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Performance and is for your personal reference only.
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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.
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 dont 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
This information is courtesy of Tracy
Performance Corvette. See addition info at alternate web site www.TracyPerf.com.
- A vacuum reading that cycles up and down slowly could indicate a fuel mixture
problem from improper carburetor or fuel injection operation.
E-mail us for all your Corvette or Chevy needs at
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Last Modified: February 11, 2002
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