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After doing some maintenance on a 2000 Porsche 911 engine (3.4l normally aspirated), I find that I can manually rotate it with a wrench on the flywheel, to a certain point but no farther and I can't figure out why it won't rotate.

The details

I removed the engine from the car and replaced the rear main seal and the intermediate shaft bearing, which may have been seeping some oil and is a notorious weak spot for these engines. The intermediate shaft bearing replacement was done using a procedure recommended on Pelican Parts. (The linked procedure is for a Boxster rather than a 911, but the procedure is essentially the same.) The chain sprocket behind the bearing is held in place temporarily by set screws to prevent it from rotating and messing up timing. Then the bearing is replaced and the set screws removed. I did all of that and then decided to recheck the timing to make sure everything was still OK. It wasn't.

With all 6 spark plugs removed, I was able to rotate the engine through about 150 degrees or so, and then it won't turn any farther. I thought that perhaps I must have made some error and that somehow the timing was slipped (but note that I don't know that this is the case or not) so I used a borescope to look inside each cylinder. However, while some valves are open, there is still clearance between every piston top and every valve bottom.

This diagram shows the orientations of the relevant pieces although this diagram is a slightly later version of my engine (mine has 5 chains; this one has 3). The intermediate shaft is below the crank shaft and the intermediate shaft bearing is not quite visible in this picture. However it's easy to visualize where it is -- it's at the far end (top of picture) of the intermediate shaft, just below the main crankshaft bearing. Porsche engine diagram

So my question is, why won't my engine turn?

Additional notes

Here are some additional notes that may help better convey essential details I inadvertently left out of the original description.

  1. I would turn it to TDC to check static timing but that's what I was attempting to do when I encountered this unexpected problem.
  2. The cam-locking procedure step that was inserted as a note into the linked procedure wasn't in the procedure I followed (which was in a printed book and not online), so the cams were not locked prior to this prepare and are not locked now.
  3. The cams can't be locked now because the tool only works when the engine is at TDC, and see point #1.
  4. I do not want to risk further possible damage by rotating the engine backwards.
  5. Based on measurements of the angles of the camshaft ends (which are visible since I've removed the protective caps), it appears to me that if the timing is off, it's not off by much.
  6. The engine is currently on an engine stand with the spark plugs and transmission removed.
  7. I'm willing and able to undertake the additional effort of removing the exhaust and cam covers, but would absolutely rather avoid it until I can first make some convincing diagnosis that requires it.
  8. I have the full sixteen volume Porsche shop manual for this vehicle.
  9. I'd really rather be driving. :(
share|improve this question
    
Great question! – Zaid Jan 7 at 19:18
    
A few things that will be useful for anyone potentially answering this question. Could you mention the steps or add a link to the Pelican Parts procedure that you followed? I have to ask whether the procedure calls for the locking of the cams and crankshaft prior to replacing the seals. I also know that the timing chain can put the cams out of time slightly due to the slack present in the chain but I'd imagine this should not be an issue if the cam and crank sprockets were always engaged. Another comment - 180° isn't enough sweep on 4-stroke engines - you have to be able to rotate it 720° – Zaid Jan 7 at 19:28
    
Also, you mention that the cams are out of time. Did you confirm this by looking at the timing marks or is this a guess on your part since the crank isn't rotating freely? – Zaid Jan 7 at 19:32
    
It would be useful to know if this is the 3.4 L or 3.6 L engine – Zaid Jan 7 at 20:05
1  
I've updated the question to try to address the questions. – Edward Jan 7 at 20:22

The answer is one of:

  • Friction

    For example, rod, main, balance or intermediate shaft, and/or cam bearings, oil or water pump, or piston rings are too tight. Re-assembly of dry clean components without using assembly lubricant will also cause excess friction.

  • Compression

    Air pressure resisting movement. Removing spark plugs will help here (in this case, already removed).

  • Interference

    If the cam(s) is/are off time, the valves can interfere with the pistons, preventing rotation

  • Locking

    Usually the state of the transmission/clutch/torque converter is the reason for locking

share|improve this answer
    
The OP indicates that the spark plugs were removed when he performed the test. I don't think the transmission is attached to the engine here (since he's replacing the rear main seal and intermediate shaft bearing) – Zaid Jan 7 at 19:45
    
I was going for the comprehensive explanation, but for clarity I'll update the answer. – kmarsh Jan 7 at 19:47
    
Because plugs and transmission are removed, it seems we can eliminate compression and locking as causes. Because I've looked with a borescope, it does not appear that valve interference is the cause and other than the repairs mentioned, nothing else has been done to the engine which was running before I removed it from the vehicle which would also seem to eliminate friction. I can't think of any other reasons it won't turn; I'm hoping one of you can! – Edward Jan 7 at 20:45
1  
Are you missing any tools? – kmarsh Jan 7 at 21:54
    
:D Good question! No, all tools, bolts, washers and the family pets are all accounted for. – Edward Jan 8 at 1:25

You need to confirm that timing is OK

This forum thread looks to be talking about the same engine as what you have (3.4 L, 5 chains).

According to the post, this is what you'll have to do:

  1. rotate the engine to TDC for Bank 1 (let's hope your 150° of freedom allows for this)

  2. check that the timing notches on the cams are parallel to the cylinder head (from this thread). I know the image here is a little small but it's the best I could find.

    Cam timing

  3. repeat 1 and 2 for Bank 2 TDC, which you won't be able to do if your engine is only rotating 150°.


I'm hoping that you can confirm that timing is out on the one side you can check. If you can't check either side then I'm not sure how else you could confirm if timing is to blame.

Based on the forum threads I've just reviewed though, I wouldn't be surprised if your timing is out.

share|improve this answer
1  
Good thoughts and I sincerely appreciate the effort. Unfortunately, I can't get it to TDC to check. What I did as a substitute was to measure the cam angles using those same notches and then calculate the timing. It seems OK, and I've updated the question to add that clarification. I will find my paper notes on that procedure and transcribe them into the question when I get some time to do so later today. – Edward Jan 7 at 21:09
    
Additional Note #9, that is the crux of the problem :) – Zaid Jan 7 at 21:11
1  
yes, but it's also an effective motivation for me to solve the problem! :) – Edward Jan 7 at 21:19

I have a 2002 Porsche 996 sometime last month I had The beginning failure of my intermediate shaft bearing The engine still running fine but you could tell he timing was off And of course heard the noise. when I try to rotate the engine , the metal filings inside the engine melted one of a chain tensioners (left side) producing a big ball of black plastic on the chain that got squished between the teeth. although the timing was Knocked off a little bit I was only able to rotate engine to a certain point and it was because of the ball of plastic Contacting the sprocket. doesn't look too bad for me but I do have a serious cleanup ahead of me and a few parts. Guess it's better than a complete catastrophic.If you have a flexible borescope try Removing the oil pump located on the camshaft left side Sliding inside you might see more stuff toward the intermediate shaft.Good luck with yours.

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