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!

Discussion
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41 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!

    Reply
    • 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.

      Reply
  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.

    Reply
  3. John

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

    Reply
  4. Bernard

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

    Reply
    • Customer Service

      Hello Bernard,

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

      Thanks,
      Becky CCRC Video Membership

      Reply
  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

    Reply
    • 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,

      Mark
      Classic Car Restoration Club Video Membership

      We’d love to have you be a part of our community. We are convinced you will enjoy the benefits of becoming a member and having access to the best instructional how to videos and professional tips. We would like to offer you a special promotion for your first year membership. https://go.ClassicCarRestorationClub.com/C13211

      Reply
  6. Robert Heaney

    My 1985 1.8L 3BBL Accord has always read a steady 16 when I’ve checked it. Altitude 4500 ft ‘ish if that pertains to reading. About 35k on engine rebuild, new cam included. Seems low ?

    Reply
    • Customer Service

      Hello Robert,

      It is on the low side but it should still be enough to operate brakes etc…
      A complete compression test may give you additional insight into your motors health.

      Hope this helps,

      Mark
      Classic Car Restoration Club Video Membership

      We’d love to have you be a part of our community. We are convinced you will enjoy the benefits of becoming a member and having access to the best instructional how to videos and professional tips. We would like to offer you a special promotion for your first-year membership.
      https://go.ClassicCarRestorationClub.com/C15187

      Reply
  7. 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!

    Reply
  8. 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

    Reply
  9. 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.

    Reply
  10. Don Nee

    I can’t get my 48Flat Head Ford to idle. It has a huge edelbrock 4 barrel. What readings should i look for when its running at higher rpm

    Reply
    • Customer Service

      Hi Don,

      Vacuum readings are most telling at idle and you should be creating 15-20Hg of steady vacuum at idle.
      Over carbureted engines can significantly lose vacuum pressure at higher RPMs as they are opening up more than the engine can use.

      Hope this helps,

      Mark
      Classic Car Restoration Club Video Membership

      Reply
  11. Robert Allen

    My 350 cui ’77 Camaro idles at 1100 rpm’s but when I put it in reverse or drive the engine drops to 800 rpm’s. Does that sound like a vacume leak. I’m going to go out and buy a gauge today.

    Reply
    • Customer Service

      Hi Robert,

      Actually your idle seems just a little high, typically it drops from 900-1000rpms to 500-700rpms in gear.
      But close to factory specifications.

      Hope this helps,

      Mark
      Classic Car Restoration Club Video Membership

      Reply
  12. Gerald

    Hi. I have a Dodge 440. Just redone and having issues. It has all Comp cam valve train , roller rockers, on 906 big valve heads, and a mild race cam, 471 is the number . I believe I have several rings not seated or installed wrong. Engine bored 30 with forged flat top pistons, chrome molly rings. Have roughly 2500 miles on rebuild. I’m oiling 3 plugs on rivers side still. What should my normal vacuum be for this mid cam’d engine?
    I’m trying to determine if oiling plugs is my rings or a possible intake leak to inner head ports? Thanks

    Reply
    • Customer Service

      Hello Gerald,

      With a mild cam your engine should produce 15-19″Hg at idle.
      I would proceed to a compression check… a lack of compression in those cylinders could point rings not seated or installed wrong or even perhaps a head gasket issue.
      Your intake shouldn’t have oil running to it, so it’s not likely the source of your problem.
      If you have good compression, it is possible valve guides or seals have failed allowing oil to pass into the combustion chamber.

      Wrench Safe,

      Mark
      Classic Car Restoration Club Video Membership

      We’d love to have you be a part of our community. We are convinced you will enjoy the benefits of becoming a member and having access to the best instructional how to videos and professional tips. We would like to offer you a special promotion for your first-year membership.
      https://go.ClassicCarRestorationClub.com/C15267

      Reply
  13. Lynn Broadman

    This is a great article. Most youg rodders may own a vacuum gauge that they bought as part of a timing light kit but they have never used it. Perhaps they now know what a valuable tool this can be

    Reply
  14. Robert Borst

    Do the vacuum readings in the chart apply to a stock/rebuilt Chevy 1952 flathead 216 ci.?

    Reply
    • Customer Service

      Hi Robert,

      Yes, these readings will apply to a stock Chevy 216 cubic inch engine.

      Thanks,

      Mark
      Classic Car Restoration Club Video Membership

      We’d love to have you be a part of our community. We are convinced you will enjoy the benefits of becoming a member and having access to the best instructional how to videos and professional tips. We would like to offer you a special promotion for your first-year membership.
      https://go.ClassicCarRestorationClub.com/C15267

      Reply
  15. Bob Carriere

    Rebuilt Chev 261 inline 6 circa 1959 stock….idle I get a steady 19 …. when I floor it in gear say 3rd gear ti drops to 5 or 6 inches growles like an old Hoover vacuum cleaner then regains its vacuum to maybe 15 – 18 stock air cleaner, standard round muffler, using YF Carter carb set up fr the 261 c.i. but you are saying that it indicates a plugged muffler or intake????? Comments please Thanks Bob C

    Reply
    • Customer Service

      Hi Bob,

      Your steady 19 inches Hg at idle would indicate a good vacuum reading for this engine.
      Your vacuum should drop under hard acceleration.
      I has a ’55 Chevy that whenever I drove it in the rain the windshield wipers would stop working when I accelerated, because the vacuum had dropped.
      When looking at the chart, please consider that almost all of the readings are “at idle”. with the exception of the Normal Motor / rapid open and close of throttle.

