In Front of the Flywheel - May - 2018

Pressure and Vacuum Diagnosis, Part II: Cranking Vacuum

Last month’s article covered the tooling and some basic theories regarding mechanical testing using electronic means. This month we’ll dig a bit deeper into cranking vacuum analysis.

When diagnosing a misfire, these techniques allow you to evaluate, in great detail, what could be causing low compression, without disassembling the engine. Often you can make these diagnostic decisions very quickly, which translates to saved time, increased profit, and a high level of customer confidence. Let’s quickly review:

A customer has a vehicle with a misfire. Initial scan tool checks obtain DTCs, identify a suspect cylinder, and gather any other pertinent information. At some point, preferably early in the diagnosis, the technician performs a relative compression test (GEARS Magazine Jan/Feb 2017).

If the vehicle has a mechanical issue, we should get a poor relative compression scope capture. Next, we connect a vacuum transducer and acquire another cranking capture (GEARS Magazine April 2018) to analyze the mechanical issue in greater detail.

Mechanical Analysis

The beauty of this technique lies in your ability to quickly connect a scope, capture a waveform, and find the exact problem without disassembling the engine.

Take this General Motors 6.0-liter engine for example: The customer’s complaint was rough running accompanied by a flashing MIL. The technician connects a Tech 2, the General Motors OE scan tool for this vehicle, and retrieves a misfire code: Cylinder number three is misfiring.

The next step is to determine if the misfire is fuel, ignition, or compression-related. After spending a moment observing some scan data on the Tech 2, the technician decides to perform a relative compression test to determine whether he needs to perform further mechanical testing.

The relative compression waveform in blue shows a single cylinder that has low or no compression (figure 1). The green trace is cranking vacuum waveform obtained using a piezoelectric sensor attached to the intake manifold at the PCV valve hose.

An obvious mechanical failure is also evident in the vacuum waveform. The last waveform in red is an ignition sync I connected to the spark plug wire for cylinder number one. This lets us determine which cylinder is which.

Figure 2 is the same waveform as figure 1, but we’ve zoomed in and adjusted the scope to make analysis easier. I’ve aligned the rulers in the PicoScope software with the ignition firing events and divided into 8 equal sections, or one for each cylinder.

I also added the firing order for the engine (1-8-7-2-6-5-4-3) to the relative compression waveform (blue), making it easier to identify each cylinder’s compression stroke.

Cylinder number 3 is missing the current rise that would normally be present during a good compression stroke. The lack of compression also aligns with the cylinder that the Tech 2 identified as misfiring. So mechanical/ compression related misfire it is. But where to next?

Next, we’ll analyze the vacuum waveform to try to determine why there’s no compression. Manifold vacuum drops when each intake stroke occurs. These pressure drops, or vacuum pulls as they are sometimes called, occur at a different time in the 720º engine cycle.

The easiest way to identify which pull belongs to which cylinder is to move halfway through the capture and label the space immediately after the halfway point with the cylinder used to sync. In this case we used cylinder one, so I added a green number 1 to the waveform.

From there, I labeled cylinders 8-7-2 and then wrapped back to the beginning of the capture to complete the 6-5-4-3 all in green. Cylinder 3, our misfiring cylinder, appears to have a dip in vacuum, which indicates it should be pulling air into the cylinder on its intake stroke. But there’s a large increase in pressure and a large drop in manifold vacuum that shows up elsewhere in the capture.

To help identify what could actually be happening, some form of visual aid will help you visualize what’s happening in the engine at each given moment. One possibility is some form of engine relationship chart.

One such option is available at (figure 3). Visual aids such as these allow you to determine where all the pistons are located in the engine at any given time, the direction that they’re traveling, and which valves are open or closed. This information can be extremely valuable when attempting to diagnose without disassembly.

Analyzing the Waveform

I zoomed in on the waveform a little more (figure 4). Now, using visual aids, we can compare vacuum changes to engine events. In this case, I’ve drawn a yellow arrow that indicates the direction of piston travel in cylinder 3 when it’s on its compression stroke.

The lack of compression in the blue waveform is obvious, but what about the vacuum change in the intake manifold? Should a compression stroke have any effect on intake manifold vacuum? Could an intake valve not sealing or closing in cylinder number 3 cause this issue? Sure.

Pressure in the intake manifold would increase as the piston pushes air back into the intake manifold instead of the intake valve holding the compression pressure in the cylinder. The next step is to explain the large increase in manifold vacuum.

The purple arrow indicates which way the piston in cylinder 3 is traveling during the large pressure decrease in the intake manifold. It just so happens to be its power stroke. If you examine the chart, or you use the intake pulls identified in figure 2, this should be the intake pull for cylinder number 2.

But why is the vacuum drop so drastic compared to the rest? Let us revisit our theory that the intake valve in cylinder 3 may not be sealing. If an intake valve were to stay open on a power stroke, wouldn’t that effectively change it into an intake stroke? And if so, wouldn’t we have two intake strokes happening at the same time?

That’s exactly what’s occurring here. Cylinder 3’s compression is pushing past the intake valve and increasing manifold pressure. Next, cylinder 3’s power stroke becomes an unwanted intake stroke causing a large increase in manifold vacuum. Time to call the customer and get authorization to remove a valve cover and possibly a cylinder head.


A relative compression capture, cranking vacuum capture, and an ignition sync all captured on one scope screen can reveal extremely useful diagnostic information.

At first, analyzing the captures will be tough. There’s definitely a learning curve involved when attempting to master these techniques. With that in mind, these tests are a perfect example of a learning tool as well as a diagnostic tool.

What I mean to say is, if you perform this test you should know right away that there is, or isn’t, a mechanical issue. What you may not be able to determine is the specific fault. The capture you obtain from the next broken vehicle probably won’t look like the one we just analyzed, so you’ll have to start from scratch.

If you can’t make sense from the waveform, that’s okay. Save the image and move on with other means of diagnosis or engine disassembly. Then, once you’ve identified the engine fault, revisit the waveform and teach yourself why the vacuum behaved the way it did.

After a while, patterns will start to emerge and specific failures will start to have their own signature. The more you practice the technique the better you get.

Next month we’ll dive even deeper and investigate some issues with a pressure transducer installed in a cylinder. In some cases, these techniques may be your next mechanical diagnostic step.

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 an issue of GEARS Magazine.