In "Engine mechanical testing: Good, better and best," June 2018, I shared two fundamental engine mechanical tests performed with a scope: cranking relative compression and cranking intake vacuum. So today, let's take it one step further.
Please keep in mind that the goal of all of these engine mechanical tests is to determine not only whether or not there is a mechanical problem present but to also determine what the problem is without engine teardown for visual inspection which is very time consuming and may yield a situation where the car owner may bail out on the repair but now the vehicle can no longer be operated unless the engine is reassembled. I’m sure many of us have been down this road and it is one I like to avoid.
First let me categorize the types of problems that may be present before testing begins and the problems that will be uncovered with these tests. Modern engines can suffer from any of the following problems; cylinder sealing issues causing compression loss, incorrect valve timing from failed cam drive systems, incorrect ignition timing, shifted or out of synch engine rotation sensor signals, intake path restrictions, exhaust path restrictions and bank to bank breathing issues - all of which can be traced down using these tests. While I have documentation on all of these issues and I could fill a small textbook with all this information, it is beyond the scope of this article to discuss each problem in detail. It is my intention to whet your appetite so you’ll invest in the tools needed to perform these tests and begin using them in your diagnostic routine. There are many excellent training classes on this subject and further research will be required to become proficient with this testing, but we must start somewhere so let’s get started now.
Start at the beginning
To begin, you must have a scope and pressure transducer along with an assortment of compression test hoses for the different spark plugs used in the many engines found in the marketplace. The first item to be aware of is there cannot be a Schrader valve in the test hose when using a transducer! If you have a gauge-style compression test kit with a hose assortment, you can use these hoses but be sure to remove the Schrader valve when using a transducer. The Schrader valve is the reason a mechanical gauge builds pressure and shows compression puffs or pulses, but this is not how pressure builds in the cylinder and you will see that the pressure pulses shown on a scope are all the same on a normal engine during cranking.
The New Way to Affect Change In Your Shop | |
 | Give us one minute of your time, and you'll learn three key ways shopping for parts online can change processes in your business. |
 | |
We will not pay too much attention to the first or last pulse in a transducer generated compression waveform because you don’t know where the piston was when the engine began to crank or stops turning. If cranking pressure peaks are varying during the test we have already uncovered a problem and one that a conventional gauge will never be able to show us! The sequence of in-cylinder pressure testing should be a cranking test first, then a running test with a snap throttle event to allow analysis of the various problems mentioned above. You must prevent the engine from starting during your cranking tests, preferably by removing fuel. Scoping the ignition event will also be needed during certain tests.
 |
| Figure 1 — A four-cylinder Honda engine set up to perform in-cylinder pressure transducer testing. A Pico WPS500 transducer is connected to cylinder #4. Many various transducers are available from different companies but this transducer does not require power supplied by the scope so it can be used with any scope and will produce very good waveforms. |
Two cranking pressure waveforms will be shown to illustrate a normal waveform and a problem vehicle. The first waveform shows normal compression pressure from the 4-cyl Honda engine shown in Figure 1. Notice the first compression event is low due to the piston being somewhere above bottom dead center when the engine began to crank but all of the subsequent compression events are at the same pressure of 175psi (Figure 2). This engine had a leaking exhaust valve in cylinder #2 and when tested, the cranking compression was only 149psi - below the 15 percent variance that would be considered a max variance on an OBDII vehicle with misfire detection.
 |
| Figure 2 — Normal cranking compression waveform from a 4-cyl Honda engine. Remember to disregard the first and last pulse. The Pico scope can scale to pressure when you select a pressure transducer but keep in mind the pressure transducer outputs a voltage and this transducer would output 1 volt per 100psi so another scope would display 1.75 volts. The range is 0-500 psi for this transducer |
The next waveform shows several good compression pulses and then a complete loss of cylinder compression (Figure 3). This 5.3 V8 GM truck engine had a cylinder #3 misfire. A compression test was performed with a gauge and the tech said the compression was the same on all cylinders on that bank. This is the same truck discussed in my June article and the problem was a broken valve spring. This problem would not escape the cranking compression test done with a transducer! If the engine is cranked over for at least 10 seconds and the compression peaks are varying more than a few psi then valve sealing issues are likely present.
 |
| Figure 3 — This cranking compression waveform shows the first two pulses at 165psi, the third pulse at 75psi and all remaining pulses at 4psi. A broken valve spring is the culprit. Because a gauge style compression tester uses a hose with a Schrader valve which traps pressure, the gauge showed about the same value as the other cylinders. |
Timing is everything
The next item I’d like to mention is timing, both ignition and valve timing. Many technicians today do not have a quick and reliable way to verify timing on most engines. Ignition timing marks are mostly a thing of the past and few techs have a working timing light. If a car comes in as a cranking no-start or lack of power, how does one eliminate a timing problem as the issue and do it quickly? The answer is to compare the ignition event to an in-cylinder pressure event of the same cylinder using two scope channels and a pressure transducer (Figure 4).
