Author Topic: 6.9 engine seized while cranking  (Read 2106 times)

raueda1

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6.9 engine seized while cranking
« on: 06 August 2019, 04:46 PM »
Preamble:  this is a fresh reboot of this https://forum.w116.org/mechanicals/6-9-ring-gear-destroyed/.  The old thread got too long and the problem seems to be something else anyway.
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Background:  After doing Fuel Distributor work I tried to start the engine.  I messed up the FD a little and flooded the engine.  I then rectified the FD and tried to start again.  The engine was firing and almost started.  It stopped abruptly however, with a hard clunk.  In the process I apparently cranked too hard for too long and burned out the starter.   I replaced the starter with a rebuilt unit from a reputable source.  On the next try the engine cranked about 1/2 a revolution and stopped abruptly with the same hard clunk.  Thereafter it wouldn't crank at all.

What I tried and looked at
  • Removed starter, cover plate on the opposite side and the plate at the bottom of the bell housing to see what I could see of the ring gear.  Everything visible looked OK.  Obviously I could only see a fraction of the ring gear.
  • I tried rotate the engine with a giant breaker bar on the 50mm bolt on the harmonic balancer.  It was impossible for me to turn it at all.  Absolutely no movement of the crankshaft. 
  • I removed spark plugs.  Even with plugs in it should turn, but why not try anyway?  No change.  It was clear that the plugs were black and sooty from running way too rich while trying to start.  I'm sure that will burn off if I ever get running again.
  • I removed one valve cover to have a look.  No obvious issues there (broken springs, destroyed chain or whatever).  I'll look at the other one tomorrow, but I'm not holding my breath.

In all cases I tried rotating CW (the right way) but also CCW just to see if it would go backwards even a tiny amount. It didn't - jammed solid in both directions.  I'm using a 24" breaker bar and lots of muscle to turn it over.  There is absolutely zero crankshaft movement.  Clearly something is jamming the crankshaft.  I think it's significant that after the first clunk the new starter DID rotate it a little but was followed by the same clunk. 

Now I've now exhausted all my logic and am at my wits end.  Clearly something is jamming the crankshaft.  I'll try any ideas, however far fetched.  Thanks for any and all help.  Cheers,
 

-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

Pete49

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Re: 6.9 engine seized while cranking
« Reply #1 on: 06 August 2019, 08:35 PM »
Drop the sump and see if a conrod has dropped and jammed it though turning it in reverse should allow movement.
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oversize

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Re: 6.9 engine seized while cranking
« Reply #2 on: 06 August 2019, 09:59 PM »
I'd pull BOTH rocker covers and thoroughly inspect the chain & guides. Check the bellhousing that you didn't drop a bolt or something in there.  If no joy you'll have to pull the motor and drop the whole sump. If not joy there then it's front covers off. If nothing is revealed it's then heads off....
1979 6.9 #5541 (Red Bull)
1978 6.9 #4248 (Skye)
1979 6.9 #3686 (Moby Dick)
1978 6.9 #1776 (Dora)
1977 450SEL #7010 white -P
1975 450SEL #8414 gold -P

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Re: 6.9 engine seized while cranking
« Reply #3 on: 06 August 2019, 10:52 PM »
Dave I would agree with the others that you should remove both valve covers and inspect thoroughly.  If everything looks good and the chain etc is all in order then I would be looking at external issues eg in the bellhousing.

Get yourself one of these cheap USB I section cameras from ebay and look down each plug hole

'76 6.9 Euro, '78 6.9 AMG, '80 280SE, '74 350SE, '82 500SEL euro full hydro, '83 500SEL euro full hydro , '81 500SL

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Re: 6.9 engine seized while cranking
« Reply #4 on: 06 August 2019, 10:54 PM »
Removing the sump in situ is possible but not easy, need to lift the engine quite high off the mounts.  Before doi g that try looking into the sump drain hole with the USB camera.

