380SL Timing Chain Stretch and Jump Question - PeachParts Mercedes-Benz Forum

ctaylor738 · #1 12-16-2002, 05:07 PM

This is technically out-of-bounds for this forum, but I'll ask here anyway since it's fairly basic mechanics and we have some great problem solvers.

On Friday, the Wayward Woman ('81 380SL) suffered a fairly catastrophic event. Started it up after a short drive and immediate loss of power and very rough idle. Got it home and discovered:

1. No compression on #1.

2. With marks on right cam/tower aligned, showing 23 deg BTDC on crank pulley (!).

3. With the crank at TDC, the cam mark is about .25" PAST the mark on the cam tower.

The engine has a lot of slop in the chain, and it is pretty clear that it has jumped. It seems to me that since the chain is turning the cam, that the chain would advance clockwise (from front) ahead of the cam gear when it jumped. Therefore, with the cam aligned with the mark, the crank should show something ATDC.

Similarly, I would expect that with the crank at TDC, the mark on the cam would be BEHIND, not PAST the mark on the tower.

I am just not getting this. Can anyone explain what is going on?

TIA

gmask · #2 12-16-2002, 05:16 PM

Quote:

Originally posted by ctaylor738
1. No compression on #1.

What do you think happened to #1? Did the valve crash the piston?

psfred · #3 12-16-2002, 08:18 PM

23 degrees is just about one whole tooth. you need to check the timing on the left bank, too -- it is possible that you jumped a tooth on the crank, although I would expect it to be late rather than early. Make sure you are reading the crank pulley correctly -- rotates toward the drivers side.

I suspect you have a bent valve in #1 unless the plug broke. Probably bent valves in other places too -- I recommend that anyone with a 380 bite the bullet at some time or another and replace the single chain with a double chain -- cheaper than the valve job when it breaks. Certainly, it is vital to check chain stretch often. Ditto for the 420, same problem.

Peter

ctaylor738 · #4 12-16-2002, 08:38 PM

Peter/Gmask -

Just finished for the evening. It was one tooth off. I reset the chain on the right sprocket and got it back to TDC with the marks aligned. Hoping against hope that the misaligned cam was keeping the valves open, I then re-ran the compression test with the same (dismal) result.

So I clearly get to replace some valves.

Started without delay to pull the right head. Alternator is off, and about half of the intake.

What a pisser.

BTW - it was BTDC, and it is a double chain already.

MikeTangas · #5 12-16-2002, 08:48 PM

I don't know why the time stamp is so far off tonight. I posted my reply an hour and forty-six minutes prior to the stamp

.

Oh well, I'll never understand computers.

MikeTangas · #6 12-16-2002, 09:46 PM

If it only jumped one tooth, it likely hit and bent valves. I agree with Peter, you need to check the left bank as well. I know when I was doing my t-chain on the 560 without benefit of a guide, I managed to jump the right can one tooth. That one tooth allowed for piston/valve contact, preventing me from turn the crank by hand. Careful thinking and work allowed me to retime the cam and continue.

Looks like you've got some head work in your future. If your 380 hasn't been converted to a double row chain, now will be a prime time to do so.

All in all, the whole matter bites

.

Just dawned on me, if the upper guides are intact, you might have worn through or broken one of the lower guides, that would create enough slack for the chain to jump. You'll likely have to pull the front cover to verify.

ctaylor738 · #7 12-17-2002, 10:36 AM

I am agonizing over the lower slide rails. According to the manual, you need to pull the oil pump to get the cover off. Since the 107 has a one-piece oil pan, This means engine removal in my garage setup. The upper rails look good - not brittle and no damage.

I am (almost) certain that the chain jumped because of stretch that I did not recognize. When I replaced the cams, the assembly marks were right on at TDC.

Right now my thinking is to go with a new chain and the upper rails. But first I will drain the oil and if there are any plastic pieces in it, then the cover comes off.

psfred · #8 12-17-2002, 09:19 PM

Jumped timing chains bite.

Check the valve clearance on #1. Dead certainty that if a valve on #1 is bent, so are the rest, though, makes me wonder what the deal is on the lack of compression.

If the valve clearance is abnomaly large (should be 0.035" or so), the valves are bent. If not, something else is wrong.

Right side isn't too bad, though, compared to the left -- Hans routinely pulls the engine to do valves, its so bad.

Make SURE you have replaced the plastic bits for the oiler tube and that you are actually getting oil dripping on the cam before you run it, or you will be replacing a camshaft next -- words of wisdom form Hans -- almost impossible to remove the cam without breaking the plastic holders for the oiler, a classic amateur repair resulting in more work (this is why I buy all my parts from him!)

By the way, you are lucky -- the usually result of excessive stretch is a broken upper rail that gets between the chain a crank gear, breaking the chain, then folding it up and driving it through the front cover on the left side.....

Good luck!

Peter

ctaylor738 · #9 12-17-2002, 10:52 PM

Thanks to all for the comments and the sympathy. I have a spare head off the original US engine that got a clean bill of health from the machine shop. MB parts guys say it will work on the euro engine, so that will speed things up a bit and save some money.

Next thing to hope for is that all the head bolts come out cleanly and that there are no surprises in the cylinder or block.

MikeTangas · #10 12-18-2002, 12:51 AM

Keeping our fingers crossed for you Chuck.

ctaylor738 · #11 12-22-2002, 06:27 PM

So here's a progress report.

The head is off, and all of the pistons have marks where the valves contacted the pistions. Other than that, you could not tell that anything happened - the valves look fine.

The removal was fairly straightforward. The intake is a pain because you need to remove the injection pipes and the manifold bolts are a PITA to get at. The head came off without any thread problems (thank you, God).

So now I have the cam mounted on the replacement head (so I can use the valve spring compressor). along with my new ball studs from the other head. Next step is new valve seals. Then it all goes back together with the new chain and upper rails.

Biggest challenge has been the nuts on the bottom of the exhaust manifold.

leathermang · #12 12-22-2002, 10:10 PM

while you have the head off... you might consider putting new studs on the bottom side of the exhaust manifold... and put copper nuts on it.. two per stud really protect it... and don't fry/corrode on the stud like steel nuts...

psfred · #13 12-23-2002, 09:49 PM

Any hints on the exhaust nut removal for the left mainifold? I'm looking at doing that little job (along with subframe mounts and upper trunion) after New Years when I get back from Florida.

Peter

ctaylor738 · #14 12-23-2002, 10:50 PM

I only did the right side, but based on that:

- Approach from below, hopeless from above
- Hopefully, you have brass nuts so with a little shot of PB blaster, you can get them off with a 1/4" drive ratchet and a 12mm deepwell. That's about all you have room to swing.

psfred · #15 12-23-2002, 11:24 PM

That's pretty much what I figured. Now, if I could just get that pesky steering box out of the way.....!

Peter