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Vacuum gauge diagnostic check

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Vacuum gauge diagnostic check


 A simple automotive vacuum gauge is a very good and cost effective way to check a number of different things going on inside of an engine.  Since an engine is basically like a big air compressor checking how well the engine pulls a vacuum can give you a good heath status check.  Unlike a compression check, checking the vacuum can get you the health of intake and throttle body seals, as well as how the catalytic converter and exhaust system are acting.  The only problem with diagnosing a problem solely based on vacuum alone is that numerous problems can cause the same bad vacuum reading.  If you find your vacuum reading to be less then satisfactory and something inside of the combustion chamber is suspected, a leak down or compression check should follow to help pinpoint the problem.


 Getting started


 Vacuum gauges can be found at most any automotive store and are fairly cheap.  Don?t worry about buying the most expensive one you can find unless you plan on using it every day.  You may also want to pick up a line reducer to fit the gauge on the brake booster vacuum line.  Another good place to find vacuum would be the cruise control vacuum line, or PCV vacuum.  When choosing a vacuum line, be sure it is absolute vacuum, not ported vacuum.  Ported vacuum means the vacuum runs through another device before the hook up, I.E. the throttle body.  You want to get your vacuum directly from the intake manifold to assure 100% absolute vacuum. 


 Hook up the gauge and start the car, check for normal operation of the engine.  Sometimes, removing vacuum lines will cause the engine to run abnormally, thus giving you false readings.  Avoid using the MAP sensor or any other vacuum that may affect engine operation.  This is why the bake booster or cruise control vacuum is a good place to start.  Before you take any readings, make sure the engine is at normal operating temperature.  At normal operating temperature and sitting at sea level a healthy engine should pull about 16 ? 23 in-HG of vacuum.  The needle on the gauge should also be quite steady.  Remember, a change in elevation or atmospheric conditions can decrease the reading.  For about every 1100 feet in elevation change the reading will drop by about one inch of mercury. 


Reading the results


 On a healthy engine at idle the gauge should read a good steady vacuum.  A low steady vacuum is usually caused by a vacuum leak somewhere.  This could be in another vacuum line or a gasket between the throttle body and intake or between the intake and the head.  If the needle displays a low, fluctuating vacuum, suspect that the intake gasket as gone bad near one of the intake ports or an injector seal is leaking.  Rapid vibrating of the needle may be a sign of a compression problem inside of a cylinder.  Do a leak down test to try to pinpoint the problem.  A large rapid fluctuation could be a sign of a blown head gasket or a nearly dead cylinder.  Do a compression check to locate the problem.


 You will also want to check for a slow return back to normal vacuum after revving the engine.  Upon rapidly revving the engine to about 3000 RPM the vacuum should drop to almost zero.  When the throttle is closed quickly the reading should jump up a bit past normal on the down rev until returning to normal at idle.  If there is a slow return and the vacuum does not rise above normal when the throttle is closed after revving the rings may be worn.  If there is an extremely long delay but the vacuum does return to normal check for a clogged exhaust system.

 This should give you a generalized idea on how to read a vacuum gauge.  Remember this is a very short article.  There are other conditions not listed above that can cause bad vacuum readings.  If you do get a bad reading do some other tests before replacing parts.


 If you have any questions about this article please feel free to post them in the forum section of this web site.