In this first of two articles on engine mechanical testing, we will introduce two fundamental engine mechanical tests that will be performed with electronic tools. This first test is the relative compression test performed with a scope and high amp current probe. The second is the cranking vacuum test performed with a scope and pressure transducer. In the next article, we will look into the engine with in-cylinder pressure transducer testing.
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Let’s look into the fundamental tests that have traditionally been performed to determine engine mechanical condition to see how times are changing. For decades, technicians have used vacuum gauges to measure intake manifold vacuum to help determine engine-sealing issues. If there was a cylinder power contribution problem suspected, the compression gauge and cylinder leakage gauge were used to determine the root cause of failure. The shortcomings of analog vacuum gauges are their inability to pinpoint a cylinder-specific problem. A vacuum gauge measures the average vacuum inside the intake manifold and some gauges are so damped they may mask a valve sealing problem. A scope and pressure transducer will display individual cylinder vacuum “pulls” or vacuum events so more detailed analysis is possible. As engines have evolved, more elaborate intake manifold designs have been incorporated on V-style engines, which can make access to spark plugs difficult and thereby render gauge-style cylinder compression testing a last resort. There must be an easier way to determine base engine mechanical health. The good news is there is a simple yet powerful mechanical test — the relative compression test. The relative compression test is not new; big-box engine analyzers performed cranking and running tests and used input from a current clamp around the battery cable to ferret out base engine mechanical problems. These large analyzers are all but gone in most shops, and many younger technicians have never seen one used at all. Many shops today have high-quality digital storage oscilloscopes available and by adding a few inexpensive probes, the scope will do a great job at displaying relative compression tests.
Inside relative compression
Let’s start with some basic theory and build on how this test will expand your diagnostic capabilities. A relative compression test relies on the fact that as a starter motor turns over a normal engine, as each cylinder is pushed to top dead center on the compression stroke, the force required to compress the trapped air increases and hence the current required by the starter motor to overcome this force increases as well. An old-style analog battery/starting/charging system tester would show the ammeter needle wavering during a cranking test so we know the current is varying in the circuit. A scope and good high-amp current probe is so fast that this changing current can be displayed on the scope. I will mention here that if the current to the starter motor is oscillating, the voltage in the circuit changes as well. It is possible to do a simple voltage test at a battery-positive voltage point and ground with a scope and see this voltage oscillation during cranking, but the current probe will give a clearer picture and will be used as the test tool of choice for this article. When you perform this test, it may be done either synced or un-synced, meaning you can add a second scope channel to an ignition trigger signal so that individual cylinders can be identified. If the engine has spark plug wires, you may need a sync probe for your scope to trigger from a specific cylinder. Sync probes are inexpensive and readily available. Let’s look at a normal relative compression test and point out the basics of how to use the test for engine diagnostics.
|Figure 1 - Relative compression test from a 2012 Chevrolet Suburban, 5.3 V8 engine. The engine was cranked over for 9 seconds.|
The first waveform (Figure 1) shows a good relative compression waveform captured from a Chevy Suburban with the raw scope settings I used during the test. The scope is set to a slow time-base and the entire test is captured. A Pico scope has very good zoom capabilities so the capture will be manipulated to show much greater detail for the analysis. If you use a scope such as a Snap-on product, you will want to perform this test at a faster time-base and then use the zoom out function to look at the entire test. The Pico scope was set to 2 seconds per division, but a Snap-on scope setting for this test may be 50 milliseconds per division and then let the scope buffer fill up. This capture is a synced relative compression test because the ignition-firing event was captured. If you only use a current probe or do a battery voltage test without the ignition trigger, you can see a problem, but you would not be able to identify a particular cylinder so you may as well just get used to connecting two leads and do synced tests. The next waveform is the same capture but with heavy manipulation of the stored patterns for better viewing.
|Figure 2 - Known good synced relative compression waveform. The current waveform in red shows a peak to valley amplitude of 31 amps, which is okay.|
In figure 2, the original capture has been zoomed in, filtered and annotated to show the engine firing order. There is a lot of data on the screen to be analyzed. Most techs are generally looking for a low current event and not much else, but there are many items to discuss about these captures. The first item to discuss is the current waveform in red. The saw tooth pattern is what we should see and each peak represents a piston reaching TDC as the starter current increases to push the piston to TDC and past, then the current drops quickly until the next cylinder in the firing order comes up on TDC. This is a “relative” compression test so it only displays each cylinder relative to the other cylinders in the engine — it is not an actual compression value. If all the cylinders have too much or too little compression, the overall test may look normal. If the engine had a valve timing problem causing high or low compression, it would affect all the cylinders unless the engine were an OHC V-style and the valve timing was off on only one side. In that case, you could have three or four high peaks and three or four lower peaks (as seen in figure 3) depending on the number of cylinders in the engine, and the test would point you in the direction of valve timing right away. Normal relative compression waveforms typically show a peak to valley current between about 30 to 70 amps. If the current is too low, suspect an engine with low overall compression and do a cylinder compression test to verify. If a chain jumped on one side of a Ford 4.6 or 5.4 V8, think of how quickly this test would lead you to verify camshaft timing.
|Figure 3 - This un-synced relative compression waveform was captured on a 1998 Ford Explorer with a 4.0 VIN E engine. The timing chain had jumped on bank 2, causing low compression on that side of the engine.|