Diagnose engine internal mechanical issues with vacuum test
Engine vacuum test is a very effective way to identify internal mechanical issues or at least points you to the right direction. OBD dash pro has an integrated vacuum test facility for vehicles that equips with a MAP sensor.
To access the vacuum test page:
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Open diagnostics screen
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Select Vacuum test from the popup. (feature only visible if ECU reports MAP data to the app)
The vacuum test screen consists of engine rpm gauge, vacuum gauge, and the diagnostics chart. Please note that the numbers used in the diagnostics chart are calibrated to sea level. If the test is done at sea level to 2000 ft (600m), the numbers on the chart can be used directly. However if the test is done on the higher altitude/elevation, you should subtract the value by 1 inch per every additional 1000 ft.
Running vacuum tests
The basic idea of vacuum test is to put the engine in various conditions, then compare the vacuum gauge needles movement to the predefined diagnostic chart
At idle, it is expected a stable vacuum reading around 18~21 inch mercury. When snap open and close the throttle, the vacuum should quickly drops, then bounce back, then drops slowly back to the initial readings. If the engine rpm is steadily increase and maintain at 2000, the vacuum reading should increase to above normal reading.