Vacuum Test – A Vacuum Test, Can Tell You More Than You Think

Vacuum Gauge Test - Tells You A Lot About The Condition Of Your Engine
Vacuum Gauge Test - Tells You A Lot About The Condition Of Your Engine

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

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

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

So, a vacuum test can tell you a lot about, an engine’s condition. Also, it is similar to a cylinder leak down test. But, by doing a vacuum test, in just 3-5 minutes you can know if an engine, is healthy or not.

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

So, each has a characteristic effect on vacuum and 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.
Vacuum Testing Gauge
Vacuum Testing Gauge

Use the information in this article to determine the engine problem. Also, you may want to read it over first, but it is self explanatory.

Engine Speed


Indication of Engine Condition

Smooth and steady idle

(800 to 1200 RPM)

Between 17 to 21 inches Engine is in Good Condition, but perform next test to be sure.
Open and close throttle quickly Jumps from 2 to about 25 inches As a result, engine is in Good Condition.
Smooth and steady idle Steady, but lower than normal reading Worn rings, but perform next test to be sure.
Open and close throttle quickly Jumps from 0 to 22 inches Confirms worn rings.
Steady idle Intermittent dropping back 3 or 5 divisions and returns to normal Sticky Valves. So, if injection of penetrating oil into intake manifold temporarily stops pointer from dropping back; it’s certain the valves are sticking.

Steady 3000 RPM

Pointer fluctuates rapidly; faster engine speed causes more pointer swing Weak valve springs.
Steady idle Fast fluctuation between 14 to 19 points Worn intake valve stem guides. As a result, excessive pointer vibration at all speeds; indicates a leaky head gasket.
Same As Above Constant drop Burnt valve or insufficient tappet clearance holding valve partly open or a spark plug occasionally misfiring.
Steady 8 to 14 inches Incorrect valve timing. Also, vacuum leaks and/or poor compression; can result in a low vacuum reading.
Same As Above Steady 14 to 16 inches Incorrect ignition timing.
Same As Above Drifting from 14 to 16 inches Plug gaps too close or points not synchronized..
Drifting 5 to 19 inches Compression leak between cylinders.
Same As Above Steady below 5 inches Leaky manifold or carburetor gasket, or stuck manifold heat control valve.
Same As Above Floats slowly between 12 and 16 inches Carburetor out of adjustment.
Blipping engine speed Quick drop to zero then return to normal reading Muffler is clear.
Blipping engine speed Slow drop of pointer then slow return to normal reading Muffler is choked or blocked.

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

Leaking Vacuum Hose
Leaking Vacuum Hose

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. Consequently, the computer gets an erroneous low reading. 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.

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.

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

  • Engine misfires
  • Fluctuating idle

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.  Also, 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. When you connect a gauge to a port on the intake; you’re measuring manifold vacuum.

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 gauge can help you find the source of your, engine mechanical problems. The vacuum gauge still remains a reliable tool for many shops, that know how to take advantage of it.

Thank You !