What does Engine Vacuum tell about Engine Condition

[OILNEWBY]

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?

 

[kreigle]

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!

 

[MC5W20]

A compression test on the cylinders will be a much better indicator of the health of your engine.

 

[1 FMF]

Can the amount of vacuum in a specific engine tell you what condition the engine is in:

relatively speaking yes, but only if you know what the readings are supposed to be. Engines will show different vacuum readings based on various things, valve timing, ignition timing, valve overlap and fuel mixture just off the top of my head. To put it in perspective, use of a vacuum gauge is only as good as how someone uses it under certain conditions and interprets the results. Much the same as oil analysis or an chasis dynomometer readings.

For what you described and how you're interpretting this particular vacuum reading off your boost/vacuum gauge, that's not going to tell your engine condition with any real certainty. It basically will only tell you if you have a significant problem, you could probably be down 25% in power and you would never be able to interpret it from a vacuum reading taken this kind of way.

If you want to know if you still have a strong motor at 114k miles, the best way to tell that is to put it on a dyno or from your top speed from a 1/4 mile run.

Vacuum gauges are best used at idle for diagnosing ignition timing, fuel mixture, intake air leaks and valve sealing but can also indicate problems at higher rpms with much of the same.

another vacuum gauge link
http://www.centuryperformance.com/vacuum.asp

 

[1sttruck]

You can also do an initial check for a plugged exhaust.

 

[OBIO3]

Altitude also has an effect on vacumn. Higher the altitude the lower the normal readings are. this is across the board readings,

 

[moribundman]

Just a comment: newer engine designs appear to create higher vacuum (low pressure, really

) than older types of engines. High vacuum and low tension ring design may contribute to high oil consumption -- due to increased blowby, I suppose. At least on my car I notice that spirited driving and especially engine breaking at high RPM will make the engine (which has low-tension rings) guzzle oil.

 

[JohnBrowning]

Vaccum reading are a good way of detecting gasket leaks especial the head gasket. It is also an indicator of engine health so long as you have a base line to compare to. Cam profiles have a huge impact on available vaccum.

If you have compression reading and vaccum reading you can get an excellent indea of the condition the engine is in.

I bar is equal to 1 atmosphere if my memory serves me correctly wich is the same as 14.7psi. In the USA we PSI in Europe Bar's and KiloPascal. Wish I had my rotoary slide rule with me right know!

 

[Gary Allan]

You can also do an initial check for a plugged exhaust.

Yes, it's the "remote view" test for a plugged cat.

 

[porcupine73]

Is pulling 1 bar vacuum pret near absolute vacuum? I would think it can't get much better than that.

 

[Gary Allan]

I dunno

I think 28.8" of mercury is about the best you can do. I forget the conversion for "neg-poundage". That is ..so many inches of water column equals so many inches (or fractional inches) of mercury.

 

[ShortBuSX]

All the info Im finding about vacuum testing applies to carbed cars...but what about fuel injection?

 

[VNT]

I would say in relative terms at idle if your reading hasnt changed then is would indicate you have no leaks on the intercooler plumbing, your rings are in good shape, and your timing is correct.

I pull about 19"s at idle in my 89 Shadow with a 2.5 turbo. Car has 145 PSI across all cylinders and less thatn 4% leak down.