What does Engine Vacuum tell about Engine Condition

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Sep 15, 2002
Las Vegas, Nevada
Hello everyone. I have a question and Engine Vacuum and how it correlates with engine condition. Can the amount of vacuum in a specific engine tell you what condition the engine is in? The reason I ask is I have a '02 WRX (100% stock, its a daily driver) with 114k miles on the clock. On deceleration it has just under 1 bar (I think 1 bar is around 14.5 lb's) on the factory installed boost guage. What does that tell me if anything about the engine condition? Since I had 2500 miles on the car I have had Amsoil ATM 10w30 with a SDF-20 filter and changed around 7500 miles per oil change. I have done many analysis with all coming back with very low wear numbers. I am curious if the money I have spent on the oil in addition to my non aggressive driving habits have helped in having a strong motor at 114k miles. What do you think?
I've used a vacuum gauge to troubleshoot NA engines, and it is a very good tool for identifying certain problems (burned/stuck valves for example). I've never owned a turbo vehicle, although my brother has tried using a combination vacuum/boost gauge to troubleshoot the APC module in his Saab. 1 bar = ~29.9in/hg (the graduations on my vacuum gauge). I think most cars should pull around that on decel, that isn't much of an indicator of anything except how hard you are engine braking. Vacuum gauges as a diagnostic tool are more commonly used at idle, and you will need something with more resolution than your built-in boost gauge. Good vacuum gauges are not expensive. The procedure and readings don't change much from engine to engine, so some links I found on Google should start you off: http://www.secondchancegarage.com/public/186.cfm http://autospeed.drive.com.au/cms/A_2393/article.html http://www.fordf150.net/howto/diagnoseengine.php Some of these tests shown (involving open throttle) are harder to perform on a turbo because the boost throws the gauge off, so you may have to see if you can bypass the turbo if you are serious about performing these tests. Also, because fo the turbo, make sure you find a gauge that can handle both vacuum and boost!
A compression test on the cylinders will be a much better indicator of the health of your engine.
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