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Old 04-02-2009, 07:52 PM   #1
streetrcer909
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Default Help With Compression Test Results!!!

if i posted this in the wrong section im sorry i just dropped my 04 sti of with 65000miles to get a compression and leak down test done to check the motor before i do my build...
The subaru dealership i brought it too said the results were horrible and if i keep driving the car like i am ill blow the motor i do drive the car hard but i dont beat the **** outta it theres a big difference...the results of the compression test are-
Cylinder 1- 132 psi
Cylinder 2- 125 psi
Cylinder 3- 130 psi
Cylinder 4- 130 psi

i just figured id check with you guys on the forums who probably know how to read these better than me and the tech i just want a second opinion...what do you guys think am i really close to blowing the motor? should i continue witih the build? if you can pm me what you think it would be easeir im not on the forums much basically just to check pms...thanks
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Old 04-02-2009, 08:26 PM   #2
LiquidForce
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517whp/516wtq (Stage pi)

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From the service manual:


Compression (350 rpm and fully open throttle):
Standard;
951- 1,147kPa(9.7- 11.7kgf/cn?, 138-
166 psi)
Limit;
834 kPa (8.5 kgf/cd, 121 psi)
Difference between c linders;
49 kPa (0.5 k g f / c d 7 psi)


I would say you are ok. There is some variation from gauge to gauge so I wouldn't worry about the numbers being slighty lower since the are all so close and within 7psi from the lowest to the highest.

The main cause of low compression is a broken ringland and with that you would see something like 130,130,130,90.
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Old 04-02-2009, 08:29 PM   #3
streetrcer909
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Default

haha thanks i have no clue how to read that but guess ill take your word on it...like i said i just wanted a few second opinions from some people on the fourms
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Old 04-02-2009, 08:43 PM   #4
LiquidForce
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You said you had a leak down test. What were those numbers? They tell a more complete story than a compression test.
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Old 04-02-2009, 09:03 PM   #5
CKxx
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Your BOV sounds lame
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Isn't it also a possibility, when number are low across the board with little deviation, that the motor was simply not warm when the test was done?

If you car sat around outside and the tech only had it running to drive it in and do the test then that would explain the low results. It also seems pretty unlikely that you would have bad leakdown issues equally on all 4 cylinders.

As was already stated, I wouldn't be concerned, but I would find a new dealer to service the car.
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Old 04-03-2009, 12:32 AM   #6
flycaster
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OK, I'll bite...how much did they want to "fix" the car?

PS - there's nothing wrong with it: a little wear, sure, but nothing wrong.

Last edited by flycaster; 04-03-2009 at 12:37 AM.
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Old 04-03-2009, 10:36 AM   #7
mick_the_ginge
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Ditto, those numbers look good to me, very small spread
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Old 04-03-2009, 10:45 AM   #8
kevinh211
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the compression numbers are fine and nothing to worry about. but what about the leakdown numbers?
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Old 04-03-2009, 12:26 PM   #9
Fongers
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Looks ok here as well. Pretty even man. If you see more than a 15# diff, I'd be a bit concerned but its really when one is like 30+ less that you know there is a problem.
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Old 04-03-2009, 03:04 PM   #10
streetrcer909
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they didnt want to fix it just told me that if i kept driving the car like i was the motor would blow in a couple months
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Old 04-03-2009, 03:11 PM   #11
Team Scream
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those are all within spec as far as i can tell....
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Old 04-03-2009, 04:27 PM   #12
hustleshark206
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Why would they tell you those result are horrible???? Maybe the leak down test has shown something we are not seeing here??
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Old 04-03-2009, 05:42 PM   #13
Chi_San
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With compression like that, I'd expect good leakdown numbers as well.
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Old 04-04-2009, 12:52 AM   #14
flycaster
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Quote:
Originally Posted by hustleshark206 View Post
Why would they tell you those result are horrible????
Uh, money maybe?
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Old 04-04-2009, 01:05 PM   #15
jarencibia
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Those numbers aren't bad. Your engine isn't going to spontaneously explode.

When auto service employees make statements like that it makes me angry.
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Old 04-04-2009, 06:16 PM   #16
sponaugle
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Quote:
Originally Posted by CKxx View Post
Isn't it also a possibility, when number are low across the board with little deviation, that the motor was simply not warm when the test was done?

If you car sat around outside and the tech only had it running to drive it in and do the test then that would explain the low results. It also seems pretty unlikely that you would have bad leakdown issues equally on all 4 cylinders.

As was already stated, I wouldn't be concerned, but I would find a new dealer to service the car.
Given my propensity to break things, I do compression tests all the time.

Engine temperature can make a difference, by my tests, of perhaps 5-10psi. A larger difference can come from battery voltage. Most shops remove the battery as it makes it easier to get to the plugs. Some people slide the battery back in place and hook it up to spin the motor,while others use a jump box or jumper cables.

In my experience, if I use jumper cables (so no battery in the car) the voltage drop across a 6 foot cable makes the starting RPMs about 100 lower, which results in compression numbers that are consistently at least 10psi less.

Some shops will test the passenger side first, then remove the battery, so the drivers side reads less. Others don’t run long enough to get a good reading. Others don’t put a charger on the battery and the voltage lowers by the time you get to the last cylinder.

Oh and my gauge at home reads about 6psi less then the one at the shop.


Jeff Sponaugle
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