RELYING ON RELATIVE COMPRESSION TESTING TO ASSESS ENGINE ISSUES:

This test uses the digital storage oscilloscope (DSO) and a high amp clamp to measure starter amperage draw at a time base fast enough to see the impact each cylinder has on that draw.

1. Synch the pattern to a given cylinder and use the firing order of the engine to identify weak cylinder(s). A weak cylinder takes less effort to spin through than a strong one does, and that means it also takes less current, and that is displayed on the scope’s screen.

2. Modern pressure transducers allow relative compression tests to be done via the exhaust pressure pulses leaving the tail pipe. Cylinders that are healthy will pass equal pressure pulses through the exhaust that the sensor can detect. A weak cylinder will show up as a weaker peak in the pattern.

3. The pattern can be synched to help identify the bad cylinder(s). Here, though, the pulse displayed will be offset from the originating cylinder. When viewing these patterns, look for anomalies rather than specifics. Some exhaust designs cause one bank to have a longer route to follow than the other, and this can reduce the amplitude of the pattern on all the cylinders for that bank.

NOTE: If there is a weakness, the pressure transducer can determine if the fault is in the valve train or cylinder.

1. Be sure to disable the fuel and ignition systems to avoid both over fueling the cylinders and accidental engine start.

2. Remove the oil dipstick and put a rubber hose over the tube, connecting the sensor to the tube.

3. Crank over the engine and watch the pattern. A pressure increase will be present each time a piston heads for bottom dead center.

If the rings are sealing properly, the peaks will be uniform and the loss of compression you found has to be caused by the top end.

SECONDARY IGNITION PATTERNS TO SLOVE ENGINE MYSTERIES:

This takes some skill, but you can master it.

1. Look at the average of the firing lines (firing voltage demand) for all the cylinders.

It’s normal for the firing line of an individual cylinder to vary as conditions vary in the combustion process. Any cylinder that has a consistently lower firing line is suspect.

2. Watch the spark line closely. No compression in a cylinder will not only have a lower firing demand, but the spark (or burn) line will be flat and longer than the others.

NOTE: When a valve doesn’t seal, the pressures built up in the cylinder have a path to escape through. It shows up in the pattern as a series of peaks in what should otherwise be a relatively smooth slope. Some turbulence at higher engine speeds is normal, though, so don’t confuse the two.

By following the three tests outlined above, you should be able to gain a better understanding of the problems the engine may be experiencing. With luck, you should be one step closer to providing your customer with the auto repair information they need.

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