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2005 FORESTER XT Auto
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Discussion Starter #1
I recently purchased a 2004 Forester XT with 91k and a blown turbo. Before purchase, I had a shop run a compression test and look at the oil to rule out major internal problems.

The test came back even, with the cylinders at 105, 105, 105, 100, respectively, and no leaks. I know that even is most important, but those numbers also seem shockingly low. Does anyone have any insight on whether this should be a concern? Anything that might bring it up?

The vehicle was tested in Colorado Springs, CO, so altitude will play a factor. It was run for 40 min before testing began. I'm unsure of what device they used, and it may read low. I've also heard that turbo'd engines run lower compression than NA.
 

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MY05 Forester 2.5 XT 5MT
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From the manual for the 2.5 XT:

Compression (350 rpm and fully open throttle):
Standard 981 — 1,177 kPa (10 — 12 kgf/cm2, 142 — 171 psi)
Limit 882 kPa (9.0 kgf/cm2, 128 psi)
Difference between cylinders
Less than 49 kPa (0.5 kgf/cm2, 7 psi)
Not sure how altitude would affect things but I would be concerned over those figures of yours - Are we sure the tester was sealing properly?

From the manual for the N/A engine:

Compression (350 rpm and fully open throttle):
Standard; 1,275 kPa (13.0 kgf/cm2, 185 psi)
Limit; 1,020 kPa (10.4 kgf/cm2, 148 psi)
Difference between cylinders;
49 kPa (0.5 kgf/cm2, 7 psi), or less
 

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2005 FORESTER XT Auto
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Discussion Starter #3
I've seen those factory spec numbers, though, all my research shows most people are between 130-150. The test was done at a reputable shop, so I can only assume they sealed everything properly to get an accurate reading, but who knows. I'm almost tempted to have it tested again at the shop doing all my mods and see what they come up with. It's just money, right? :|
 

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2017 VW Golf SportWagen 5MT
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Altitude will affect things because your ambient atmospheric pressure will be lower a mile up vs. sea level. Ever tried running at 10,000 feet elevation lol?

I wouldn't sweat it yet. I'd talk to the shop that did the test to get their opinion on reasons why the numbers are low. It may be normal.

If the tester wasn't sealing properly, it wasn't sealing while testing all 4 cylinders because results are even. I don't know how these tests are performed on a fuel-injected car -- on my carbureted car I have to make sure the throttle butterflies are held open, maybe your throttle body plate was shut?

Here you go: http://www.coloradoevo.com/forums/showthread.php?t=4683



According to that info, if you were to do a compression test on your car at sea level using the same gauge/process, result would be 105 / .8359 = 125.6 PSI.

Stan
 

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2005 FORESTER XT Auto
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Discussion Starter #5
Altitude will affect things because your ambient atmospheric pressure will be lower a mile up vs. sea level. Ever tried running at 10,000 feet elevation lol?

I wouldn't sweat it yet. I'd talk to the shop that did the test to get their opinion on reasons why the numbers are low. It may be normal.

If the tester wasn't sealing properly, it wasn't sealing while testing all 4 cylinders because results are even. I don't know how these tests are performed on a fuel-injected car -- on my carbureted car I have to make sure the throttle butterflies are held open, maybe your throttle body plate was shut?

Here you go: Compression Test - Altitude Factors



According to that info, if you were to do a compression test on your car at sea level using the same gauge/process, result would be 105 / .8359 = 125.6 PSI.

Stan
Right, altitude definitely affects the numbers, and those subaru factory specs are based on sea level elevation, so I wouldn't expect to get anywhere close to 170 here in CO, even brand new.

The shop didn't seem concerned at all really. They said they came in low, but even, and there can be a number of factors causing it to read low - none of which could be their equipment or method, I'm sure ;).

I'm not overly concerned at this point, just opening up a discussion to see if others have some different insight or perspective.
 

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2017 VW Golf SportWagen 5MT
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You could have them do leak-down test.

The car sat for a while, right? May just needs to get nice and hot for rings to start sealing better. I'd try to get it running and go from there.

Stan
 

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Discussion Starter #7
You could have them do leak-down test.

The car sat for a while, right? May just needs to get nice and hot for rings to start sealing better. I'd try to get it running and go from there.

Stan
When I first scheduled, I requested a leak down test as well, but they told me they didn't do it because all the cylinders came back even and didn't warrant further testing. I wish they had done it anyway, but too late now.

The car has been sitting awhile, about a year and a half, and has only been started and driven on and off trailers, and into shops during that time, probably under 2000rpm. But I was thinking that also. They said they let it run for 40 minutes before starting the test.
 

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2017 VW Golf SportWagen 5MT
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I'd be more inclined to listen to them more than any posts on the Internet, kudos to them for saving you money by not doing the leak-down test. You can always have it done later, you need to get the car on the road either way.

Stan
 

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05 Forester XT
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My 05 xt at 4982ft (fort collins) gave me 110 on all 4.
The local shop (who does built motors) said it was fine. What you want is all the cylinders giving you close to the same numbers.
 

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SG9 Forester XT Stick Shift
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Compressions ratio are multiplication of atmospheric pressure. So higher elevation definitely would reads lower. This is the same exact reason NA engine lost much more power at higher elevation.
 

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Hi all. I have 2005 XT with an electronic fuel throttle (drive by wire). I need to do a compression test. Will the throttle fully open up if I floor the pedal during cranking? I've seen many raise this question regarding Subarus but haven't seen a definite answer.
 

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MY05 Forester 2.5 XT 5MT
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@grantlee1972 I also have a 2005 XT with the same throttle. when I did a compression test, I just removed the fuel pump fuse from memory and all was fine.
I believe you can also disconnect the crankshaft position sensor which is just under the alternator from memory for the same effect..
 

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@TMX. Thanks for that :) I did disconnect the fuel pump relay under the glove compartment. Should I disconnect the crankshaft position sensor also? What will that do for the process?
Thanks again :)
 

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MY05 Forester 2.5 XT 5MT
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@grantlee1972 From memory disconnecting the sensor prevents spark - But if you've removed the fuel pump fuse/relay, then you don't need to worry about the sensor :)

I've attached the page from the Subaru workshop manual to give you an idea how your results compare.
 

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