O.K., many here recall I bought this poor old Coupe some years ago and have been fiddling with endless problems both known and hidden ever since .
I rebuilt the turbo charger on the tailgate of my old GMC pickup truck a couple years ago, it was fairly easy as promised by Ken of Mercedessource who supplied the overhaul kit and new shaft .
I don't have metered compressed air and so didn't know how to test the wastegate, now I discover the diaphragm is bad along with the "A.I.R. valve on the upper part of the turbo and the ALDA too, ALL THREE are leaking vacuum or pressure 
.
.So, off I go to learn what's what and teach myself how to service, repair and adjust all this stuff .
Basic wastegate, ALDA and A.I.R. testing is simply : remove the hose at the far end (away from the wastegate, ALDA or A.I.R. valve) and gently apply vacuum or suction , it *must* hold vacuum or the internal diaphragm is ruptured and the device will not work properly until it is repaired then adjusted, symptoms include : lack of power, smoking out the exhaust, low fuel economy and you angrily thinking 'why did I ever buy this old piece of junk ?!' .
Happily these are all fairly easy and simple repairs due to Mercedes good engineering practices ~ they knew these were top of the line cars that would get seriously heavy duty use worldwide (maybe not so much in North America and Canada 
) and so need servicing of these imprtant and poorly understood parts / systems .
) and so need servicing of these imprtant and poorly understood parts / systems .So, my wastegate doesn't hold vacuum, that means a double whammy of both no boost control and loss of boost due to the internal leaking between the intake and exhaust manifolds .
Simple temporary fix : disconnect and plug the hose leading to the turbo, this instantly gives the engine more power but it also allows the turbo to spool up to TWENTY POUNDS PLUS of boost if I romp on it, a very bad thing indeed for engine life and reliability .
My temporary overboost solution is to mount a boost pressure gauge under the edge of the left speaker grille and keep an eye on it, ease off the throttle whenever it goes past 13 # of boost .
The next step was to replace the leaky ALDA valve on top of the injection pump with a good used one (I tested it before removing from the junk yard car) installing it then adjusting it *slightly* leaner by loosening the 10MM ATF lock nut and turning the slotted adjuster screw 1/4 turn _clockwise_ to lean out the idle and off idle mixture a little bit .
The performance is a bit improved and the fuel economy rose by about 1 - 1/2 miles per gallon, once I properly fix the wastegate and adjust the total boost and the ALDA I expect the car to get at least 26MPG's, possibly more, I'm a bit of a lead foot so maybe not .
I stumbled across a 1984 Diesel Coupe the other day and took it's A.I.R. valve apart, it had a good blue diaphragm I hope to install soon .
I'm still not c;lear on what this 'A.I.R.' valve does exactly, I was told it helps shifting, making it smoother or something, who knows ? .
To - day I was harvesting a brandy new W126 driver's side window lift for Tom's '87 30SDL and decided to take a look at an old 1980 W116 300SD, it was very greasy but most of the engine was untouched so I decided to try Ken Bergman's tip on taking the wastegate diaphragm out ~ I first suction tested it, good, now to find the tiny end of the double circle flat retaining ring so I can take the good used diaphragm out using a sharp pointed tool ~ Ken says to use a small flat bladed screwdriver ~ Ken's video is very nice and clear, unfortunately after messing about with it under the sun for two hours I gave up and dismounted the turbo complete, then it was a snap to get the locking ring out and the wastegate apart, good used diaphragm in my hand .
However...........
I didn't really understand how the boost pressure is retained in the intake manifold for power or allowed to dump into the exhaust flow so I took the flat plate off the exhaust manifold and discovered a big exhaust valve inside the exhaust manifold, the diaphragm has a large spring that pulls is shut and it takes (depending on the adjustment) over 10# to pop it open, it's supposed to be a fairly LOUD 'click' when it pops open under heavy boost .
HOWEVER ~ this valve was suck about 1/2 way open in spite of the rest of the wastegate being in good shape, this no doubt made the big, heavy W116 slow and sluggish off the line along with horrible fuel economy (think you're 197-anything malaise era American sled when the EGR valves failed, like that) , I got my ballpeen hammer and gently tapped the valve and it came out, it looks O.K. and I'll clean off the accumulated carbon deposits on it, I expect the same valve in my Coupe and prolly every other old smoky Mercedes Diesel you've ever seen has this same issue even if there's not a ruptured diaphragm in the wastegate .
Anyway, lots leaned and much food for thought here ~ I have some non EGR intake and exhaust manifolds I've been thinking of putting on my Coupe, maybe now is the time .
Stay tuned, more to come, hopefully some pictures too .
-Nate
Seriously tired and beat up right now
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Posted by: Nate <vwnate1@yahoo.com>
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