'76 6.9 Euro, '78 6.9 AMG, '80 280SE, '74 350SE, '82 500SEL euro full hydro, '83 500SEL euro full hydro , '81 500SL

rumb

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Re: 6.9 engine seized while cranking
« Reply #5 on: 07 August 2019, 07:15 AM »
This is a tough one!  As others have said I would start with the other valve cover and look for broken chain guide pieces that may have fallen and jammed somewhere. One of those usb cameras could help look down into engine. You could a look at both cam alignment marks to see if they are somewhere near the same on both sides. Something broken in valve train? Chain jump I would think most likely at start up whereas rod I would think fail at high rpm.
Overall I would rate valve train damage as most likely, something on the backside of engine 2nd and rods last.
I cant remember doesnt the 6.9 have an inspection plate on the front bottom of bell housing?
'68 250S, '77 6.9 euro, '91 300SE, '98 SL500 '14 CLS550

raueda1

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Re: 6.9 engine seized while cranking
« Reply #6 on: 07 August 2019, 09:10 AM »
This is a tough one!  As others have said I would start with the other valve cover and look for broken chain guide pieces that may have fallen and jammed somewhere. One of those usb cameras could help look down into engine. You could a look at both cam alignment marks to see if they are somewhere near the same on both sides. Something broken in valve train? Chain jump I would think most likely at start up whereas rod I would think fail at high rpm.
Overall I would rate valve train damage as most likely, something on the backside of engine 2nd and rods last.
I cant remember doesnt the 6.9 have an inspection plate on the front bottom of bell housing?
Thanks to all for the comments and ideas.  I'll start with the other valve cover.  My new borescope has been ordered and will arrive today.  The real fun will begin tomorrow.  Robert, yes there is such a cover plate.  Unfortunately what can be seen looks good. 

Seems like an engine manual would be helpful.  The manuals here just seem to stop with the 4.5l engines. I'll start scouring the interwebs, but if anybody has a lead on an 6.9/M100 engine manual please do let me know.  German is fine.

I'm amazed that after all the triple digit speeds, high temp desert driving, etc etc, that just starting the car in a garage has so utterly screwed it up.  I'm being punished.  Good grief!  Regardless, I'll post findings as they're found.  Thanks again,
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

daantjie

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Re: 6.9 engine seized while cranking
« Reply #7 on: 07 August 2019, 09:47 AM »
Dave there is a 100.985 service manual in the Library under "Technical".  It also has a Combustion and I think Emissions sections too.  It is transposed from micro fische so the pics are useless but info is good.
Daniel
1977 450 SEL 6.9 - Astralsilber

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Randys01

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Re: 6.9 engine seized while cranking
« Reply #9 on: 07 August 2019, 08:29 PM »
As soon as I read the 1st sentence I thought "hydraulic lock". it is easy to do with K jetronic. However, removing the spark plugs should have allowed the motor to turn.
...mm

OK......b4 the borescope arrives take a piece of wire and carefully wrap some tissue around it tightly.  lower into a the plug holes and see if it comes out wet. keep going until all 8 are done. If they are dry then drain  the engine sump oil only from the sump. Ordinarily you only get about 3 litres.
Examine the nature of the oil..does it seem super thin and watery?  did you get significantly more than 2..5 l to 3 litres?
With the plugs out and the sump drained........disconnect the fuel pump relay.  and crank the motor.
If it's no go, then you have deeper problems but I would eliminate any possibility of hydro lock b4 hitting the panic button.
Do not force anything with crowbars/ whatever if she won't turn on the starter.




raueda1

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Re: 6.9 engine seized while cranking
« Reply #10 on: 07 August 2019, 09:42 PM »
As soon as I read the 1st sentence I thought "hydraulic lock". it is easy to do with K jetronic. However, removing the spark plugs should have allowed the motor to turn.
...mm

OK......b4 the borescope arrives take a piece of wire and carefully wrap some tissue around it tightly.  lower into a the plug holes and see if it comes out wet. keep going until all 8 are done. If they are dry then drain  the engine sump oil only from the sump. Ordinarily you only get about 3 litres.
Examine the nature of the oil..does it seem super thin and watery?  did you get significantly more than 2..5 l to 3 litres?
With the plugs out and the sump drained........disconnect the fuel pump relay.  and crank the motor.
If it's no go, then you have deeper problems but I would eliminate any possibility of hydro lock b4 hitting the panic button.
Do not force anything with crowbars/ whatever if she won't turn on the starter.
Interesting idea.  What exactly is hydraulic lock?  Fuel filling one of more cylinders so it can't crank?  And then fuel gets into oil and thins it out?  Or the same but with coolant?
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

Randys01

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Re: 6.9 engine seized while cranking
« Reply #11 on: 08 August 2019, 12:33 AM »
Hydraulic lock occurs when one cylinder or more fill with petrol. This condition can occur when  mucking around with the FD on K jetronic resulting in a continuous stream of fuel squirting into the cylinders. I have encountered this problem myself...as have quite a few other folk ...who have delved into the wonders of K jet.
The condition can also occur if we get water into a cylinder..eg blown head gasket. This happened to me 2 days ago when a Nissan  popped the head gasket..water was drawn into the cylinder and damn near locked the motor.

