Oscilloscope Waveform Library: Description

The figures in the tables below are reference waveforms collected from known-good vehicles and faulted vehicles. Each figure is accompanied by a description. 

Normal	Abnormal
Courtesy of GENERAL MOTORS COMPANY 8 Cylinder Engine 4 Cylinder Engine 6 Cylinder Engine	Courtesy of GENERAL MOTORS COMPANY 3 Cylinder Engine - Low Compression Low Compression - The waveform repeats every 3 compression strokes, indicating a current and repeatable compression fault. 8 Cylinder Engine - High Starter Load Exhaust Valve = Stuck Closed - The waveform repeats every 8 compression strokes, indicating a current and repeatable compression fault. When one cylinder displays double the relative compression, this is an indication of a stuck closed exhaust valve on the companion cylinder.
NOTE: An engine with little to no compression variance between cylinders will have waveform valleys that are even. If the engine compression is equally low across all cylinders, the oscilloscope pattern will appear good. The total amplitude (range from highest to lowest point of the waveform) is NOT an indicator of good compression. Operating Conditions:  Fuel System - Disabled {Automatic Transmission} Apply & Hold - Brake Pedal {Manual Transmission} Apply & Hold - Clutch Pedal Engine - Cranking - For 5 - 10 s

Normal	Abnormal
Courtesy of GENERAL MOTORS COMPANY 8 Cylinder Engine 6 Cylinder Engine 4 Cylinder Engine 3 Cylinder Engine	Courtesy of GENERAL MOTORS COMPANY 3 Cylinder Engine - Low Compression Low Compression - The waveform repeats every 3 compression strokes, indicating a current and repeatable compression fault. 8 Cylinder Engine - High Starter Load Exhaust Valve = Stuck Closed - The waveform repeats every 8 compression strokes, indicating a current and repeatable compression fault. When one cylinder displays double the relative compression, this is an indication of a stuck closed exhaust valve on the companion cylinder.
NOTE: An engine with little to no compression variance between cylinders will have waveform valleys that are even. If the engine compression is equally low across all cylinders, the oscilloscope pattern will appear good. The total amplitude (range from highest to lowest point of the waveform) is NOT an indicator of good compression. Operating Conditions:  Fully Applied - Accelerator Pedal @ Wide Open Throttle {Automatic Transmission} Apply & Hold - Brake Pedal {Manual Transmission} Apply & Hold - Clutch Pedal Engine - Cranking - For 5 - 10 s