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Use a Vacuum Gauge to Troubleshoot Your Car's Mechanical Problems

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How to Use a Vacuum Gauge

A vacuum gauge helps you diagnose many car mechanical problems without using expensive tools or taking apart the engine to analyze each component. For example, a vacuum gauge will track down worn out piston rings and cylinder walls, a damaged head or manifold or throttle body gasket, a carburetor in need of adjustment, a restricted exhaust system (e.g., a clogged catalytic converter), and many other mechanical components causing engine performance problems.

Within the last few years, vacuum gauges have slowly been replaced by a number of specialized and expensive tools. Yet, this simple and inexpensive tool can still save you a huge amount of time and money when troubleshooting potential engine mechanical problems. And many professional car technicians still use it.

But how a simple vacuum gauge can reveal so much in just a few minutes? It all has to do with the way vacuum works in your engine — and how automotive engines are designed, of course.

A vacuum gauge can help you diagnose a blown head gasket.

A vacuum gauge can help you diagnose a blown head gasket.

How Engine Vacuum Works

If we think of a vacuum as the absence of air, this is what happens (more or less) inside your car engine when running. As pistons go up and down during their intake and exhaust strokes in their cycle (drawing of the air-fuel mixture into the cylinder and expelling of exhaust gases), a relative vacuum environment is created inside the intake manifold. This is because there's always a piston at its intake point in its cycle, whether you're operating a 4, 6, or 8 cylinders engine.

Automotive technology uses the difference in atmospheric pressure (air weight) between the outside and inside of a car's intake manifold to measure engine vacuum. Although atmospheric pressure has been established at 14.7 psia (pounds per square inch absolute) at sea level, a common vacuum gauge will use this level as a starting point (for 0 vacuum — negative atmospheric pressure) and use inches of Mercury (in-Hg) as a unit of measurement (some gauges include measurements in kilopascals kPa and kilograms per square centimeter kg/cm2 as well).

A common internal combustion engine uses this vacuum to meter the amount of fuel needed during engine operation, and many engine devices may use an intake vacuum to operate too like the EGR valve, brake booster, purge control valve and many other emission control devices — although the tendency is to delegate control to the car's computer for better engine efficiency. Therefore, upsetting vacuum control can translate into one or more engine performance problems.

A vacuum gauge can detect a clogged muffler or catalytic converter too.

A vacuum gauge can detect a clogged muffler or catalytic converter too.

Mechanical and System Problems Affecting Vacuum

To operate properly, an internal combustion engine needs a relatively sealed environment to create and control a vacuum. However, as your engine accumulates miles or sees little or no adequate service, vacuum issues begin to develop. These problems may originate in the cylinder head or manifold gaskets, intake or exhaust valves, cylinders, pistons or piston rings, ignition timing, fuel injectors, vacuum hoses, and many other components and systems.

Vacuum problems symptoms may include high oil consumption, exhaust smoke, poor fuel economy, hard-to-start issues, rough idle, engine stalling and many others.

Using a vacuum gauge to diagnose any of these symptoms can save you time — and money — when you have difficulty locating the problem source.

Some Problems You Can Diagnose Using a Vacuum Gauge

Sticking valves

Ignition problems — late ignition, misfire

Hard starting

Clogged PVC system


Worn out piston rings

Worn valve guides

Rough idling

Leaking vacuum hoses

Weak or dead cylinders

Leaking intake manifold gasket


Leaking valves

Blown head gasket

Poor fuel economy

Clogged catalytic converter

Clogged muffler

Preparing for a Vacuum Gauge Reading Test

Before you begin your diagnostic, you should know that even a trained car technician may misread a gauge during a test. So you need to pay close attention to the behavior of the dial as you conduct the test. However, the vacuum gauge remains an excellent troubleshooting tool when dealing with mechanical problems for a reason. Car technicians still take advantage of this tool and, when necessary, use it along with other specialized tools to help confirm a diagnostic.

So this strategy will help you focus on specific areas of your engine with potential problems.

  1. Open the hood of your vehicle and find an accessible vacuum hose connected to the intake manifold of your engine. You'll use this hose to plug in your engine vacuum gauge.
  2. Start the engine and let it idle for about 15 or 20 minutes to bring it up to operating temperature. Or run an errand but take your car for a 20-minute drive on the highway. Then, back in your garage, turn off the engine, and open the hood again.
  3. Set the parking brake and block the wheels using wooden blocks. This will prevent your car from accidentally moving as you run your diagnostic test.
  4. Set your transmission to neutral, if you have a standard transmission, or park if you have an automatic one.