      Wrench Safe,

      Mark
      Classic Car Restoration Club Video Membership

      Reply
  16. Michael Brower

    I suggest connecting the vacuum gauge as close to the center and carburetor as possible, especially with a lopey cam. You could pick up somewhat extreme intake runner pulses if you connect too close to the front or rear of the intake manifold.

    Reply
  17. Kin Walkup

    Checking for vacuum leak I’ve always used carburetor cleaner to spray all over area where the vacuum hoses are connected and the intake manifold and carburetor if you have a vacuum leak you will see the RPM raise and you can find a vacuum leak pretty easy by spraying that area again until you pinpoint the exact location. It is save me a lot of time and headaches. My dad always told me to tune my car by a vacuum gauge set it to the highest vacuum and then back it down.
    Do have a problem though with a Pontiac 455 motor. Just fresh rebuilt with mild cam shaft. When you go to accelerate from stop it Ping’s very bad sounds like a mule in a tin Barn every set the timing numerous times all the way up all the way down recheck top dead center everything checks out to be proper factory specs has a 650 Edelbrock carburetor. Also what I found out it has low compression heads as it is a 75 motor. Have not been able Define problem with pinging. Number one plug shows it is not firing in the hole as it has carbon buildup on the top of a flat top piston. The only plug that is fouled. Pull the plug out and ground it and it is firing. All other plugs are fine no carbon buildup on any of the other Pistons used camera down spark plug hole to check engine compression all running around 120 125 per cylinder. Any ideas what might be the problem? Have talked to numerous people and the mechanics no help so far.

    Reply
    • Customer Service

      Hello Kin,

      You may have addressed this already, but do you have fresh fuel in the car?
      I have had fuel in cars go bad in as little as 8 months and would ping hard under load.
      as long a we’re talking fuel, I would try a higher octane fuel to see if that made a difference.

      Another thought is carb jetting/metering rods/springs…. with your new rebuild and cam, is your engine looking for more fuel or less fuel?
      A tuning kit is available for the 650 carb that include all of these parts for around $30.
      It would be interesting to see if it was jetted up or down if it would make a difference.

      It is curious that only number one plug is fouling… what is unique about this cylinder? Is there oil getting past the valve seals?
      The fact you have good compression and spark to number one left me scratching my head. and I don’t really have an answer other than to think perhaps it clears itself up once the engine is running right.

      Not sure if I have a smoking gun for you, but at least a few more things to check out.

      Wrench Safe,

      Mark
      Classic Car Restoration Club Video Membership

      We’d love to have you be a part of our community. We are convinced you will enjoy the benefits of becoming a member and having access to the best instructional how to videos and professional tips. We would like to offer you a special promotion for your first-year membership.
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      Reply
  18. John Stafford

    ‬ Hi, Thanks for your write up.
    I have just purchased a 2003 subaru wrx STI. The car runs well, no misses but it uses oil, about 1 ltr in 1800 klms running interstate at about 3000 rpm most of the time. The motor was supposedly rebuilt 70,000 klms ago, so I did not expect any issues with it.
    Oil pressure is 27psi on hot idle at 700 rpm, 57psi at 2000 rpm, 71psi at 3000 rpm.
    After a hot run and the car has been parked a few minutes and restarted there is plenty of grey smoke out the back. This seems to go away after a little driving. It does not happen if the motor is started from cold. There is a little smoke if you look hard on normal running.
    I just did a compression test with the thottle fully open and it had (4) 112, (2) 100, (1) 100, (3) 110. All the spark plugs are burning cleanly.
    I put a vacuum guage on it and the reading is about 14 with a vibrating on cold but it increased to 17 vibrating when warm. The vibration of the need was only about 1 on the guage.
    I am guessing that the engine has been worked hard and is just showing signs of ware in the rings and in the Valve guide seals. I do not intend to do anything at this stage, just keep the oil up to it. I am using either 10-60 Castrol Edge or 15 – 50 Penrite 10 tenths oil.
    Do you have any insite that I am missing or any advise for me?
    thanks again,
    Regards John in Australia.

    Reply
    • Customer Service

      Hi John,

      Thank you fro your patience, here is the response from the expert’s:

      The Subaru wrx STI is notorious for oil consumption, and 1 ltr in 1800 klms is not unheard of.
      When the turbos are kicked in these engines have a fair amount of blow-by that is sucked in by the PCV system and burned up. In fact, Subaru even says its not unusual to see a quart or so loss between 3k mile oil changes.

      Wrench Safe
      Mark
      Classic Car Restoration Club

      Reply
  19. Jonathan

    I have a 1996 Chrysler Concorde 3.3L. for some reason the engine runs even though I have turn the key off. To stop it I have to pull out a fuse that will shut it off. Then I have a hard time starting it after a few hours.

    Reply
  20. William Scott

    Thanks for the great info. Could you possibly answer this question; I used my vacuum gauge and it had a reading: 25 inches. What would cause it to be so high?

    Reply
    • Customer Service

      Hi William,

      Thank you for your patience, here is the response from the expert:

      High vacuum readings (25″ +) at idle are generally a sign that your engine is working hard to get air/fuel into the engine.
      This can be commonly caused by a restrictive air cleaner, dirty/clogged air filter, choke stuck or partially closed, or undersized carburetor.
      Wrench Safe,
      Mark
      Classic Car Restoration Club

      Reply
  21. jklw77

    great site found u when looking for info on vacuums gauge I have not used in 30 years, neighbor has a run down 76 dodge pick up with a 318, looking and running like crap . was going to help out and yes I am a bow tie guy,1st car 57 B210 sedan 60 over 283……..

    Reply