Vacuum Test – A vacuum test, can tell you, way more than you think.

Using A Vacuum Gauge, To Test The Condition Of Your Engine
Using A Vacuum Gauge, To Test The Condition Of Your Engine

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

Above all, one of the easiest and cheapest ways; to check an engine for serious issues, is to do a vacuum test.

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

A vacuum test, can tell you a lot about, an engine’s condition. In addition, by doing a vacuum test; in just 3-5 minutes, you can know if an engine, is healthy or not.

So, a vacuum gauge shows the difference between; outside atmospheric pressure and the amount of vacuum, present in the intake manifold. Firstly, to check manifold pressure with a vacuum gauge; you need to locate, a vacuum port in the manifold or throttle body.

Vacuum Port
Vacuum Port

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, 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.

Doing 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.
Testing Gauge
Testing Gauge

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

Leaking Hoses
Leaking Hoses

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, an erroneous low reading.

Vacuum Readings
Vacuum Readings

As a result, that raises the normal 14.7:1 air-to-gasoline ratio; causing the engine to run leaner than it would, in normal operation. Finally, the engine ­computer ­dithers the mixture ratio; back and forth several times per second in the vicinity of 14.7:1. That’s why, a vacuum test, has many benefits.

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

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 indeed richen up the overall mix; in an attempt to bring the excess oxygen in the exhaust, back to the appropriate low level.

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

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

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
Check Engine Light

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

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. 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.

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

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

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. Furthermore, these systems include, power brake boosters, a/c vacuum motors and some emissions controls.

So, ported vacuum, is used to control vehicle systems, in relation to, engine load. As a result, these include; old-fashioned 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 test, can help you find the source, of your engine mechanical problems. That’s why, the vacuum gauge still remains; a reliable tool for many shops, that know how to take advantage of it.

Thank You !