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Introduction to In-cylinder Pressure Testing

This exciting diagnostic method opens new possibilities in drivability troubleshooting.
Wednesday, May 29, 2013 - 06:21

Every decade or so, a new automotive technology is discovered that is truly game changing. The use of pressure transducers in automotive service bays is one of the most exciting discoveries of the 21st century. This innovative technology saves repair shops time and money on a grand scale.

This technology can be used to check engines, transmissions, power steering, brake systems, EVAP systems and air conditioning systems. Basically any system that uses pressure can be analyzed by the use of a pressure transducer. These transducers measure physical pressure changes, negative or positive, and convert these changes to an electrical output signal. Pressure transducers need a power source and ground source, and they will produce a voltage signal that is proportional to the physical quantity applied. An oscilloscope is used to display and analyze the signal output produced from the pressure transducer by graphing the pressure changes over time thereby identifying changes that occur within the system. 

Figure 1 – Here, the in-cylinder pressure pattern is overlaid on a cam spec sheet to illustrate how the pattern changes through the 720° combustion cycle.

Pressure transducers allow the technician to see the inner working of the internal combustion engine without disassembly. In order to check the spark ignition internal combustion engine, three pressure transducers are used: one in the cylinder, one in the intake and one in the exhaust. To place the one in the cylinder, remove the spark plug from the cylinder head (be sure to ground the spark), then install a compression test hose with the one way check valve removed and place a 300 psi transducer on the compression hose. The -30 Hg vacuum transducer on the intake manifold will be centrally located on the vacuum port close to the throttle body.  Place the exhaust 25 inch/H20 transducer hose in the end of the tailpipe. With these transducers in place, the engine will be operated in three different modes: crank no start, idle, snap throttle and each of these engine operating conditions will produce different pressure waveforms on the oscilloscope and will use different techniques to diagnose them with.

The engine under these three operating conditions can be checked for camshaft to crankshaft timing problems, variable camshaft timing problems, intake and exhaust valve sealing problems, both consistent or intermittent, valve spring problems, piston ring sealing problems, worn camshaft lobes, restricted exhaust problems, ignition timing problems or cylinder misfire identification. As you can see, this list includes some of the toughest problem to diagnose. These difficult diagnoses will become routine in your service bay with just a little understanding of the pressure changes that occur within the engine.  

Let us start by analyzing the idle compression waveform as seen in Figures 1 and 2.  Figure 1 is a camshaft chart with the compression waveform overlaid on the cam card. Figure 2 is a basic compression waveform produced at closed throttle at low rpm. The large pink lines divide the compression waveform into 180-degree divisions of crankshaft rotation or the strokes (intake, compression, power, exhaust) of the engine, and the small pink lines divide the crankshaft rotation into 30-degree divisions as seen in Figure 2. The large pink line in the middle of Figure 2 represents when the piston is at 360 degrees of crankshaft rotation at Top Dead Center (TDC). The intake valve opens just before this point. The crankshaft is in rotation so the piston is in motion, the piston moving away from the cylinder head increasing the volume within the cylinder. This in turn creates a low-pressure area contained within the cylinder, which pulls a negative pressure (vacuum) against the closed throttle plate. This reduction in pressure can be seen from G, which is atmospheric pressure to I, which is negative pressure.

This drop in pressure should start at the TDC point and fall rapidly to I and this pressure change should occur before the two small pink markers after TDC or 60 degree after TDC. I indicates the lowest pressure obtained during the intake stroke, whereas J indicates the average pressure during the intake stroke. The intake duration is from G to K, note that K occurs after the intake stroke ends at the Bottom Dead Center (BDC) mark. The intake pressure stays low after the BDC mark occurs even through the piston is in an upward movement.

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