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Old 11-21-2017, 09:45 AM   #1
Scooby Newbie
Member#: 472151
Join Date: Aug 2017
Default 2005 Subaru sti 0 compression in cylinder 1

I was wondering if somebody could help me figure this out please? I bought this with 85k miles on it a few months ago with the intent to rebuild the motor with forged internals and other upgrades and what not. The problem was it would have a weird knock at idle. Probably a bad bearing or something o thought. Still it ran as good as you would expect it to. A couple weeks ago I was having grounding issues with the battery. I fixed it and when I started it it was throwing cylinder misfires and was running terrible. I ran a compression test on the cylinders and 2, 3, and 4 were all good. The thing is I was seriously getting 0 compression from cylinder one. I did the test 5 times and got the same thing every time. I decided it was probably time to rebuild anyways. Pulled my engine and heads and nothing was obvious. Valve seats look fine nothing's burnt which was what I thought it would be. Then pulled the pistons and the rings were still intact. Now if I was getting 0 compression and take the block apart shouldn't this be an obvious thing to find for air to be leaking that bad. Anyways before I pulled the pistons I flipped the engine to where I could fill cylinder 1 with oil. I let it sit all night and it didn't leak any oil I did the same with the head on the valve seats and nothing leaked. I have all the parts I need to just about finish the rebuild and put it back in, but I don't really want to do that until I know I'm not going to have the same problem I had when I pulled it out.
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Old 11-21-2017, 12:55 PM   #2
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Join Date: Dec 2002
Chapter/Region: Tri-State
Location: USA, North NJ, 07456
1998 Legacy 2.5GT
Silver Sleeper BK, 5MT


Well, since the gauge showed decent values on the other cylinders, it is likely working.
Yes, with "0" compression on a cylinder, you would expect to see something wrong once it was apart.
Could be as simple as a dropped valve guide that can hang a valve open at times, thus no compression. You should be able to look in the valve ports and see a dropped guide.

Before pulling the engine, doing a leakdown would have been best, then it would tell you if it was valves or ring/piston related.

What was the room temp when you did the oil test?
What weight oil did you use?
If a heavy oil in a cool room you may not get much leakage, but using kerosene flows pretty good in any temp.
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Old 11-22-2017, 01:26 AM   #3
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Member#: 477892
Join Date: Nov 2017
Chapter/Region: NESIC
Location: Waterbury Center, Vermont
2000 Impreza Wagon


Could be a slightly bent valve. Even though the seats look ok, it takes a perfectly straight valve with a perfect valve face-to-stem transition for the valve to seal correctly.

Did the compression gauge read anything at all during the test? 0 is low lol Either the valves in that cyl were bent, the rings were shot, you have a massive hole in the piston (which you said you didn't) or your timing belt jumped on that cam and the valves were timed incorrectly.
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Old 11-26-2017, 05:44 PM   #4
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Member#: 472151
Join Date: Aug 2017

The oil that I use in the block is 5w 30 but the oil I used in the cylinder and head was 5w 20 to see if anything was leaking. I was thinking it could still be a bent valve I'll check that tomorrow. The rings and pistons were all perfectly fine and the pressure gauge did kind of move every few turnovers but that was it. The temperature was around 50 degrees F when I did the test as well. I have all the parts to finish the rebuild now, but I don't know that I want to finish it put it back in just to have to pull it out again because the problem isn't fixed.
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