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can a cam chain jump teeth and still turn over?

As the title suggests, is it possible?

I ask as i have experienced a sudden drop (disappearance) of compression on all cylinders.

Engine turns over, i have spark, i appear to have fuel (smell/wet plugs/fuel pump working) but have no start. I'll do a CIS system pressure test shortly but the drop in compression is my big concern.

The car is a ROW 1980 911SC i imported from japan. I have had it about 12 months but driven it probably 5-6 times as it isn't yet registered. As part of the import process here in Australia, we need to get Australian Design Compliance. Before driving it to the tester i checked the compression on all cylinders, and got a 125-130psi reading on all cylinders in (dry and cold). I assume it would be higher if wet and warm. Ahem.

I drove the car to the compliance assessor who in the process of assessing it needed to drive it. They called suggesting it wouldn't idle under 2000rpm, and then again later that it wouldn't start.

i trailered it home and it now has no, repeat, NO compression. It has spark, fuel, but no start. It turns over, and having taken the inlet cam covers off the cams are turning but no compression. I am wondering if it has had a tensioner fail, jumped a cog and now wont get compression but will turn over and not start.

Intuitively this seems unlikely, as a valve would hit the piston to stop it turning over. But i cant explain the no compression, the lack of starting with fuel and spark.
Old 01-02-2018, 02:16 AM
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A tensioner failure on one side would not affect the other bank. Could have been over-revved and bent all the valves. There would be a lot of extra valve clearance on the bent ones if that was the case. Pop the other covers and go through the adjustment procedure one cylinder at a time. No need to use a feeler gauge for now, just work the rockers by hand and see if any are really loose. Really tight isn't good either obviously, got to have at least .004 clearance.
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Last edited by john walker's workshop; 01-02-2018 at 05:59 AM..
Old 01-02-2018, 05:53 AM
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I had this happen on one bank, due to a crankshaft oil plug that came out and jammed in the chain (I guess). I noticed the engine lost power before it lost oil pressure. Broke a bunch of rocker arms and the engine needed to be rebuilt
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Old 01-02-2018, 12:36 PM
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No way would I have let them drive it w/o me there. Sounds like they money (yours) shifted it.
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Old 01-02-2018, 06:08 PM
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Do a leak down.

Wouldn't hurt to check the cam timing, but as said, sounds like bent valves.
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Old 01-02-2018, 06:22 PM
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I feel sick thinking about it.....
Old 01-02-2018, 09:21 PM
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Any recourse against the agency and the genius tester who did this to your engine?
Old 01-03-2018, 03:22 AM
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We're just ass-u-me ing right now, until further inspection.
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Old 01-03-2018, 05:48 AM
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if the timing has changed, one side will be different than the other, if low compression do a leak down to check for bent valve or do as JW said.

125-130 is low. could be your tester?
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Old 01-03-2018, 06:59 AM
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Quote:
Originally Posted by john walker's workshop View Post
A tensioner failure on one side would not affect the other bank. Could have been over-revved and bent all the valves.
In reality, if there is no compression, the valves are all bent. Before coming to this awful conclusion, I would check your compression gauge first.
Old 01-03-2018, 01:33 PM
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Agreed a leak down would be the next check, as well as checking valve clearances for bent/damaged valves.

I had previously looked at doing a leak down but couldn't get to the crank pulley nut to turn it to TDC (engine mount give no clearance with a two belt pulley)

Anyone got a tip on turning the thing over ?

cheers

Ben
Old 01-03-2018, 03:08 PM
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Quote:
Originally Posted by 88-ben-88 View Post
Agreed a leak down would be the next check, as well as checking valve clearances for bent/damaged valves.

I had previously looked at doing a leak down but couldn't get to the crank pulley nut to turn it to TDC (engine mount give no clearance with a two belt pulley)

Anyone got a tip on turning the thing over ?

cheers

Ben
Put your socket on the upper pulley; you may need to put pressure on the back side of the belt or squeeze it together if it slips when you turn it.
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Old 01-03-2018, 04:12 PM
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Quote:
Anyone got a tip on turning the thing over ?
Put it in 5th gear and push the car forward.
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Old 01-03-2018, 04:43 PM
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Push in on the right side of the belt as you crank the alternator nut clockwise.
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Old 01-03-2018, 05:28 PM
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I love how so many of these posts just dead end with no explanation.
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Old 02-22-2018, 02:46 PM
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A follow-up explanation would be great so we all can learn. This forum works best when information moves both ways. Rob
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