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

Vacuum Test
Vacuum Test

A vacuum test can tell you more than you think.

One of the easiest and cheapest ways to check an engine for serious issues is to do a vacuum test.

A vacuum test can tell you a lot about an engine’s condition, similar to a cylinder leak down test.

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 present in the intake manifold.

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

Manufacturer’s install ports on their manifolds for lots of different reasons.

You simply need to find one small enough for the vacuum gauge line to slide onto firmly.

There are many factors that can affect the amount of vacuum created. Some of these are, Piston rings, Valves, Ignition system, Fuel control system and Emission Controls.

So, Each has a characteristic effect on vacuum and you judge their performance by watching variations from normal. Also, It is important to judge engine performance by the general location and action of the needle on a vacuum gauge. 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 Test Gauge
Vacuum Test Gauge

Use the information in this article to determine the engine problem. So, You may want to read it over first as it is self explanatory.

Engine SpeedReadingIndication of Engine Condition
Smooth and steady idle

(800 to 1200 RPM)

Between 17 to 21 inchesEngine is in Good Condition, but perform next test to be sure.
Open and close throttle quicklyJumps from 2 to about 25 inchesEngine is in Good Condition.
Smooth and steady idleSteady, but lower than normal readingWorn rings, but perform next test to be sure.
Open and close throttle quicklyJumps from 0 to 22 inchesConfirms worn rings.
Steady idleIntermittent dropping back 3 or 5 divisions and returns to normalSticky Valves. 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 idleFast fluctuation between 14 to 19 pointsWorn intake valve stem guides. Excessive pointer vibration at all speeds indicates a leaky head gasket.
Same As AboveConstant dropBurnt valve or insufficient tappet clearance holding valve partly open or a spark plug occasionally misfiring.
Steady 8 to 14 inchesIncorrect valve timing. Also, Vacuum leaks and/or poor compression can result in a low vacuum reading.
Same As AboveSteady 14 to 16 inchesIncorrect ignition timing.
Same As AboveDrifting from 14 to 16 inchesPlug gaps too close or points not synchronized..
Drifting 5 to 19 inchesCompression leak between cylinders.
Same As AboveSteady below 5 inchesLeaky manifold or carburetor gasket, or stuck manifold heat control valve.
Same As AboveFloats slowly between 12 and 16 inchesCarburetor out of adjustment.
Blipping engine speedQuick drop to zero then return to normal readingMuffler is clear.
Blipping engine speedSlow drop of pointer then slow return to normal readingMuffler is choked or blocked.

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

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. That raises the normal 14.7:1 air-to-gasoline ratio, causing the engine to run leaner than it would in normal operation. 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

Well, it will quickly detect the extra air, and in response the computer will richen up the mixture. 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.

Leaking Hose
Leaking Hose

Today, With all the vacuum hoses running everywhere, there are plenty of places for leaks to crop up.

The ducting that runs between the throttle body and the MAF sensor, often 3-inch-diameter rubber, can also degrade.

A leak in this duct isn’t technically a vacuum leak, it’s a metered air leak.

Finally, If extra air slips past the throttle body without being accounted for by the computer, you’re running lean.

More About Engine Vacuum Test

Remember that engine vacuum is just air pressure lower than atmospheric pressure. The starting point to evaluate engine vacuum is 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

Vacuum drawn from an opening ahead of the throttle is called ported vacuum. 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.

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

Ported vacuum is used to control vehicle systems in relation to engine load. 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.

Vacuum Test – Conclusion

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.