Author Topic: 6.9 engine seized while cranking  (Read 4498 times)


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  • W116 Enthusiast
  • Location: Salt Lake City, Utah
Re: 6.9 engine seized while cranking
« Reply #75 on: 21 June 2020, 07:51 PM »
That engine bay is amazing. Those rocker covers.....  8)
F__k yeah!  They were polished  by a local shop, more or less to a mirror finish.  Why MB just painted them black is beyond me, especially when that's what's lurking underneath.  Too flashy I guess.  But damn......
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0


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  • W116 Enthusiast
  • Location: Miami & São Paulo
Re: 6.9 engine seized while cranking
« Reply #76 on: 22 June 2020, 02:04 AM »
so how did the crank measure up?     ...and congrats on getting it all back and going.!
Thanks! Crank was fine.  I went through that last fall, can't recall details.  I didn't mess with the mains but replaced rod bearings.  Oil pressure is excellent, but possibly a function of 30 weight break-in oil.

The elusive smooth idle.  hehehe!!.........yeah..............I've spent a 100 hours on this if I've spent 2 minutes.
I've got mine pretty good across the entire temp spectrum but just be satisfied with hot idle in drive at 650 rpm.
The key is hot control pressure and evenness of fuel delivery at the injector nozzle.

I have designed and built my own fuel head and injector test bed.  It's a fascinating science but as these babies are getting older, the cast iron FD's are proving snarky in reliable and consistent fuel flow due to wear.  10000's litres of fuel washing thru them over 30 years is actually wearing them out.
Fuel deliv at idle is down to 6cc per min. max variation is 1 cc. that's on just the delivery. Then there are nuances of the injectors...dirty..cracking pressure.. leakage.
In hindsight I think the FD rebuild actually went well.  I had help from an unnamed collaborator on this board with mucho experience with the iron FDs.  The original diaphragm was quite deformed from wear and the metering ports uneven.  The polishing process fixed all that, which is key to even flow (I guess).

If we go back to the beginning. air flap clearance..idle fuel ratio..........these have little range and must be spot on.
All of these moments are truth are almost exclusive K jet considerations..but of course you still have the basic engine fundamentals that need to be correct....We  haven't come to air leaks yet..!   evenness of compressions.......accuracy of valve lift and timing.
As I said.......if you can achieve a reasonably smooth hot idle take a bow and unless you are OCD about it, be content with near enuff is good enuff.
Turns out my WUR, which had been rebuilt by the famous CIS Flowtech, was substantially out of whack.  It seemed ok before the rebuild, but no longer.  Maybe running the system without the FD metering piston toasted it, who knows.  Among other things cold pressure was close to zero.  Anyway, I've got a box of WURs, one of which was the high-altitude version out of a parts car.  I swapped and guess what? Everything instantly improved instantly. All the pressures are spot on.  Idle is much, much better.  And stable.  Before it would be fine, then go bad at the next traffic light, then ok again.  I got a Gunson CO meter in the mail and will play with that.  In the meantime the car is running well. Just gotta get the transmission rod right.  So tedious - it's close but not perfect.  Finally, I looked for vacuum leaks with a smoke tester.  No leaks, system seems to be tight.

Next will be checking lifters and cam timing, but I'm no big rush.  The car got a new chain, rails and sprockets.  During assembly timing was pretty close (way better than it had been), but not perfect.  Somebody had messed with the woodruff keys and done it backwards if I recall.  Anyway, I want to get some mileage on it first so it's settled in.  Small stuff remains.  One headlight wiper doesn't work and neither do headlight washers.  Cruise control is kaput.  AC needs recharging.  Suspension light problem needs diagnosis.  But none of that prevents motoring fun in the meantime.  Most of my friends have long questioned my sanity.  This removes all doubt.  ::)  And so it goes....

Great progress. Congratulations!
1978 - 450SEL 6.9 - 3170
1978 - 450SEL 6.9 "Parts Car" - 2973