Deceptive engine knock
10-19-2009, 08:35 PM
#1
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Deceptive engine knock
A few weeks ago my engine started to sound like a diesel truck. The noise seemed to be coming from the drivers cyl head. I could feel a vibration when I touched the cam cover and distributor cap. I removed the cam cover and dist cap to inspect. Everything looked good, so I towed it to my shop.
Put it on the lift and removed the belly pans. Fellow lister Eturbo924/Eric came along for the diagnosis. When I started the engine his mouth dropped. The noise was louder and I could feel knocking on bottom of oil pan. Was a very stressful moment at first.
The vibration when touching dist cap kept calling me back. I removed cap, rotor and upper timing cover. I was amazed at what I found. The cam bolt was loose, so I tightened it and noise was gone. What happened was the cam hub had cracked and broke in 2 pieces. This caused the bolt to be loose. The rotor screws were holding everything together at that point. I replaced both hubs and reset my cams.
I was lucky, it could have been much worse. Strongly suggest replacing cam hubs if your in there.
Put it on the lift and removed the belly pans. Fellow lister Eturbo924/Eric came along for the diagnosis. When I started the engine his mouth dropped. The noise was louder and I could feel knocking on bottom of oil pan. Was a very stressful moment at first.
The vibration when touching dist cap kept calling me back. I removed cap, rotor and upper timing cover. I was amazed at what I found. The cam bolt was loose, so I tightened it and noise was gone. What happened was the cam hub had cracked and broke in 2 pieces. This caused the bolt to be loose. The rotor screws were holding everything together at that point. I replaced both hubs and reset my cams.
I was lucky, it could have been much worse. Strongly suggest replacing cam hubs if your in there.
10-19-2009, 09:08 PM
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Those are problematic at best.
You dodged a bullet on that one, it could have broken a cam, bent valves, done all sorts of things.
They were on back order when I built my engine.
You dodged a bullet on that one, it could have broken a cam, bent valves, done all sorts of things.
They were on back order when I built my engine.
10-19-2009, 09:37 PM
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Paul, are the replacement hubs aluminum? Or steel?
10-19-2009, 09:48 PM
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But, the originals are AL right? Or are they steel to start?
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10-19-2009, 09:54 PM
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They were steel also. Had they been aluminum it would have shredded filings everywhere. The new hubs are factory Porsche from the dealer. The package had a manufacturer date of 8/2009. I'm hoping the newer hubs have modern technology steel and are stronger than 80's steel.
10-19-2009, 10:05 PM
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That's a big hunk-o-steel to be broken like that.
10-19-2009, 10:27 PM
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Zeus,
Put that on eBay and you will probably get $1000 as a piece of art.
Glad you ducked on that one.
Roger
Put that on eBay and you will probably get $1000 as a piece of art.
Glad you ducked on that one.
Roger
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Does it have the "Do It Yourself" manual transmission, or the superior "Fully Equipped by Porsche" Automatic Transmission?
George Layton March 2014
928 Owners are ".....a secret sect of quietly assured Porsche pragmatists who in near anonymity appreciate the prodigious, easy going prowess of the 928."
Does it have the "Do It Yourself" manual transmission, or the superior "Fully Equipped by Porsche" Automatic Transmission?
George Layton March 2014928 Owners are ".....a secret sect of quietly assured Porsche pragmatists who in near anonymity appreciate the prodigious, easy going prowess of the 928."
10-19-2009, 10:59 PM
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Am I missing something?
10-19-2009, 11:12 PM
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IIRC this seems like a not-that-common failure mode. But, the question in my mind is why? With the big bolt properly tightened and with the rotor bolts even and not over-tightened to the point of snapping there shouldn't be a lot of force on that piece. Or IOW, the cam belt shouldn't be dragging the sprocket around via force on that piece and thus the keyway.
Am I missing something?
Am I missing something?
Harmonics maybe.