But having now read your earlier threads on starter motor -to ring gear- to fuel distrib, something tells me this is a problem that is all related and has been some time in the making. Seemingly, the case for conventional "cylinder hydraulic lock "seems to weaken partic as you have removed the plugs. ie the lock up is not in the cylinders. You have removed all the plugs ?...yes?

 It is drawing a long bow, but it is possible to jam a motor from turning if there is too much oil/fluid in the sump.
Hence my suggestion to drain it. As u are aware the 6.9 is dry sumped. This is a bit misleading coz of course it has a sump but only to collect and scavenge. Oil supply management is drawn from the reservoir tank.
It is theoretically plausible and possible for the sump/crankcase to overfill and lock the piston downstroke. There are a couple of ways this can happen...all of them bizarre.
Impossible I hear the experts chortle!

Ordinarily in this motor, the scavenge pump would keep the sump at the right level even if it had filled with surplus petrol draining from the cylinders past the rings and into the sump..
But what if the scavenge side of the pump is not returning the oil to the reservoir.? The reservoir would be empty and the sump would be full.
So you need to determine that the oil level in the reservoir is in the right range [yes I know..that level is determined when the engine is running but we can't run the engine ]HOWEVER...a static dip of the tank will tell you if it's emptyish.
If heaps comes out of the sump  then you have the answer.

Resolve "crankcase hydraulic lock" before moving on to the next scenario.
...and yes...it is a long shot.
 

TJ 450

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Re: 6.9 engine seized while cranking
« Reply #12 on: 08 August 2019, 12:37 AM »
I hate to say it, but with the sheer volume of fuel K-Jet is capable of delivering, hydro-lock is likely.

It can and will bend the conrods, a car Nathan now owns had this occur and the engine was rebuilt as a result.

Tim
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1969 300SEL 6.3 1394
2003 ML500

daantjie

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Re: 6.9 engine seized while cranking
« Reply #13 on: 08 August 2019, 07:57 AM »
Man, the suspense is killing me here :o
Daniel
1977 450 SEL 6.9 - Astralsilber

raueda1

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Re: 6.9 engine seized while cranking
« Reply #14 on: 08 August 2019, 09:53 AM »
Thanks for all this, extremely illuminating.  I've seen several cars with hydraulic lock from water - driving through floods, at the beach.  It once happened to my wife when her car was submerged.  But it never occurred to me that it could happen from fuel.  Anyway, see more below....

Hydraulic lock occurs when one cylinder or more fill with petrol. This condition can occur when mucking around with the FD on K jetronic resulting in a continuous stream of fuel squirting into the cylinders. I have encountered this problem myself...as have quite a few other folk ...who have delved into the wonders of K jet.
This most definitely DID occur, at least briefly, and the engine was cranked until it was clear that it wasn't going to fire.

But having now read your earlier threads on starter motor -to ring gear- to fuel distrib, something tells me this is a problem that is all related and has been some time in the making. Seemingly, the case for conventional "cylinder hydraulic lock "seems to weaken partic as you have removed the plugs. ie the lock up is not in the cylinders. You have removed all the plugs ?...yes?
Yes, removed plugs.  But the borescope arrived.  Same day delivery from Amazon!  Sso I'll be having a look inside anyway shortly.

It is drawing a long bow, but it is possible to jam a motor from turning if there is too much oil/fluid in the sump.
Hence my suggestion to drain it.  As u are aware the 6.9 is dry sumped.  This is a bit misleading coz of course it has a sump but only to collect and scavenge. Oil supply management is drawn from the reservoir tank.  It is theoretically plausible and possible for the sump/crankcase to overfill and lock the piston downstroke. There are a couple of ways this can happen...all of them bizarre.  Impossible I hear the experts chortle!

Ordinarily in this motor, the scavenge pump would keep the sump at the right level even if it had filled with surplus petrol draining from the cylinders past the rings and into the sump...
But what if the scavenge side of the pump is not returning the oil to the reservoir? The reservoir would be empty and the sump would be full.
Interesting.  What scenario(s) would cause this to happen apart from pump failure?

So you need to determine that the oil level in the reservoir is in the right range [yes I know..that level is determined when the engine is running but we can't run the engine ]HOWEVER...a static dip of the tank will tell you if it's emptyish.  If heaps comes out of the sump  then you have the answer.
We'll know the answers in a few hours!  Keeping my fingers crossed that this IS the case, and nothing else is broken.  Thanks again.  This mystery deserves a youtube channel with millions of followers.  Cheers,
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0