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Old 09-19-2018, 03:30 PM   #1
NASIOC Supporter
Member#: 30669
Join Date: Dec 2002
Chapter/Region: Tri-State
Location: USA, North NJ, 07456
1998 Legacy 2.5GT
Silver Sleeper BK, 5MT

Default Compression test, leakdown test, how and what...

OK, I may make this into a "Nuts and bolts" thread (advance search, "nuts" in title, my username for my others).

What are the tests and how to do them.

Good link (from a search on here, good info linked in post 1)....https://forums.nasioc.com/forums/sho...light=leakdown

To start, get tools ready.
Compression tester (if you want that).
Leakdown tester (if you want that), compressed air (air compressor of some sort, the higher the pressure the better).
Spark plug socket and associated flex tools, extensions, ratchet, etc.
Tools to make it easy to get to plugs. Usually a 10mm to at least remove washer tank on older Subaru.
Fully charged battery and/or decent charger on battery for consistent cranking speed.
Some "about" 30wt oil in a squeeze bottle or oil can ( for compression wet test).
Socket for crank bolt (Subaru is 22mm or close US).
Preferably breaker bar for crank bolt socket.
Helper, not needed, but helps.......thus, "helper"......
A way to record info.....pencil and paper works.

Have tools ready.

Stupid notes......
Basic "rule of thumb" on compression numbers. A good shop manual is best, but lacking that, take.....
Air pressure (figure about 14PSI near sea level, less in Denver).
Compression ratio (called CR from now on)
Multiply them and "add a bit" due to dynamic air filling.
So, typically a turbo engine is about 8.5:1 x 14 = 119PSI.
Add in fudge, say 130+PSI.
For a NA, say 10.5:1 x 14 = 147PSI.
Add in fudge, say 155PSI.

Use numbers from FSM or other sources. I am just giving a ballpark.
You are looking at minimum numbers as well as spread between high and low. Usually it is a 10% spread. Many factors affect max number, mechanical tends to ONLY affect spread.

Get engine up to temp. Shut off.

Pull all plugs.

Install compression tester, throttle full open, crank maybe 6 times.

[question for some here, does a DBW go full throttle with key on, engine off, pedal to the floor?]

Record number.
Pull tester, squirt a shot glass or so of oil through the spark plug hole, install tester, repeat test.
Record number (this is a wet test, checks piston rings).
Move to next cylinder, repeat above until all cylinders have 2 numbers.

Move to leakdown test.

In some aspects, you WANT the piston at TDC.
Max combustion pressure is where the piston is near the top of the power stroke with max pressure (combustion pressure). This will yield max cylinder wall wear, thus max ring leakage.
Piston can be almost anywhere, sorta depends on holding it (TDC or BDC is best, pressure will hold it).
Again, near TDC on power is best!

[note for others, what is the FSM procedure, engine in car. I will assume all timing covers on....how do you determine cyl 1 TDC power, then how much to swing crank bolt to the next cylinder which is what....?, etc.....]

Crappy way is to move to next cylinder, breaker bar to spin crank, look for numbers but no air through exhaust, intake, oil fill.
Not the best.
I am hoping for........(paraphrased.....assumptions....)
Plugs out, remove DS timing cover, align cam mark/marks, rotate Y*, check cylinder x, rotate crank Y*, check cylinder y, etc.....

When a cylinder is under pressure, listen......
Hissing through intake, maybe intake valve/valves issue.
Hissing though exhaust, maybe exhaust valve/valves issue.
Hissing though oil fill cap, maybe rings/ringlands.

To me, this is "automotive mechanics 102".
Know the system, understand the mechanics.
This works on pretty much any IC engine regardless of number of cylinders, etc.

Hoping some Subaru techs can flesh out specifics on leakdown test crank info.
I have a full FSM for my '98, but I had to buy a '96, then the addons.
I now have 9 3 ring binders of info, makes it hard to deal with.
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Old 09-19-2018, 03:31 PM   #2
NASIOC Supporter
Member#: 30669
Join Date: Dec 2002
Chapter/Region: Tri-State
Location: USA, North NJ, 07456
1998 Legacy 2.5GT
Silver Sleeper BK, 5MT


Saved for updates.
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Old 09-19-2018, 03:33 PM   #3
NASIOC Supporter
Member#: 30669
Join Date: Dec 2002
Chapter/Region: Tri-State
Location: USA, North NJ, 07456
1998 Legacy 2.5GT
Silver Sleeper BK, 5MT


Saved for updates.......hope I can edit first post.
Seems like I can edit some a year later here, others I am locked out in 30 seconds.

Yes, I have noted to site staff.......
Charlie-III is offline   Reply With Quote

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