How to do a 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 use a vacuum gauge.

A vacuum gauge can tell you a lot about an engine’s condition, similar to a cylinder leak down test . In just 3-5 minutes you can know if an engine is healthy or not.

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: Brake Booster, PCV tube, EGR Switch, A/C vents, and on and on. You simply need to find one small enough for the vacuum gauge line to slide onto firmly.

A vacuum test shows the difference between outside atmospheric pressure and the amount of vacuum present in the intake manifold. Piston rings, Valves, Ignition system and the Fuel control system all affect how much vacuum is created.

Other parts affecting the combustion process (emission devices, etc.).
Each has a characteristic effect on A and you judge their performance by watching variations from normal. It is important to judge engine performance by the general location and action of the needle on a vacuum gauge, rather than just by a vacuum reading.

CHART OF VACUUM TEST READINGS

Engine Speed Reading 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 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. 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. Excessive pointer vibration at all speeds indicates a leaky head gasket.
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. It must also be remembered that vacuum leaks and/or poor compression can result in a low vacuum reading.
Steady 14 to 16 inches Incorrect ignition timing.
Drifting from 14 to 16 inches Plug gaps too close or points not synchronized..
Drifting 5 to 19 inches Compression leak between cylinders.
Steady below 5 inches Leaky manifold or carburetor gasket, or stuck manifold heat control valve.
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.
Slow drop of pointer then slow return to normal reading Muffler is choked or blocked.

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

  • Degrade engine performance.
  • Compromise driveability.
  • Turn on your Check Engine light.
vacuum test
vacuum test

Here’s why:

A vacuum leak is downstream of the device that measures the incoming air—the mass airflow (MAF) sensor—which means the engine actually ingests more air than is measured and 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 that stoichiometrically correct 14.7:1.)

What about 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—but that will force the other cylinders to be too rich, which may cause a whole menu of issues—misfires, unstable idle, etc.—that might set a trouble code and turn on the Check Engine light

And with some vehicles using dozens of vacuum hoses, and the potential for cracks in multipart intake manifold systems, 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. If extra air slips past the throttle body without being accounted for by the computer, you’re running lean.

Snapping the throttle and getting a drop to 0 and then a climb to only say 20-22 will possibly indicate worn rings.

  • Basically, if it only snaps to about where it was or just a hair above, you may be dealing with worn piston rings. It should snap temporarily higher (3-4 inches or so) and then settle back.
  • A sticky valve can be spotted by seeing a vacuum reading that occasionally drops from normal by about 4 inches of mercury.
  • A burnt valve will behave the same as a sticky valve, but will drop much more, say 6 inches of mercury (and then return back to normal, in a cycle).
  • Worn valve guides can be detected by a vacuum reading that vibrates back and forth between 14 and 19 or so very quickly.
  • Weak valve springs if you suspect weak valve springs, rev the engine to say 4000 rpm and see if the needle swings back and forth rapidly. Raise the RPM to 5000 or even 6000 rpm, if your valve springs are weak, you’ll see the swinging get wider as revs increase.
  • A low reading of say 5 inches of mercury would be a vacuum leak
  • Extremely advanced ignition timing or incorrect camshaft timing would cause a very low reading.

A vacuum test can tell you more than you think.

Head gasket issues can be found by seeing a floating needle between 5 and 19 inches of mercury. Other causes are a intake manifold gasket leak or a faulty injector. You can check the oil for coolant if you see this behavior however (and vice versa) to confirm your diagnosis.
Really strange behavior such as a vacuum indication that shows a slow movement between various readings could mean a leak in the intake manifold, bad idle air/fuel ratio, or perhaps a clogged PCV system.

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