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Premium Member
2015 ML250 Bluetec and 1987 560SL
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3,581 Posts
Pull your rockers and make sure they're all in good shape. Dollars to donuts you have one that looks like the one on the top:

2610212


The one on the bottom is what it should look like. When I had this problem, the cams were fine.
 

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1986 560SL
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36 Posts
Why would a worn cam tower make a knock? Should I just release the followers and rotate the cam?
I'm new to these old SLs but not to 70's Datsuns (510, 240Z) which had a cylinder head design that was more or less copied from Mercedes-Benz. Those cams also turned within Aluminum towers just like these motors, and never were a problem, even at high revs.

I would try the cranking with plugs out, in case the problem is a loose wrist-pin. With plugs out, all "up" strokes will be without much resistance from compressed air, so a loose pin might click every revolution rather than just on the exhaust stroke. A friend had a Pontiac V6 with a really loose pin (we supposed) which sounded like the piston was hitting the head.
 

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'88 560SL '91 420SEL '73 280SEL '59 220S '73 450SL '74 450SL '86 560SL
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47 Posts
Discussion Starter #28
I have heard that there is more than 'a bit of oil'. Just looking at what the oilers put out is impressive. I am planning on pulling the followers and inspecting. Waiting on the spring compression tool.
 

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Premium Member
1986 560SL with M120 V12 Engine, 1988 560SL Stock
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9,732 Posts
Here is the latest. The flex plate is fine. There is no noise during cold crank with plugs in. Any reason to try it with plugs out? The knock is every other rotation. Attached is a trace of the audio. The engine is idling fast 1500 rpm. The trace shows about 13 knocks/sec which is 780 rpm.
Why would a worn cam tower make a knock? Should I just release the followers and rotate the cam?
So it sounds like cam shaft speed. Might try removing one cylinder of rockers at a time. By the way hows your compression.
 

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Premium Member
1973 450SL, 1999 c230
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914 Posts
Use a stethoscope and try to isolate the noise. Whatever is making that noise is already roched, a few more revs won't make any difference. Use a long screwdriver place up to your ear and push it against each part of the engine until you can find it. Watch out for moving parts and Electronicals....
 

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'88 560SL '91 420SEL '73 280SEL '59 220S '73 450SL '74 450SL '86 560SL
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47 Posts
Discussion Starter #33
Latest update:
I tried a stethoscope and cannot locate the knock. It seems to be everywhere I listen.
Waiting on the spring compressor to remove the followers.
Below is a link to audio files with and without plugs. I do not hear any knocking in either one.
https://drive.google.com/drive/folders/1r9eTDDkqisR3vZqCUhajHY-dYFVuHH6d?usp=sharing
If I push down on the compensators, they do not move at all.
Ran compression test, cold engine, no plugs. All cylinders are 170 to 175.
 

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'88 560SL '91 420SEL '73 280SEL '59 220S '73 450SL '74 450SL '86 560SL
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47 Posts
Discussion Starter #34
Got the followers out. number 3 cylinder is definitely worn. The cam does not show any signs of wear.
This is the #3 follower. Don't know if it's intake or exhaust but it's the one towards the rear of the engine.
# 3 follower shows deep wear:
2610530

The neighbors also show that the outer 'hard surface' is breaking through. the dark spots on the right hand follower are breaking through. I noticed other that have the same type of damage;
2610531

The cam doesn't seem to have any significant wear. this is the # 3 cam lobes:
2610532


Next step is to take off the timing chain sprocket and see what the cam towers are like.
 

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1986 560SL
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36 Posts
Any intention of starting the engine with those lifters out (being certain that the hydraulic lash unit can't come apart)?
 

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'88 560SL '91 420SEL '73 280SEL '59 220S '73 450SL '74 450SL '86 560SL
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47 Posts
Discussion Starter #36
Not sure I understand. The engine won't start with half the followers removed.
 

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'88 560SL '91 420SEL '73 280SEL '59 220S '73 450SL '74 450SL '86 560SL
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47 Posts
Discussion Starter #37
Removed the timing chain sprocket. the cam turns smoothly and I don't have any play in the cam with the followers removed.
Should I consider replacing the followers only? I'll check the left hand side for signs of the same of course.
 

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1986 560SL
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36 Posts
Not sure I understand. The engine won't start with half the followers removed.
I misunderstood that half the engine was disabled- I thought that just two valve followers were removed where you suspected the problem, in which case it would run (poorly) on seven.
 

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Premium Member
2015 ML250 Bluetec and 1987 560SL
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3,581 Posts
If any of the followers are other than smooth on the top, replace them. They're cheap. And I guarantee that replacing that dished one will cure the knock.
 
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