Engine Vacuum Troubleshooting

Years ago, A good friend introduced me to using a vacuum gauge to diagnose engine problems, in-fact it was often the fist tool he would reach for when confronted with a poorly running engine. I’ll admit to being a little skeptical at first, but over the years I have become convinced that engine vacuum is a great source for information in helping diagnose problems inside the engine. Of course, high-performance engines with lopey camshafts often make little vacuum but even then the vacuum gauge can give you insights to the inner workings of your engine.

Simply put, the vacuum gauge has proven itself time and time again to be an invaluable tool in troubleshooting engine problems.

The key to using a vacuum gauge to diagnose engine problems is to understand exactly what the gauge is telling you. When armed with this knowledge you’ll be able to quickly discern between simple tuning issues to potentially more severe mechanical problems.

Good quality vacuum testing gauges are available at most auto parts stores and online resources, and are relatively inexpensive. Before beginning any vacuum testing, a visual inspection should be made of the entire vacuum system. Check all hoses, hose connections, and all open ports on carburetors and intake manifold are plugged (note: some cars also have vacuum operated heat/ac controls).

To get started, hook the vacuum gauge to an intake manifold vacuum source. Manufacturers 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. This is also done with a tee on an existing line or pulling a line and connecting it direct (for example, the vacuum line to the transmission can be used). Start your engine and allow it to come up to operating temperature before testing.

Common Vacuum Test Results:

Normal Engine: On most engines, accelerate to around 2000 rpm and then quickly release the throttle. The engine should snap right back to a steady 17- 21″hg vacuum.

Steady low between 5-10″hg vacuum: This indicates that the engine has a leak in the intake manifold or the intake gasket.

Steady low between 10-15″hg vacuum: This reading indicates late valve timing. There’s a chance the vehicle has jumped timing. Check the timing belt or chain depending on the application.

Steady low between 15-18″hg vacuum: This low reading indicates retarded ignition timing. Advance the timing on the distributor to correct this problem, and recheck vacuum.

Fluctuating Needle: A fluctuating needle indicates there’s a problem with a valve or a there’s an engine misfire.

Needle drops during acceleration: If the needle drops steady during acceleration there’s a restriction in the exhaust or intake. This is typically due to a clogged muffler or exhaust system.

Also see vacuum gauge chart.

Print it out and hang it on your tool box, and you’ll never second guess what your gauge is telling you!

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16 Responses to “Engine Vacuum Troubleshooting”
  1. Darwin C

    great advise, I had trouble with my 1986 dodge d150 318 the carb was replaced and it ran good for awhile, then when I would take off from a stop it would not have power. I put the vac gauge on and the trouble was in the intake manifold would you believe it was the carb mounting gasket that I installed, it had a flaw in it from the manufacture the mounting bolts were torqued correctly. wit another new one it is running great. Thanks again!

    • Max technology

      Let me give you more info. You can spray some water , around carburator, if it has a leak gauge will detect it. You can do a lot of things .

      with it i have discovered servo brake leaks with this tool.

  2. Carl Smith

    Being Chief Tech, I use vac gauge often on standard equipment and race cars in their tune up. You did excellent write up.

  3. John

    It should be noted that a high performance camshaft will make the idle vacuum readings lower than the 19-20″ you show.

  4. Bernard

    How can I print off the chart? I’m getting an error that the requested image is not available.

    • Customer Service

      Hello Bernard,

      This has been fixed, and you should be able to print off the chart now.

      Becky CCRC Video Membership

  5. Al Stokes

    What is normal manifold vacuum for a Willis/Knight sliding sleeve engine?
    My readings are very low.
    Al Stokes, Huntsville, AL, 256-539-2058

    • Customer Service

      Hello Al,

      The sleeve valve engines are notorious for low vacuum pressure, but there are things to check that can affect vacuum like proper engine timing.

      I have seen them run as little as 4 inches Hg, but more commonly between 6-12 inches Hg.

      Freshly rebuilt engines tend to have lower pressure than those that have a few thousand miles on them.
      W/K owners claim the engine needs to carbon up a little to operate properly.

      I’ve even heard of enthusiasts adding oil to their fuel to break-in new engines.

      Wrench Safe,

      Classic Car Restoration Club Video Membership

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  6. Carl Heringer

    Nice article! Can’t wait to hook my 6.0 LS up and see if I can figure out why it’s so hard to start after it’s warmed up!

  7. kenneth gehres

    I have all the info on engine vacuum troubleshooting but there is no info for reading the vacuum gauge for a engine that has a 3/4 race cam thanks ken

  8. Tom Barto

    I had a very similar vacuum guage in 1967 that I used on my ’55 Chevy. I came with a chart very similar to this one. Thanks for posting this. I will print the chart out. Very good information.