Compression Test Help

[OldManSTI]

I am currently trying to do a compression test on my STi, and I'm not sure what is wrong.

It took me over three hours to pull three spark plugs out last night, and I got the fourth one out this morning (stupid tiny work space!). Because of that, the car isn't warm. I have all of the spark plugs out (which is what I read I'm supposed to do) and after testing two cylinders I think I have a problem somewhere: I'm only gettting 90psi! I rented the compression tester from an auto parts dealer here, so that could be the problem. Or could it be that the engine isn't warm, and that's messing up the results? Or, am I doing this wrong, and I DO need to have the other plugs in place? Or (worst case scenario) is my engine screwed?

Help me Obi Wan!

 

[IGOTASTi]

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[OldManSTI]

So I looks like I'm doing everything there, other than the engine being warm. When it takes me 3+ hours to get everything apart, I can't imagine ever being able to make that happen! :?

 

[IGOTASTi]

Takes practice my young apprentice. ?

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[HolyCrapItsFast]

When I do the test I disconnect the line going to the brake booster. Also when you crank the engine all plugs need to be removed as shown above and you crank it till the needle on the tester stops advancing

If you can't get above 90 psi then check all cylinders and as long as they are all the same or close then it is probably just because it is cold and they are good.

 

[Raven32]

Only thing I can think of is that the throttle pedal is not all the way down when cranking the engine, or low battery


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[OldManSTI]

So I put everything back together and warmed the engine up, then spent about an hour pulling the plugs back out. When I tested the engine was still warm to the touch. That being said, here's what I got:
1=95psi
2=99psi
3=97psi
4=100psi

Maybe I can blame the elevation... *gulp* So these seem really low. Do I have a big problem? It doesn't eat oil, and it runs fine, but I'm really concerned that I have a major problem lurking inside my engine.

 

[HolyCrapItsFast]

Honestly I think it is something with either the tester itself or some other influence. Altitude can also lower the numbers. :tup:

If there is no oil consumption and the duct work is fairly dry then you are good to go.

How do the old plugs look!

 

[Batmobile_Engage]

The first thing that strikes me when I see your numbers is how close they all are together. Generally speaking, it's difficult to judge engine health based on compression numbers as compared to what they "should be". I've never seen anyone get even close to the ~160psi that is suggested. Everything would have to be just right...the test would have to be performed perfectly, the engine would have to be as the exact right temperature, test performed at sea level, etc. etc. etc...

So, low numbers aren't necessarily a bad thing. When you should worry is if one or more cylinders is drastically different than the others. If I remember correctly, the text book figure is "7% different that the average of your other compression numbers".

To me, your numbers look fine...

1=95psi
2=99psi
3=97psi
4=100psi

But if they were something like...

1=95
2=99
3=97
4=85

...then I would worry about cylinder 4.
I hope that makes sense.