Lesson 3/Learning Event 2
Start the engine. When it has run long enough to reach operating temperature, look at the vacuum reading.
Normal vacuum readings will vary between different engine models, so be sure to check the proper technical
manual for the exact normal readings. Specifications given in the technical manual are for sea-level operations.
At higher elevations, the gage readings are lowered about 1 inch of mercury for every 1,000 feet of increase in
altitude. For example, Denver, Colorado, is about 5,000 feet above sea level. So the vacuum reading obtained
on a vehicle at Denver will be about 5 inches less than a reading obtained on the same vehicle at sea level.
The normal reading for most engines is about 20 inches of mercury with the engine at idle speed. A steady
reading in this range indicates that the engine is running as it should at this speed.
As a further check for normal engine operation, open and close the throttle quickly. If the engine is running as
it should, the reading will drop to about 2 inches at wide open throttle. Then it will quickly go to about 25
inches when you close the throttle and the engine is slowing down. (The black needle on the gage (in the
illustration) indicates the normal reading and the white needle indicates the needle travel.)