How to Hook Up and Use a Vacuum Gauge

The Vacuum Test

  1. Unplug the vacuum hose from the intake manifold and connect it to the tee fitting that comes with your vacuum gauge (if your engine uses a carburetor, don't connect the gauge to it. You need direct vacuum from the intake manifold).
  2. Then, connect the other end of the tee fitting to the intake manifold.
  3. Now, connect the gauge to the tee fitting.
  4. Keep the vacuum gauge and hoses away from moving engine components like pulleys, drive belts, and radiator fans.

Vacuum Gauge Readings

To accurately diagnose your engine's mechanical condition, pay close attention to the next points:

  • First, use a professional-quality, at least 2 inches in diameter, vacuum gauge for better accuracy. It'll respond better to minor fluctuations in vacuum produced by leaking valves and weak spring valves, for example.
  • Second, when taking your reading, pay close attention to the number(s) the needle on the dial point to.
  • Third, if the needle fluctuates at all during your reading, note the speed it moves (slow, fast), range of movement (e.g., about two in-Hg), and movement pattern (e.g., jumps back and forth, or moves and remains steady).
  • Also, an engine in good mechanical condition will read between 15 and 20 in-Hg on a vacuum gauge at sea-level altitude — this is a general standard measure; some technicians will go as far as 14 and 22 in-Hg. As a rule, your vacuum gauge reading will drop about one in-Hg for every 1000 foot of altitude after the first 2000 feet above sea level.
  • So adjust your readings accordingly. For example, if you're at 5000 feet above sea level, and your reading falls between 12 and 18 in-Hg (dial on gauge fairly steady), your engine doesn't show mechanical problems.
  • Then, keep in mind that factors like weather conditions, the particular engine design (turbocharged, supercharged, variable valve timing) and other computer-controlled technology will affect your vacuum gauge readings. In general, the more technologically advanced an engine, the higher your vacuum readings will be.

OK, now you're ready to do the test.

  1. First, locate and remove the fuel pump fuse or disconnect the distributor from the ignition coil to disable the ignition system so the engine won't start.
  2. Maintain the throttle wide open — use a screwdriver, if necessary.
  3. Have an assistant crank the engine for about three or five seconds. Vacuum should hold steady at approximately 1 to 4 in-Hg.
  4. Replace the fuel pump fuse or reconnect the ignition coil to the distributor, and remove the screwdriver from the throttle.
  5. Now, have your assistant start the engine. Vacuum should remain fairly steady between 14 and 22 in-Hg. This means your engine is in fairly good condition, taking mileage into account.
  6. Quickly open and close the throttle. The needle on the dial should move quickly to almost zero and then up between 21 and 27 and back to the previous idle reading.
  7. Have your assistant raise and hold engine speed to about 2500 rpm. Vacuum should hold fairly steady between 19 and 21 in-Hg.
  8. If your vacuum gauge readings deviate from the previous patterns: Pay close attention to the position of the needle at the beginning of each test and jot down this number on a notepad. If the needle moves from its initial position and then stops, note the starting and ending point (distance in in-Hg) and whether the needle moved fast or gradually. Note whether the need follows a pattern, for example, it jumps irregularly or regularly, whether it fluctuates slightly or widely or remains steady once it drops.
  9. If your vacuum gauge comes with its own reading chart, compare your vacuum gauge readings to the chart. Or take a look at this vacuum readings chart at Look for the patterns that most resemble your readings and check the potential cause for the abnormal readings described below that pattern.

Once you have established your diagnosis, you may want to further test those systems or components pointed out by the reading chart, based on your results. The troubleshooting section of the repair (service) manual for your specific vehicle make, and model will help you here. The repair manual provides step-by-step troubleshooting procedures for many systems and components, which will help you confirm your reading diagnostics, if necessary.

You can find an inexpensive, aftermarket manual for your vehicle at most auto parts stores or online. Another possible source is the reference section of your local public library.

Take Advantage

A vacuum gauge will help you find the source of your car's mechanical problems at a low cost and within a few minutes. It'll guide you to specific systems and components for further testing, when necessary. Remember that you can continue your troubleshooting procedure with the help of the vehicle service manual for your particular car make and model. Today, the vacuum gauge has been replaced by newer technology as one of the primary tools in a shop. But don't underestimate its effectiveness. The vacuum gauge still remains a reliable tool for many shops and DIYers that know how to take advantage of it. And you should too.

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This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

© 2015 Dan Ferrell