29069 Calahan Rd, Roseville, MI 48066
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Vacuum Diagnosis

The  following information is copyright Tracy Performance and is for your personal reference only.

We Offer These Suggestions For Vacuum Diagnosis


Proper engine operation should result in an even vacuum reading from 15 inches to 20 inches of vacuum with 15 inches of Mercury being more appropriate for high performance type engines.

  • A vacuum reading that cycles up and down slowly could indicate a fuel mixture problem from improper carburetor or fuel injection operation.  TracyPerf©98


  • Low vacuum readings may indicate intake valve, manifold, or carb gasket leaks including internal or external manifold leaks.  Spray carb cleaner around suspected areas to test for leaks.
  • Plugged catalytic converters, stuck heat riser valves, plugged intake crossover heat passages, crushed exhaust pipes, plugged mufflers, (check out that mouse nest in the exhaust!), and similar restrictions will usually lead to very low vacuum readings.
  • Excessively carbon in engines or those that did not get their oil changed often enough may result in sudden vacuum reading changes when valves stick. It could happen at any engine speed and may temporarily cure itself by quickly changing engine speeds.
  • Lower vacuum readings directly related to engine misfiring may be a result of a burned exhaust valve.
  • Constantly changing vacuum readings at low engine speeds may indicate a problem with worn valve stems, guides, or seals. Higher engine speeds tend to temporarily cancel out this problem.


  • High-speed vacuum readings that change may be a result of broken or tired valve springs, while low engine speed vacuum readings tend to be constant.  TracyPerf©98


  • High performance or race cams tend to result in lower vacuum readings.
  • Make sure that the cam and lifters are matched, (solid/solid, hydraulic/hydraulic, roller hydraulic/roller hydraulic, solid roller/solid roller), etc. Strange things happen when these items are mismatched, although some will actually run that way. Cam indexing/timing will also affect these readings. Also check for sloppy timing chains, and improper or changing ignition timing.


  • Make sure you have a good vacuum gage that gives repeatable readings.  Remember your high school science class experiments, where you have to go back to the original test to verify your numbers and don’t make changes until you are sure of your first set of readings.  Who said all of this really sucks? I want you to write vacuum 100 times on the blackboard!  Good luck.  TracyPerf©98

This information is for experienced licensed mechanics!


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