First post of this thread shows the layout of the overboost (switchover valve) system.

Your broken line is the one marked 56 in the line drawing in figure 3.

The banjo bolt hard line on your ALDA, as you trace it backwards, it runs to the switchover valve which is attached to the bulkhead (next to the brake booster).
The other hard line attached to the switchover valve (with a small rubber connector) is the broken one.

If you trace the broken line to the PS side of the engine, it should be attached to the rear of the intake manifold.

Your broken line is now venting the boost to the atmosphere preventing the ALDA from permitting the full range of fuel delivery by the IP.

__________________
78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

Very helpful! I will lay eyes on it again this evening when I return home. Thank you thank you!

Something I'm a little worried about.. If the line broke off flush with the banjo bolt, I wouldn't be able to slip the vacuum line over a male end. Would I need t remove the banjo bolt and insert a fresh hard (plastic) line?

Dang, Alec - yer a bit of OK. Having ditched all this stuff I'm useless on this kind of thing but it's good to know that there are folks on the Forum who can and will help out.

Dan

Any line that doesn't collapse or break will work. Max boost pressure isn't high and many cars need to have the boost adjusted back to specs - usually higher. Destructions are in the archives.

RE: installing the valve cover, have all of the linkage moved out of the way. You may find it helpful to tie the injector return lines back out of the way. Don't pull so hard that you create leaks. I use small zip ties bought from Harbor Freight for next to nothing. Practice R&R without the gasket and it becomes easier.

Regarding the gasket not leaking, make sure that is properly and easily seated on the cover. I've taken to cleaning the cover with whatever degreaser is handy then installing the gasket. If it jumps off, it needs to be shifted on the cover. Giving the gasket a little stretch seems to let it seat. I set the cover on a piece of glass to make sure that the gasket sits flat.

My most recent attempt at getting a leak-free seal was to clean the sealing surface of the head and apply a little Hi-Tack to both the head and bottom of the gasket. Don't install the cover until the sealer becomes tacky.

This is the 1st time in a while that the head is completely dry. I haven't tried taking the cover off yet or cleaning the Hi-tack off but will probably this weekend when I do another valve adjust.

Some don't have trouble with a leaking VC but both of my 617s do. I've looked carefully at the covers and and also tried the cover from the parts car. Neither car has much blow by.

Your results may vary.

__________________
85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

Easy-peasy, just insert the appropriately sized vacuum line where needed for a temporary/permanent fix.
The car will run fine, the hard line just doesn't degrade as fast as rubber would from exposure to heat, oil, and time.



On my 78 W116 300SD, the car had remnants of two broken hard boost sense lines when I bought it in January 2014.
Both had snapped at the banjo fittings, one at the end of the intake manifold and the other at the ALDA.
The PO left the remnants of the hard lines on the barbed ends of the banjo fittings and installed vacuum lines over the ends.

In the photo from April 2016, you can see the vacuum line attached to the banjo bolt on the ALDA, and the remnant of the hard boost sense line that goes to the rubber connector on the right of the switchover valve.


The top white plastic part of the switchover valve is the vent that opens to release the overboost pressure.
The electric overboost signal comes from the boost sensor on the intake manifold, and the sensor wire is plugged into connection on the left of the switchover valve.

The other hard boost sense line that would attach to the bottom of the switchover valve is absent...the vacuum line coming off the bottom of the switchover valve is the replacement for that line.



Many have eliminated the switchover valve and plumbed the boost sense line directly to the ALDA.
Some have taken it further and removed the ALDA altogether and run the boost sense line to a mechanical boost gauge (or plugged the intake manifold boost fitting).



Thanks for the compliment, Dan. I'm still learning.
I appreciate what this active forum had taught me, and contribute where I can.

Attached Thumbnails

 

__________________
78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

All great info! Alec, you were spot on. I swung by the local parts shop and grabbed a few extra feet of vacuum line just to keep in the glovebox. I was able to old, crusty tip off of the banjo bolt very easily and replaced it in about 2 seconds.

Last night I started the car up for the first time in a little over a week. Seems to run better. Could be the new vacuum pump, or a few of the other broken vacuum lines that ended up getting replaced as I found them. Coincidentally, the car had struggled to turn off if it hadn't been running for long and now she turns right off. Accidental repair :-)

'Goose - NO, NO, NO!!! NOT an accidental repair. You are now an Internet expert having actually made something work (unlike MOST Internet experts). You are now qualified to dis all over anyone's post on any automotive topic.

At least, those seem to be the rules as far as I can tell.

Dan

This link will help(disguised so it's not pulled)

dieselgiant (D0t) com/mercedesaldaboostsystemservi.htm