Ernst,

The switch you refer to fuel enrichment switch is called a switchover
valve.  If its buypassed (as in your case), loss of power is not one
of the symptoms as a result.  The black smoke upon accelaration is the
unburned fuel.

When were the fule filters replaced?  How is the air filter?  Might
want to do some "Italian tune-up."  Also you might want to consider
diesel purge.

If the above suggestions don't work, you might need to look at
injection timing, ALDA adjustment, etc.

anil bharucha
1985 300SD 197,xxx miles

One never knows what all was done to a used car.

Black smoke is a sign of unburned fuel - too much fuel for the available
air. As some moron has already messed about nothing can be taken for
granted and all possibilities need to be checked.

These engines have a poor oil recovery system for the crankcase fumes
and the engine's paper air filter gets soaked with oil and can't let the
air pass through to the engine - as the engine gasps for air smoke is
the result. If this is the case install the newer one piece crankcase
vent hose that's formed for this purpose. If that's already installed
then the next step is inside the valve cover which we'll get to later.

The plastic line from the intake manifold is in fact a pressure line
that transmits the turbo's boost pressure to the injection pump. The
disconnected switchover valve is a safety valve to prevent a run away
engine condition in the unlikely case of too much turbo boost (boost is
limited by the wastegate, a 10 psi pressure relief valve). From the
switchover valve the line runs to the ALDA which adds fuel to the engine
as the boost increases. But go back to the point of connection at the
intake manifold, this is called the "banjo fitting" and gets clogged
with soot and oil, so remove the hollow bolt and clean it and the
fitting with some wire so the boost pressure can be passed to the ALDA.

The ALDA is what idiots always mess up in their attempts to make these
relatively small diesels perform like Corvette V-8s. The ALDA has an
adjustment (bench set at the factory) to balance the need for
acceleration vs. minimum smoke. Inside the ALDA are some sealed brass
bellows that contract ever so slightly as boost increases. This slight
contraction is transmitted via some levers to the fuel rack to add fuel
to the engine, for fuel, not air, makes the power.

If the air filter was found to be OK then you need to back off the ALDA
adjustment to cut the smoke. The adjustment is at the top center of the
circle of the ALDA the device on the aft end of the injection pump to
which the boost line is attached. The adjustment to reduce fuel is to
turn the screw IN. Be very conservative and make only small adjustments
of say 1/4 turn, then drive it, adjust it, drive it. The smoke will be
reduced as you progress. But you should know at the outset that someone
may have broken the brass bellows and no amount of adjusting will fix
that, only new Aneroid capsules from Bosch.

The other possibilities: Needs a valve adjustment so badly that it has
poor compression and makes smoke. The valves should be adjusted after
15K to 20K miles.
Throttle linkage needs to be adjusted so throttle opens faster when
accelerator is pressed.
Four speed transmission is not starting out (from dead stop) in first
and / or up shifting too soon. Adjust transmission / throttle position
cable.

If you do a valve adjustment (I can send you some instructions) the
crankcase vent baffle in the inside top of the valve cover should be
checked to ensure it's relatively clear to allow fumes to pass. The
valve timing and condition of the timing chain can also be checked then.

So those are the possibilities. e-mail me if you have questions
Tom

All the replies with advice are very much appreciated.
I started out with replacing the 3 buffer bumper supports for the
aircleaner as it was almost disconnected and was then able to check the
turbo by turning it with my fingers; seemed ok. The airfilter was dirty
although not excessively oily and was replaced. Changed the oil and filter
and front brake pads.
Interestingly, when I took the car for a testdrive it drove quite well
with good power and did not have the sluggishness and smoke as before, so
I thought I had licked that problem. However, a couple of days later I
took it out and the same problem of smoke and no power had returned. It is
obiously an intermittend problem.
In the meantime I started to hook the vaccuum lines back by using my 300CD
as an example. When I took the car for another drive it ran ok but then
would not shut off and that had to be done under the hood manually.
I also found a severed vacuum line (main yellow) in the driver's door
which I repaired, but door locks still dont work automatically.
My next step is to follow Tom's advice to clean out the banjo fittings and
the oil line to the turbo. Then back to checking the vacuum lines
Tom, your input is invaluable!
I will continue to update this saga which will hopefully help others as
well.
Thanks,
Ernst