Yeah, I must have been sleeping. The loss results in a drop of manifold pressure. Whether the turbo can make up for the loss before the wastegate opens is the question.

No compelling argument, whatsoever.

Don't want to sound totally dumb, but is the best way to check by pulling off the fresh air intake hose or by seperating the turbo from the outbound flex exhaust connection?? Also, not sure how to inspect the wastegate without pulling the entire turbo off the manifold. Any advice here? Would attach photo of my turbo setup but I can't figure out how to reduce the file size. I think the waste gate is just inside the intake portion according to the Haynes diagram. Or is it the round canister thing that is attached to the hard rubber hose you guys said could be leaking?
Thanks,
Gene

That's it.

I'd still like to know what PSI boost I should expect from a properly operational turbo on this car?

The service manual for the 617.95 engine says 0.7 - 0.8 bar at 4000rpm, transmission in S or L. In round figures, 10-12 psi.

That round canister has a strong spring inside it that should keep the wastegate fully closed until sufficient manifold pressure builds up to open it. It's possible that the wastegate itself has some carbon that prevents it from fully closing, or that the spring failed and the gate is just flopping around. However, normally, these are pretty rugged devices and I'd be inclined to double check to be sure the hose is intact on both ends and has no cracks.

Hey,
Pretty good troubleshooting done here, good job.
I would replace that wastegate hose. Just for peace of mind. then getr to adjusting the boost pressure. I guess you have a KKK turbo?

Here are some interesting links for you:

Turbocharger Wastegate Adjustment:

No Turbo Boost Pressure

Rebuilding Turbo

Danny

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Not sure which make turbo I have. If I could figure out how to reduce the file size of my pictures, I could attach it.
Gene

Where does the excess air escape from the wastegate?

P E H

It doesn't. The wastegate does not vent excess boost pressure. The excess boost pressure opens a valve which diverts exhaust gas from the turbine (bypasses the turbine) to reduce turbine speed and thereby reduce boost pressure.

Update: Good and bad news. I think I may have found the cause of my low pressure boost (max 5 PSI). I pulled the back plate off the tubo housing that lets me see the waste gate valve. I then applied light pressure directly to the waste gate valve. The waste gate valve began to open with only 4-5 PSI, hence this may be the cause of low intake boost pressure and corresponding lack of activity by the ALDA. I adjusted the waste gate spring tension and valve now opens at 11 PSI.

Bad News: I snapped off one of the studs that holds the metal shim and cover plate over back side. Broke an undtold number of drill bits trying to repair it as these small treaded studs are made out of krytonite. Finally made it through, and retapped. Tomorrow I hope to finish this saga and if I am right, I should pull more turbo boost and the car will run like it used to.
Gene

It does beg the question of how the spring tension could suddenly drop dramatically. I'm wondering if the spring failed in some fashion and it's not providing the proper tension. Hopefully the further compression of the spring will cure it, however..........it's still a question.

Tango,

That shows how much I know about the turbo/waste gate system. It just seemed to keep the manifold pressure from exceeding the maximum pressure, some of the compressed intake air would be bled off.

Now that I think about it, it must be more efficient to slow the turbo to keep the manifold pressure from exceeding the limit.

Other questions: What is the name of that cylindrical part, the one sort of looks like a large metal in-line fuel filter? What does it do and how does it work. Where is the wastegate bypass located?

I've had my 300SD for 16 years and never had any trouble with the turbo system, so I didn't learn much about it.

P E H

Regard "cylindrical part, the one sort of looks like a large metal in-line fuel filter", this is the pressure mechanism that activates the valve that acts as a wastegate and protectects the turbo from overboosting you engine. If you look, you should see a hose (looks like heavy black rubber fuel line), that runs from the front of your intake manifold to the gizmo you described. Inside the gizmo is spring loaded diaphram. Should the tuboboost reach approx 11 PSI, it pressurizes the diaphram and opens a valve (just like an intake or exhaust valve) that allows some of the gas to circumvent the tubo and blow straight out the ehaust pipe. This chamber and valve seat is all cast into the turbo housing, hence out of sight. Behind this part of the casting is usually a plate that can be removed and exposes the bottom of the valve and valve seat, as well as the internal bypass chamber.
Gene

The official MB nomencleture is "Boost Pressure Control Valve."