2 captured repliesDiscussion pagination complete · Source accounts, not a confirmed diagnosis
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Discussion summary
YOUR RESEARCH2004 C4S 3.6 - 50% leak-down at TDC
Leading reported evidence
Replace when the valve is moved
Suggestion or failed attempt
No successful repair reported for this finding.
0
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0
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1
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Suggestion
The big clue for me is that the leak noise changes when the valve is moved.
Leading evidence: 'Replace when the valve is moved' (no explicit attributable resolution; this is a reported action or suggestion). Across 1 matching discussions, The most frequently discussed action is 'Replace when the valve is moved' (1 discussions). For that action, 0 discussions report a resolution and 0 report an unsuccessful attempt. No action has an explicit, attributable resolution report under the current rules. These are forum reports and suggestions, not a confirmed diagnosis.
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2 eligible posts · 0 unverified posts · 1 discussions match the subject in their title or opening post.
911uk: 1 discussions
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Interesting situation and hopefully can get some guidance. , great compression leak-down at mid-stroke, leaky exhaust valve.
Lumpy idle but ran fine once given gas. Pulled plugs and Cylinder 1 was toast while all others healthy. No misfire codes.
Did a leak-down test. All cylinders at 95/100 at TDC except for Cylinder 1 which is at 50/100. If I turn the crank (not sure of the actual degree), I can get 95/100, just not at TDC.
With the headers removed, it’s very apparent that it’s leaking from one of the exhaust valves. If I wiggle the valve, the sound of the air changes.
From my research it sounds like it might be a broken spring or bad lifter. Does that sound likely to you? There are no signs of valve contact on the piston when scoping, so I don’t think it would be a bent valve.
I’m planning on removing the cams this weekend to see if anything sticks out as broken.
On a track day back in 2005 with my dad’s old 2003 Boxster I accidentally over-revved and did something to wear a cam lobe and he had to have the cam replaced - problem is he kept quiet about it and didn’t tell me until years later so I wouldn’t feel bad. He’s dead now so I can’t ask him if he recalls the symptoms. That said, I’ve never over-revved this car as far as I can recall.
That does sound like cylinder 1 has an exhaust valve that isn’t always seating properly. The fact it leaks badly when the piston is at the top of its stroke (TDC), then improves when you move the crank slightly, feels more like something on the valve side than the bottom end.
I’d be thinking along the same lines as you: spring, lifter/follower, valve seat, or possibly a slightly bent valve. I wouldn’t totally rule the last one out just because there’s no obvious piston mark. Pulling the cams and having a look sounds like a sensible next step. The big clue for me is that the leak noise changes when the valve is moved.
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