Reaching for that compression gauge? Grab your scope instead!
The Pressure Peak
This pattern represents two complete running revolutions of the crankshaft with all the cylinder events you are familiar with captured. Unlike a mechanical gauge, the pressure transducer/scope combination charts the pressure changes in the cylinder from the compression stroke, through the power stroke, exhaust stroke and, finally, the intake stroke where we start all over again. Point A is TDC, short for Top Dead Center, of the compression stroke. Running compression pressure measures about 86 psi — more than enough to support combustion. Running pressures of less than 40 to 45 psi will cause a misfire due to lack of compression.
On the way to B, you'll see a nice, curved pocket at the base of the downward slope. This is the expansion pocket, and its pressure is the result of the air quantity remaining above the piston as compared to what it started with. At this point, the cylinder still should be sealed and contain the same quantity of air it did at the end of the intake stroke. Line F compares the two. A significantly lower expansion pocket is one indication of poor ring seal and is more apparent on a cranking test because of the overall higher pressures present. It is my thought that poor ring sealing will allow air to pass by into the crankcase under compression. There is, then, less air remaining above the piston as it moves downward. If the valves are sealed, the result is a lower pressure area when the piston nears the bottom of its travel.
At The Bottom
The exhaust valve begins to open just before B, or Bottom Dead Center (BDC) of the power stroke. The result is the upward ramp you see as the pressure in the cylinder begins to equalize with the pressure in the exhaust manifold. Generally, the center of the ramp will be near the BDC mark.
The red box, G, represents a general range of -10 to +15 degrees of BDC that is considered acceptable.
This is an indication of cam timing, and anything outside of this box bears a closer look. Yes, I know, I can compare the cam/crank sensor signals synch or visually verify the timing marks line up. Ever see a crank key shear? You can also use this relationship when testing variable valve timing designs. Simply activate the solenoid and watch the ramp to see if it moves as it should.
The exhaust plateau is between B and C. This pressure will vary with load. It might be close to what you have been taught is acceptable exhaust back pressure at idle, but will rise significantly with throttle opening. However, it can be used when testing for a restricted exhaust on those engines using dual cats. Measure snap throttle pressures on each bank and compare the two. A major difference between the results is one clue that the exhaust is plugged on the bank with the higher reading.
The Intake Stroke
Just before C, or TDC on the exhaust stroke, the intake valve opens. Remember, though, that the exhaust valve is still open as well. This is the point of valve overlap and can make positively identifying when the intake opens difficult. However, about 20 degrees after TDC, the exhaust is no longer open and the resulting downward ramp is the effect of the intake beginning to fill the cylinder all by itself.
As with the exhaust ramp, this is an indication of cam timing and the center of the ramp should occur between 10 and 30 degrees after this TDC point, shown by the red box H. If the engine is a single cam design, the shift in cam timing should be close to equal on both the exhaust and intake ramps if a belt has jumped a tooth or two.
That brings us to D, or BDC of the intake stroke. The cylinder is now filled, and the relative pressure here should be about the same as the lowest point of the expansion pocket we talked about earlier. From here, the piston travels upward on the compression stroke, E, and the pattern repeats itself if all is well.
Have you ever dealt with an intermittent miss caused by sticking or heavily carboned valves? Try using this method to identify this problem by setting your scope's time base way up and focus on just the pressure peaks. Most modern scopes are fast enough to catch the lower compression that will naturally result.
A Few Examples
How about a 2006 Dodge Caravan with a definite miss? A relative compression test like the one seen in Figure 3 was performed using starter current draw, and the low peak was the first indication that one cylinder wasn't pumping up as it should.Back to the good pattern. Compression pressure in that cylinder was a little high, and in hindsight, provided additional clues as the cause of this failure. The tech I was assisting got customer authorization to remove the head and found the pistons heavily carboned.
Another tech also performed a relative compression test with similar results. His cranking compression pattern, though, revealed something even more unique. After finding normal compression readings in the cylinder he thought was weak, he decided to move to the cylinder corresponding to the highest peak on his relative compression test.
See anything unusual in the waveform that technician took (Figure 6)? Instead of an exhaust plateau, he saw this exhaust peak, which was equivalent to a second compression event!The exhaust valve wasn't opening, and every time this piston reached TDC of its exhaust stroke, its mate in this four cylinder engine was reaching TDC of its compression stroke, making it that much harder for the starter to turn the engine over and causing the high peak on his earlier test.
Of course, the bad cylinder wasn't going to run with all the remains of the last combustion event still in the cylinder.
Last is a 2003 Jeep 4.0 with a misfire only when cold. Using the transducer and a high time base, this tech captured the culprit (Figure 7). The orange line represents minimum pressure needed to support combustion and it isn't long after start up to see it disappear. The cause? A hydraulic lifter that went solid, holding the exhaust valve open.Using a pressure transducer and a scope to test engine compression doesn't eliminate the need for your traditional gauge and leakdown tester. It is another diagnostic tool at your disposal when the need arrives, and can find problems conventional testing methods can't — at least not as easily, and certainly not as fast. Easy and fast! Both make life more pleasant — and productive — in the bays.