Vacuum Test Basics – What To Check And How To Do It

So, even a tiny leak, as small as 0.020 of an inch can:

Leaking Hoses
Leaking Vacuum Hoses

So, by doing a vacuum test, in just 3-5 minutes, you can know if an engine, is healthy or not. A vacuum gauge shows the difference between, outside atmospheric pressure and the amount of vacuum, in the intake manifold.

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Vacuum Test Basics - What To Check And How To Do It
Vacuum Test Basics – What To Check And How To Do It

First, doing a vacuum test, can tell you more about your engine, than you think.

Because, it is one of the easiest and least expensive ways, to check an engine for serious issues.

So, engine vacuum, is defined as, any pressure lower than atmospheric pressure, that is produced in each cylinder, during the intake stroke.

Well, it will quickly detect the extra air and in response, the computer will richen up the mixture. Because, it’s a self-correcting closed-loop system. Unfortunately, the leak may cause, the nearest cylinder, to run leaner than the others. The (ECM) will then richen up the overall mix. All, in an attempt to bring the excess oxygen in the exhaust, back to the appropriate low level.

Vacuum can also vary, in different areas of the engine:

  • Above the throttle valve
  • Below the throttle valve
  • The intake ports
  • The exhaust ports

Consequently, that will force the other cylinders to be too rich, which may cause a whole list of issues:

Remember that engine vacuum, is just air pressure, lower than atmospheric pressure. So, the starting point to evaluate engine vacuum, is at the intake manifold.

So, there are many factors that can affect, the amount of vacuum created:

  • Piston rings
  • Valves
  • Ignition system
  • Fuel control system
  • Emission Controls

Because, each has a characteristic effect on vacuum, you judge their performance, by watching variations from normal. Furthermore, it is important to judge engine performance, by the general location and action of the needle, on a vacuum gauge. Rather, not by just a vacuum reading.

How To Do The Vacuum Test:

  1. Connect the vacuum gauge hose, as close to the intake manifold as possible and start the engine.
  2. Run the engine long enough, to reach normal operating temperature.
  3. Note: the location and action, of the vacuum gauge needle.
Doing The Test
Doing A Vacuum Test

To clarify, vacuum drawn from an opening, ahead of the throttle is called, ported vacuum. So, throttle opening affects ported vacuum, opposite to the way, it affects manifold vacuum. For example, at closed throttle, manifold vacuum is at its peak. But, there is no significant vacuum, at a port ahead of the throttle plate, when the throttle is closed. Vacuum appears at such a port, only when the throttle opens.

Consequently, many vehicle systems need, a steady supply of low-pressure air, under all engine operating conditions. And, these systems include, power brake boosters, a/c vacuum motors and some emissions controls.

Therefore, when you connect a gauge to a port on the intake, you’re measuring manifold vacuum. That’s why, a vacuum test, has many benefits.

Consequently, manufacturer’s install ports on their manifolds, for lots of different reasons. So, you simply need to find one, small enough for the vacuum gauge line, to slide onto firmly.

So, ported vacuum, is used to control vehicle systems, connected to, engine load. As a result, these include, out of date distributor vacuum advance diaphragms and carburetor assist devices. They also include, many emissions control devices and transmission shift points. So, under some engine load conditions, ported vacuum may equal manifold vacuum, but, it can never exceed it.


So, a vacuum leak is downstream of the device, that measures the incoming air, the mass airflow (MAF) sensor. So, this means, the engine actually ingests more air, than is getting measured. As a result, the computer gets, a wrong low reading.

Vacuum Test Results
Vacuum Test Results

As a result, a leak in this duct, isn’t technically a vacuum leak, it’s a metered air leak.  So, if extra air slips past the throttle body, without being accounted for by the computer, you’re running lean.

More About, Engine Vacuum Testing

Furthermore, this might set a trouble code and turn on, the Check Engine light.

So, that raises the normal 14.7:1 air/fuel ratio, causing the engine to run leaner than, normal operation. Finally, the engine ­computer ­dithers the mixture ratio, back and forth several times per second in the area of 14.7:1. That’s why, a vacuum test, has many benefits.

How Does This Affect, The Exhaust Mounted Oxygen Sensor (O2)

So, a vacuum test, can help you find the source, of your engine mechanical problems. That’s why, the vacuum gauge still remains, a good tool for many shops.

So, today, with all the vacuum hoses running everywhere, there are plenty of places, for leaks to crop up.  Furthermore, the ducting that runs between the throttle body and the (MAF) sensor, often 3-inch-diameter rubber, can also degrade.

Check Engine Light On
Check Engine Light On

Firstly, to check manifold pressure with a vacuum gauge, you need to locate, a vacuum port. Usually, in the manifold or throttle body.

Finding Vacuum Port
Finding Vacuum Port