I recently looked at a 2004 Toyota Highlander 3.3 V6 that another shop was struggling with diagnosing a lack of power complaint. When test driven, the vehicle seemed like it was starting off in third gear but the transmission had no codes and was shifting through the gears. Wondering about a possible torque converter problem, I decided to make sure the timing was correct on the engine and found the problem. The timing was severely retarded due to a grooved-out crankshaft timing belt sprocket which also has the crank sensor trigger wheel cast into the sprocket. This technique has uncovered many similar problems and is a simple test to verify a very important relationship that is all too often taken for granted because timing is no longer adjustable.
 |
| Figure 4 — This ignition timing relationship waveform was captured on a 3.3 V6 Toyota during a mild brake torque. The scope rotational rulers are used to measure the ignition signal and show the ignition pulse in blue occurring 14 ATDC. This means the timing is retarded about 22 degrees because the scan tool timing PID was reading 8 degrees BTDC. |
Some readers may think this is advanced diagnostic testing and would only be used in rare cases but nothing could be further from the truth. A nearby shop towed over a 2010 VW CC they were stuck on. Considerable time had been spent trying to diagnose a cranking no-start on this 2.0, GDI turbo engine with no answers. After verifying a few basics, a cranking in-cylinder waveform was captured and analyzed. The waveform quickly pointed out what I will call an exhaust path restriction (Figure 5). The scope rulers are showing the 4-stroke cycle and there is a high-pressure pulse at the point where the exhaust stroke is ending and the intake stroke is beginning that measures 117 PSI (Figure 6). This is caused by the exhaust cam being advanced nearly 90 degrees due to a jumped timing chain. I will point out that older engines that jumped time usually resulted in retarded camshafts but today’s engine with their complex camshaft drive mechanisms can have cams that jump and end up advanced or retarded. Exhaust cams that are out of time have very little effect on cylinder compression. When an intake cam is out of time there will be a large effect on compression, retarded cams will lower compression and advanced cams will raise compression. When checking any V-style or opposed engine with two banks, if one bank has different compression than the other you should immediately suspect camshaft timing. Advanced intake cams are why an engine can have too much compression, if the measured compression is above specifications suspect an advanced intake cam possibly from incorrect installation, a jumped chain or frozen cam phaser assembly.
 |
| Figure 5 — This cranking compression test from a 2.0 VW engine looks odd with what looks like a double compression event. The waveform will be zoomed in for a closer look. |
 |
| Figure 6 — This is the classic waveform for what should be called an exhaust path restriction. Pressure on the exhaust stroke should be the same as atmospheric pressure which would be the zero point on the waveform scale. This was caused by an advanced exhaust cam and it prevented the engine from starting. |
If this exhaust pressure pulse was seen on only one cylinder expect a worn-down cam lobe or plugged exhaust manifold runner. A clogged catalytic converter could produce a similar waveform but expect the pressure rise to begin earlier due to the transducer seeing the pressure in the manifold as soon as the exhaust valve opens. I will mention here that some late-model DOHC engines that phase both camshafts may have no valve overlap when the cams are not phased and can produce a pressure pulse at the end of the exhaust stroke and this can be normal. It should be clear that an in-cylinder pressure test is an excellent way to measure exhaust backpressure! It is almost always easier to remove a spark plug than to take out an oxygen sensor to measure exhaust backpressure.
