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Cam Timing Slipped & Bad Actuator?


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Throwing this one out to the masses.

I had an opportunity to diagnose a 1999 996 C2 (not my own) which has some very odd symptoms.

The car is idling somewhat rough, but not alarmingly so, and there is a slight pop (not quite a backfire) from bank 1 exhaust (psgnr side pipe). That is pretty much the only unusual noise, aside from some slight "pinging" noise coming from bank 2. The owner reports its down on power throughout the range.

I scanned the stored codes (no CEL) and found the following:

p1531 Camshaft adjustment, bank 1

P0341 Camshaft Position Sensor 1 - Signal Implausible, Short to Ground, Short to B+

p1340 Timing chain out of position, bank 1, Below lower limit or above upper limit

Ok, probably bad actuator right! So I looked at the cam deviation, both very steady.

Bank 1 25 degrees

Bank 2 3 degrees

Everything makes sense so far, the actuator is stuck "open" or in the relaxed advanced intake cam position (which is 25 degrees TDC by design).

Here's where it gets very very strange.

I cleared the codes, and then ran the engine again, at idle and 3000 rpm. I actuated variocam on bank 1 manually with Durametric at warm idle the engine responded as I am used to for this test with a wildly fluctuating idle and a whoooshing noise. Bank 2 behaved the same way.

Head scratching time. Re-read the codes:

p1340 Timing chain out of position, bank 1, Below lower limit or above upper limit

P0341 Camshaft Position Sensor 1 - Signal Implausible, Short to Ground, Short to B+

?????? How could this be, actuator seems to be working in both banks, 25 degrees in bank 1 (stable), AND a p1340 ????

So I rotated the engine to TDC, and pulled the green caps to look at the exhaust timing slots.

Bank 2 was straight up.

Bank 1 EXHAUST appeared to be about 12 degrees advanced at the cam (25 deg at the crank).

What is strange to me is that variocam advances the intake cam only since the exhaust cam is fixed to the IMS tube with the drive chain. If an actuator fails, I can't surmise a reason why an actuator failure would cause an exhaust cam to advance by 25 degrees TDC.

So in other words, variocam appears to be working correctly in bank 1, but that bank has somehow managed to advance itself (both intake and exhaust) to 25 degrees. Although the P0341 might indicate that we can't rely on the intake cam position and need to check the allocation manually.

I halfway want to just re-time the engine and see what happens, but the other half of me suspects doom and gloom, unfortunately, and I guess with wild speculation until the cam covers are pulled, a huge variety of possibilities.

Cam sprocket was fine but then somehow slipped after 3000 miles on the car since internal engine work

Cam to cam chain slipped, jumped several teeth

Exhaust cam to IMS tube jumped several teeth

Guide rail, tension, or cam pad problem (too much slack?)

IMS tube bank 1 sprocket slipped

Seizure in valve train forced exhaust cam sprocket to slip exactly 25 degrees (coincidence that variocam advance is also 25 degrees?)

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First response too cryptic .If I read your description correctly the exhaust cam timing has definately slipped .The reason for the question on the bolts was to ascertain if the sprocket could have been loose and the slot end stoped it (not very likely ).

This sounds very similiar to the Kellison issue .However .a failing tensioner could also be a potential candidate as well as one of the paddles .It also might be instructive to understand what work was done 3k ago .Just to determine what was touched .

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I'll look at kellison's thread again, thanks.

Yes, highly improbable the bolts would have worked themselves loose & backed out, considering the engine was running perfect for 3000 miles before ths happened after the internal work. Even say in the situation where someone forgot to torque just one of the bolts, I can't see it happening. It should have happened almost immediately after the engine was started and not 3000 miles later.

The work I understand was basically a LNE ims bearing, valve job, new lifters, cam pads, variocam chains, etc. They stopped short of breaking open the crank case.

I am thinking at this point either the sprocket was forced to slip violently, despite correct torque and thread locker, which I might see some tell tale signs of scraping or dragging on the sprocket face OR the slippage is at the other end, for example insert/interference fit failure on the IMS tube 1-3 chain sprocket.

I'm not sure if the piece on the cam that the cam sprocket bolts into is also press/interference fit and could rotate as well or if the sprocket bolts direclty into the cam and there's no way that it could move execpt at the sprocket. That's a far fetched idea anyways.

The cam would probably break before it twists that much.

Certainly it could be a broken ramp, causing relaxed tension, but I dind't hear any unusual noises from the chain box area.

I don't think the sprocket is at the max adjustment, because if I remember right I think there is more than 12 degrees (@ the cam) of adjustment avialable, maybe 2x to 3x that. Unless someone didn't re-install it with the bolts near the center of the slots. This is something I was wondering too and will pull the pump before pulling the cam covers.

Just strikes me as very odd that the amount the DME is reporting the intake cam is off by exactly the same amount of the variocam advance, and the exhaust cam seems to be just the same.

Edited by logray
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Here was the thread, thanks again:

http://forums.rennlist.com/rennforums/996-forum/621739-996-cam-timing-tools.html

The car I'm looking at didn't have an IMS failure though, the retrofit was elective surgery.

In reading LNE's site and this thread above leads me to believe the sprocket on the ims doesn't usually slip unless there is a bearing failure.

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