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Old 10-22-2008, 08:49 PM   #1
IllNastyImpreza
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Default can someone please explane Leakdown vs compression test?

somewhere along the lines I fell off the boat with this one...

anyone ??
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Old 10-23-2008, 09:41 AM   #2
DiscoGsus
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Compression Test: Testing for compression in the cylinder using the starter to crank the motor with no spark/fuel. Plug a peak-pressure sensor into the spark plug hole, turn the crank so the piston is at bottom than tap the starter. Piston will compress the air in the cylinder as it goes towards top dead center. If your car is broken then you won't get the stated compression numbers. Thinks like broken rings, valve seats, pistons, messed up timing, and headgasket all will give you lower numbers.

Leakdown: Pressurize the same system through the spark plug hole with the piston at the top. If something is wrong, then the air will leak out, and you won't be able to hold a high pressure in the tested cylinder. Its more useful than a compression test because it helps to diagnose the problem somewhat.
-Air out the exhaust = exhaust valve/valve seat problem.
-Air out radiator cap = headgasket to the water jacket
-Air out oil cap /bubbles = headgasket to oil, or broken rings/pistons.
-Air out intake = intake valves

Both test are done on a warm engine, because as the motor warms up all the metal parts expand a little and the rings/valve seals become as good a seal as they are ever going to.

Edit: I'm a rere

Last edited by DiscoGsus; 10-23-2008 at 11:10 AM.
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Old 10-23-2008, 09:44 AM   #3
crampfan
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I assume that you know what a compression test is. A leak down test is like a compression test in reverse. the engine is rotated to TDC compression stroke. (both valves closed) the crankshaft is locked so it can't move. a hose is installed in the spark plug hole and regulated air is pumped into the cyl. ( to keep it simple) lets say 100 lbs. of pressure is pumped in and the leak down gauge reads 80 lbs. that will mean that 20% is leaking past the combustion chamber, it could be leaking past valves, rings, head gasket, etc. you can tell where it is going by listening to the exhaust, intake , crankcase, etc. I use a leakdown tester to find head gasket leaks by removing the radiator cap and watching for a rise in coolant level. It never fails to ID a gasket failure.
leak down test will tell you alot more than a compression test, the only time I use a compression tester is to ID the cyl. I need to do a leak down on. with most modern engines anything more than about 15% is worth looking into, and if you are above 20% you are broke.


The engine must be at TOP dead center compression, with the piston at the bottom, how will you take into account cyl wall wear? I have found major difference in readings between TDC and BDC

Last edited by crampfan; 10-23-2008 at 09:49 AM. Reason: miss info in above post
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Old 10-23-2008, 11:11 AM   #4
DiscoGsus
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Quote:
Originally Posted by crampfan View Post
The engine must be at TOP dead center compression, with the piston at the bottom, how will you take into account cyl wall wear? I have found major difference in readings between TDC and BDC
You are correct, I mispoke (and edited to change it.)
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Old 12-23-2008, 11:45 PM   #5
werix
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is the fuel pump or the pressure of the gaz coming in the cylinder have an effet on compression test ? ( sry im a noob trying to understand all of this )
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Old 12-24-2008, 12:22 AM   #6
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Quote:
Originally Posted by werix View Post
is the fuel pump or the pressure of the gaz coming in the cylinder have an effet on compression test ? ( sry im a noob trying to understand all of this )
nope, you don't start the engine during either test, in fact you have to make sure you can turn it over with the starter during a compression test, but it MUST NOT START. (so no fuel, and no pressurized gas entering the system) You are just measuring how well the cylinder can compress the intake charge.
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Old 12-24-2008, 12:39 AM   #7
SubeN'Siren
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How do you manage to do this on a "warm" engine without burning the crap out of you're hands?
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Old 12-24-2008, 06:02 PM   #8
beegmon
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I dont mean to thread hijack here, but I am brand new and dont have the required post count to start a new thread.

I was hoping some one could give me their take on the following compression + leakdown results I just got. They look good to me.

05, STI, stock, 47000 miles

Stone Cold (Compression check only):

#1 -- 130 psi

#2 -- 130 psi

#3 -- 130 psi

#4 -- 130 psi

Hot (Compression + Leakdown -- Dry)

#1 -- 135 psi -- 0%

#2 -- 130 psi -- 9%

#3 -- 130 psi -- 5%

#4 -- 130 psi -- 1%


Hot (Compression + Leakdown -- Wet)

#1 -- 140 psi -- 0%

#2 -- 135 psi -- 1%

#3 -- 135 psi -- 0%

#4 -- 140 psi -- 0%


The leakdown losses are not doubt coming from the rings since the wet tests sealed them up.

I got the test down because I am seeing really rough idle and missing up to around 3k. I have checked for vac leaks, replaced the MAF and O2 sensors. I havent pulled the timing belt cover off and check and see if the belt jumped any teeth, but from the looks of it I dont think thats the issue either. The plugs are new, the coil packs are all good, and the injectors check out fine as well.

In any case any input on the above numbers would be great. Thanks!
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Old 12-25-2008, 01:57 AM   #9
valetudospang
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Quote:
Originally Posted by DiscoGsus View Post
Compression Test: Testing for compression in the cylinder using the starter to crank the motor with no spark/fuel. Plug a peak-pressure sensor into the spark plug hole, turn the crank so the piston is at bottom than tap the starter. Piston will compress the air in the cylinder as it goes towards top dead center. If your car is broken then you won't get the stated compression numbers. Thinks like broken rings, valve seats, pistons, messed up timing, and headgasket all will give you lower numbers.

Leakdown: Pressurize the same system through the spark plug hole with the piston at the top. If something is wrong, then the air will leak out, and you won't be able to hold a high pressure in the tested cylinder. Its more useful than a compression test because it helps to diagnose the problem somewhat.
-Air out the exhaust = exhaust valve/valve seat problem.
-Air out radiator cap = headgasket to the water jacket
-Air out oil cap /bubbles = headgasket to oil, or broken rings/pistons.
-Air out intake = intake valves

Both test are done on a warm engine, because as the motor warms up all the metal parts expand a little and the rings/valve seals become as good a seal as they are ever going to.

Edit: I'm a rere

How do I get the "Piston to the Top"??

Merry Christmas!
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