On To Running Compression
Everything to this point has been cranking tests but once we start the engine there is quite a bit more information presented. Let’s take a look at a running compression waveform and point out what it is showing us. We will look at the basic 4-stroke cycle (Figure 7) starting with the power stroke and continuing from there. Remember that we are viewing the change in cylinder pressure as the piston is pulled up and down in the cylinder by the crankshaft, there is no combustion in the cylinder so we call the first event the expansion stroke, not a power stroke. There are two basic reasons the pressure changes in the displayed waveform, the piston changes direction or a valve event occurred (valve opens or closes). Once you understand the basic 4-stroke cycle and what the waveform represents some pretty powerful diagnostics can be generated.
 |
| Figure 7: Normal running compression waveform with callouts. The blue arrows represent the direction of piston travel through the 4-stroke cycle. Valve overlap occurs in the area of the 360-degree ruler on the screen. |
The next waveform was captured on a 2010 Dodge Challenger with a misfire code complaint and a slight tick from the engine. The scan tool would show some misfire counts on cylinder #3 and sometimes there were none. Relative compression and cranking vacuum tests looked normal. There was very little fuel trim correction so it did not appear to be injector related. It was decided to do an in-cylinder test after noticing an abnormality in the running vacuum waveform. It is always a good idea to capture a waveform from a known good cylinder to compare to a problem cylinder. After testing cylinder #1 (Figure 8) the transducer was moved to the problem cylinder.
 |
| Figure 8 — Running compression waveform from cylinder #1 on a 2010 Dodge Challenger 5.7 Hemi engine. This is a normal pattern from a good cylinder comparing in-cylinder compression to running vacuum from a Sen-X Technologies First Look transducer. |
The cylinder #1 waveform shows a pressure rise in the vacuum waveform that is just in front of the compression peak, this would be the vacuum event for cylinder #3. When the transducer is connected to cylinder #3 the problem is obvious (Figure 9). The waveform shows the vacuum pressure rise lines up with where the intake valve opens. The in-cylinder waveform shows a 23psi pressure pulse at the end of the exhaust stroke which is not normal. When the intake valve opens, this trapped pressure is released into the intake manifold and can be seen with the intake pressure transducer. The waveform cursors are measuring the difference between exhaust valve opening and closing at 168 degrees, way below what normal duration would be on a cam lobe in a Hemi! This engine has a worn-out exhaust cam lobe on cylinder #3 and needs a cam and lifters. You can see the damage from the picture (Figure 10) of the camshaft after removal.
 |
| Figure 9 — Running compression and vacuum waveform from the misfiring cylinder # 3 on the 2010 Dodge Challenger. Note the exhaust pressure pulse and vertical rise in the vacuum waveform and their relationship to each other as highlighted. |
 |
| Figure 10 — The worn exhaust cam lobe is the second from the left in this picture. |
Another Hemi Misfire Resolved
The next problem vehicle is a 2011 Dodge Ram pick-up with another 5.7 Hemi engine. This truck idles fine but misfires under hard acceleration. Normal parts replacement has failed to rectify the problem, spark plug, ignition coil and fuel injector were all tried to no avail. The truck is driven and only cylinder #1 misfires under load. A relative compression test shows cylinder #1 has slightly higher compression (Figure 11).
 |
| Figure 11 — Relative compression test from the 2011 Dodge Ram 5.7 Hemi engine. Cylinder #1 has the highest peak. Compression is not the problem, right? |
After seeing a higher compression event for cylinder #1 and knowing this is the problem cylinder a running compression test is performed. The result is quite revealing. A good cylinder (Figure 12) will be compared to the problem cylinder.
 |
| Figure 12 — Running compression test from cylinder #3 with a throttle snap is shown. This is a known good cylinder |
 |
| Figure 13 — This zoomed capture of a cranking compression test from cylinder #3 is measuring the point of intake valve closing which occurs on the compression stroke. |
The good cylinder shows a compression peak during a snap throttle application reaching over 330 PSI and the intake valve closing at 45 degrees after bottom dead center (Figure 13). Comparing the same test from cylinder #1 shows peak compression during a snap at only 287 PSI (Figure 14) and the intake valve closing at 20 degrees after bottom dead center (Figure 15).
 |
| Figure 14 — Running compression test from the problem cylinder showing lower peak compression during a throttle snap. |
 |
| Figure 15 — Zoomed cranking compression waveform showing early intake valve closing point. |
The early intake valve closing and reduced peak cylinder pressure during a snap throttle illustrate a worn intake camshaft lobe causing an intake path restriction. Cylinder pressure is a function of airflow and the effective compression ratio which is determined by the point at which the intake valve closes so that pressure can be built. When an intake valve closes earlier than normal, the effective compression ratio increases and will cause higher compression values. It must be clear at this point that engine mechanical problems cannot escape detection when using the tests performed in this article. Once these tests are mastered your accuracy in determining engine mechanical problems will be spot on without requiring engine disassembly.