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Background... I have a '99 Impreza Outback Sport with 200k miles. It's had a slow oil leak for about 35k miles, that hasn't been serious enough for me to learn how to deal with it. This winter it started having issues where it wouldn't produce much power when it was cold, and way less than usual when warm. After about $800 in parts and 30 hours in my own labor (work moves slower at sub-freezing temps) I think I'm nearing the end of my journey.

I did a compression test after double checking that the timing belt was aligned correctly and got some somewhat disturbing results:

Cylinder 1: 185 PSI
Cylinder 2: 80 PSI
Cylinder 3: 135 PSI
Cylinder 4: 135 PSI

I figured that since the car was it's 200k mark, and the head gaskets have never been looked at, as well as they were leaking oil, that they might have just lost compression.

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So, after getting my driver side head off the engine today, I started looking at how my valves are doing. Apart from being coated in a nice layer of carbon that I have to figure out how to clean I can see where at least half if not more of my compression issues are coming from.

It turns out that my head gaskets were mostly leaking oil, and I couldn't find any issues with the cylinder seal or coolant seal.

After looking for ways in which I can clean my valves, I found multiple references that every 100k miles the valve clearance needs to be checked. I know this has never happened. Looking at the valves none of them are obviously bent, however only the cylinder with 185 PSI of compression actually has the valves close all the way at TDC, otherwise the rest of them vary to some extent.

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So my questions are:

Before putting everything back together, what would be the best way to clean all the carbon buildup off the heads and valves?

The valve adjustment process doesn't seem difficult compared to what I've been doing so far, so should I just do them myself?

Any other concerns that I should address at this mileage, with the car where it is now (Heads off, but engine still in the car)?
 

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01 Outback H6 VDC, 97 GT wgn w/ ej22, 98 OBW w/ej22
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sending the heads to a machine shop, to be cleaned and checked, $135 in my area, will clean all the surface carbon. this will also check to see if they ''seal''.

if they do not seal, then they need to be either re-seated, (in theory, you could do this) or replaced. replaced is rare if they are not bent or burnt.
 

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I would pull the engine and replace the oil separater plate. That's probably where your leak is coming from and it will only get worse as time goes on.
Then you'll have to pull it anyway.

O.
 
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