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Old 05-29-2010, 12:56 PM   #1 (permalink)
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Default low compression on #4

OK, here goes. 99 Forester with 2.5 SOHC. Put in new head gaskets, timing belt and everything that goes along with the job. After starting I have rough idle and po304 code. Have only 35 psi in number 4, but it will hold 125 psi during leakdown. Took timing belt off and reinstalled seemed to run better, still rough idle but no po304. runs great on highway but can hear spark knock sometimes. po170 code started showing up so i changed both o2 sensors to no avail. runs rougher than ever and po304 is back. still low compression in #4 any ideas?
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Old 05-29-2010, 02:02 PM   #2 (permalink)
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Any issues before the timing belt install?
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Old 05-29-2010, 03:13 PM   #3 (permalink)
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had oil leak on the back of the left head and close to 100,000 miles so i did the gaskets and timing belt. had the heads surfaced, changed water pump, all idlers.
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Old 05-29-2010, 03:18 PM   #4 (permalink)
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Did you check the valve clearances? Also what percentage of leak down did you have on this cylinder anything over 10 percent is a problem.
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Old 05-29-2010, 03:56 PM   #5 (permalink)
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If the heads were dis-assembled to be machined the valve clearances need to be checked. Assuming you didn't install the HG on that side wrong and don't have the T-belt off by one or more teeth I don't have much for you.

-To me the 35psi comp test but holding pressure on the leak down test would indicate a valve issue, adjustment, sticking or cam timing

-Check the wires for the #4 injector to see if one was pulled out/cut while things were being moved around.

-try swapping plug wires with #4 to see if it follows.
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Old 05-29-2010, 04:34 PM   #6 (permalink)
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new wires, plugs and injectors when HGs changed. also valves adjusted and cams checked. i will go back and readjust valves, maybe i got a tight one. way under 10 percent on leakdown. i drive this car to work which is 350 miles round trip and it gets 26-28 mpg and runs great when driving but is getting worse on idle, shakes car noticable. have put almost 8,000 miles since HGs.
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Old 05-29-2010, 06:22 PM   #7 (permalink)
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If your leak down is way under 10 Percent you should have more than 35 psi during the compression test unless it is a tight valve. Are you doing the compression and leak down test with the motor hot?
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Old 05-29-2010, 07:09 PM   #8 (permalink)
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Check the valve adjustment. If the valves were for some reason set way loose, like if they were mistakenly set while the cam was at some point in the valve lift, that could explain the poor comp. test numbers but the good leak down numbers.

Only other thoughts I have are if the cam timing is off as was said beforeor if the valve adjuster lock nuts cam loose and the adjusters backed off.

How does cylinder #2 check out for the comp and leak down test? If that is also poor that would point to cam timing. If it is good the issue is narrowed down to cylinder #4.
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Old 05-30-2010, 03:05 PM   #9 (permalink)
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adjusted the valves and took a compression test.
1) 120
2) 120
3) 165
4) 35
i kinda leaning toward timing. is there a good site that shows how?
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Old 05-30-2010, 03:43 PM   #10 (permalink)
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Well the only timing you can set is the timing belt The marks on the cam sprockets and crank sprocket all go to the 12 o clock position. Try a can of engine restorer http://www.restoreusa.com/faq.html

You add a can of this stuff and if it stops the p0304 you need to re-ring the motor or rebuild it I have seen this stuff stop mis-fires for almost 4000 miles I could not believe it!
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Old 05-30-2010, 04:35 PM   #11 (permalink)
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it's hard to believe it needs an overhaul but i'm leaning that way also. it ran fine until the HG and timing belt job. the cylinders looked great with no ridge. the engine does not smoke and no blow by from the crankcase. i'm not as familiar with subaru as other makes but this stumps me. thanks all i'll post the out come.
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Old 05-30-2010, 07:38 PM   #12 (permalink)
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Quote:
Originally Posted by jackdog View Post
adjusted the valves and took a compression test.
1) 120
2) 120
3) 165
4) 35
i kinda leaning toward timing. is there a good site that shows how?
With it being just one cylinder I doubt it is timing. If it was cam timing it would affect both the #4 and the #2 cylinder. I would redo the leak down test on the #4 and look/listen to the intake and exhaust valves for any air passing through them, same with listening to the crank case via the oil cap.
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