Vacuum testing

When the engine turns the movement of the pistons combined with the opening and closing of the valves work as a pump. It pumps air from the intake and pushes it out the exhaust. When the throttle is closed a vacuum or a low pressure area is created in the intake manifold because the engine pumps the air from the intake manifold and the throttle butterfly prevents air entering from the atmosphere. Vacuum can be defined as a area that has less pressure than its surrounding atmosphere.

Experiments


We connected the vacuum gauge to the intake manifold between the map sensor and the manifold. When the engine idled the vacuum was at 16in. Hg (inches of mercury). When the engine speed increased the vacuum went down to about 6 in. Hg. We then disconnected one spark plug wire (HT lead) and saw that the vacuum gauge had a small fluctuation. Then were removed one spark plug which made the gauge fluctuate a lot between 10 and 15 in.Hg.

Reflection


The vacuum gauge can be used to check that the intake manifold has the correct vacuum. It can also indicate if there are leaks in the manifold, if there is a cylinder that is misfiring or if there is a mechanical fault (air leak past the valves not sealing properly, damaged piston/rings. Can use a compression tester to find the fault) within one of the cylinders (removing one spark plug could simulate this). If the vacuum starts to get less after about 15 seconds there may be a obstruction in the exhaust.