under the radar.

Another advantage of a pressure transducer over a conventional gauge is the transducer can provide detailed information during a running compression test. To perform this test, re-enable the fuel system and start the engine. The result is a waveform that displays all of the strokes in the four-stroke cycle.

With some practice, you can use this information to find faults like restricted exhaust, incorrect valve timing, compression issues that aren’t present during cranking, improper valve lash adjustments, and even incorrect ignition timing.

To summarize, an additional tool or two added to your oscilloscope’s arsenal of adapters can open the door to these valuable and detailed testing techniques. In the next issue, we’ll cover engine cranking vacuum testing in detail, including how to interpret your test results.

Engine or electrical diagnostic issues you’d like to see addressed? Let Scott know. Send him an email at and you just may have your question covered in a future issue of GEARS Magazine.