# Question: What should a vacuum gauge read at idle?

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Idle vacuum for most engines is about 18 to 22 in. -Hg, but some may produce only 15 to 17 inches at idle. (Remember what we said about experience.) If vacuum is steady and within these ranges, the engine and fuel and ignition systems are operating normally.

## How will a vacuum leak affect the gauge reading at idle?

With the engine at idle, the vacuum gauge pointer will drop sharply every time the leak occurs. If the leak is between two cylinders, the drop will be much greater. You can determine the location of the leak by doing a compression test. Remember, engine problems can affect transmission performance.

## What is vacuum at idle?

What is Idle Vacuum? Idle Vacuum is the intake manifold vacuum when the engine is idling. The higher the vacuum, the more closed the throttle plates and the lower the amount of air and fuel required to keep the engine running an no load. Idle Vacuum is is a measure of how well the engine can idle.

## Why is vacuum high at idle?

If vacuum is steady at idle but lower than normal, the ignition or valve timing may be retarded. Higher-than-normal vacuum at idle is a common clue to overly advanced ignition timing, while low vacuum can indicate retarded timing. Low vacuum also can be an immediate clue to a plugged exhaust.

## What gauge measures vacuum and pressure?

A vacuum gauge is a pressure gauge used to measure pressures lower than the ambient atmospheric pressure, which is set as the zero point, in negative values (e.g.: −15 psig or −760 mmHg equals total vacuum).

## What gauge measures high vacuum?

For high vacuum, from 10-3 Torr to 10-6 Torr, either cold cathode or Bayard-Alpert hot cathode gauges are used. Both require frequent calibration. For ultra-high vacuum, below 10-6 Torr, cold cathode or Bayard-Alpert hot cathode gauges are also used.

## What should intake manifold pressure be at idle?

The vacuum inside an engines intake manifold, by comparison, can range from zero up to 22 inches Hg or more depending on operating conditions. Vacuum at idle is always high and typically ranges from 16 to 20 inches Hg in most vehicles.

## What should vacuum be at idle PSI?

about 18 to 22 in.- Idle vacuum for most engines is about 18 to 22 in. -Hg, but some may produce only 15 to 17 inches at idle. (Remember what we said about experience.) If vacuum is steady and within these ranges, the engine and fuel and ignition systems are operating normally.

## What is a good vacuum pressure?

At atmospheric pressure, the value 0 in. -Hg is equivalent to 14.7 psia. At the opposite reference point, 0 psia, — a perfect vacuum (if it could be attained) — would have a value equal to the other extreme of its range, 29.92 in.

## How does a high vacuum gauge work?

This vacuum gauge contains a hermetically sealed, evacuated, thin-walled diaphragm capsule which is located within the instrument. As the vacuum pressure reduces, the capsule bulges. This movement is transferred via a system of levers to a pointer and can then be read off as the pressure on a linear scale.

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