How To Diagnose Common Engine Problems with a Vacuum Gauge

Automotive Vacuum Gauge Engine Tests

One of the easiest and cheapest ways to check an engine for serious issues is to perform a vacuum gauge test. You can determine whether or not an engine is healthy in just several minutes.

Automotive Vacuum Gauge
Automotive Vacuum Gauge

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, etc. 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 system all affect how much vacuum is created, as well as other parts that may affect the combustion process (emissions devices, etc.). 

Each has a characteristic effect on the running of the engine and you judge performance / problems 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 by a single reading.

Vacuum Gauge Readings

White needle indicates Steady hand – Outline needle indicates fluctuating needle

Vacuum Gauge Readings

How To Connect and Use a Vacuum Gauge

Vacuum Gauge Test Reading Chart

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.

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A vacuum gauge test will help pinpoint the source of automotive mechanical problems at a low cost and in a short time. The results of a well performed vacuum gauge test will guide you to specific systems and components for further testing, if needed.

Today, the vacuum gauge has been replaced as one of the primary tools in a shop by newer technology — but don’t underestimate its value and effectiveness. The vacuum gauge still remains a reliable tool for many shops and wise Do-It-Yourselfers that know how to take advantage of it…

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