Edinsf

I would bet a nickel that the cause of the problem is the timing chain spun on the cam gear because of the cam gear loosing the tops of their teeth. This is what can happen when when the car has loooong intervals between oil changes.

nizzki

Could be, although i've done more frequent oil changes than the porsche's recommended interval. I did one ~3k miles ago.

Edinsf

^^^ not saying you nizzki but if you bought the car used and not new the previous owner might not have changed the oil frequently.
Because of my sometimes short drives, I change my oil at 3000 miles. It's a cheap price to pay for engine longevity. What oil do you use?

race911

Which has happened exactly never with the several hundred 911 engines I've either had my hands in or have seen apart. Granted, 95+% of them were little ol' normally aspirated engines of the '70's and '80's......... But at least a couple were 935s that ran boost beyond belief.

Macster

Based on my understanding it is not possible. There is not enough clearance between the valves and the piston for the piston to move up and down if the valves are open any. With a camshaft not turning some valves have to be open. (Unless the open valves have lost their heads from the piston contacting the valves and the engine can then spin in which case it would be running -- if one can call it that -- on 3 cylinders, the 3 cylinders of the unaffected cylinder bank.)

Regardless, my advice would be to not run the engine any at all. Let the teardown happen and let the shop determine what is wrong. The engine may be salvageable for much less than the cost of a full rebuild or engine replacement.

race911

Guess you've never torn into a 911 that came into your shop with a tensioner that laid down. They'll still limp along on three. Yeah, Olden Days, but same thing.

But back to basics. Someone with actual expertise on a DOHC air cooled based can correct me, but I don't see any timing chain breaking. Losing time? Sure. Look at how they cheaped out on holding the cam sprocket in place early into the 993 production. Before that, it was held in place by a Woodruff Key. While I've not personally seen a 993 engine with merely torque of a 12mm bolt failing, I know of a couple of instances where it's been lost and the cam stopped spinning.

Edinsf

You're talking ancient history. Things are different now.
The year is 2014 and oil has changed and so has the recommended oil change intervals.
Sticking to Porsches recommended oil change intervals is a recipe for disaster.

Macster

Doesn't make any sense.

The layshaft (sort of an intermediate shaft) is gear driven from the crankshaft. These gears do not lose their teeth except possibly under anything but the most rare of failure scenarios. (Any gear teeth failures are most often the result of some other failure that causes metal fragments to get caught in the gear teeth which can cause all sorts of horrific collateral damage to the gear teeth, the gears, and other hardware.)

There have been a few reports of one of these gears becoming loose on its shaft and then spinning which upsets the cam timing. It would be a lucky failure to have this occur and *not* have the cam timing so disturbed that no valve/piston contact occurred.

It is I believe possible for any of the several cam chain sprockets to suffer the same failure mode and again it would be a very lucky failure to have this occur and have the cam timing change *not* cause valve/piston contact.

There is also the rare failure that has one cam chain/sprocket that jumps one or more teeth.

This changes the cam timing for the affected cam or cams and because the change in timing is relatively small (and can happen in a timing direction that has some margin for error) valve and piston contact do not occur. But the engine will run very poorly and possibly run for all practical purposes on one bank.

With any interference engine -- that is any engine in which the pistons can not go up and down if the valves are not closing and opening in proper time with the piston movement -- it doesn't take much valve timing error to result in valve/piston contact. Except on the most well exceptional cases, if valve/piston contact happens almost invariably you can kiss the engine goodbye.

race911

So you have repaired DOHC timing sprocket failure on this generation of engine?

What shop do you run, and how many of these cars do you have in your customer base?

Macster

The air-cooled engines may be non-interference engines in which case a non-turning cam would still have an engine that runs albeit one that runs poorly.

AFAIK, all of the water cooled Boxster, Cayman, 911 engines are interference engines. A non-turning cam is not going to let the engine run because of valve/piston interference with one possible exception. The contact breaks the valves and then piston can continue to go up and down although with valve heads embedded in its crown or possibly in the top of the combustion chamber.

Timing chain breakage is rare.

A cam drive gear or sprocket coming loose and allowing a cam to jump time or even stop spinning altogether is also rare.

But they can happen.

In any event, I look forward to reading what the OP learns about his Turbo engine's ills. I hope the problem isn't that severe and the engine can be resurrected without breaking the bank.

Edinsf

^^^talk to Jake Raby. You might learn something.
Have a great day.

race911

They all are. And don't confuse the issue by bringing in M96, M97, 9A1, four and eight cylinder water cooleds, or even pushrod Porsche engines.

Kevin

I have two customer engines sitting here torn down with broken timing chains.

It is rare but it is happening. My advice to the OP is FIRST, pull the oil filter, if you see metal in the canister and or filter STOP.. The engine is a write off at this stage. The monies that you spend in labor to tear down can be applied to a new engine or good used. IF the chain breaks, it will WRAP itself around the intermediate shaft, the bundle of chain IMS mass will tear into the engine case webbing. The two engine cases that I have are cracked due to the rotating mass of chain and IMS.

race911

I take it this was directed at me, since it was posted basically at the same time as the other.

I'm happy to talk to experts when I'm in need. In 1980 it was Bruce Anderson, Jerry Woods, and Jim Pasha when I was 18 and had a broken 911 engine. Been a long, long, long list over nearly 35 years. Pete Zimmerman (and Walt) were life savers a couple of times through the '80s and '90s.

But what I asked was whether you personally owned a shop, and your first hand expertise dealing with cam drive (either camshaft or intermediate shaft end) sprocket failure. All you could do was direct me toward a name shop owner, then sign off dismissively. Just the kind of customers I loathed walking in the door.

race911

Thanks so much. It's knowing there are guys out there like you who will help me transition from decades of air cooled fiddling into this generation of 911 that dropped in my lap.

Quick question though--is it the DOHC adaptation to the original engine design that's the problem? Seems we were fine, even on stupid horsepower 930s, with the -3.2L ramps/tensioners/gears.