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  #1  
Old 08-21-2008, 02:40 AM
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300SDL 1 PSI Boost 1K Miles After Dealer Replaces Trap Oxidizer ?

Hello everyone -

I thought I might get some feedback from anyone who has been down this road.
Purchased this 300SDL about 1 year ago and did considerable work to get it road worthy. About 6 months ago took it to the local MB dealer to have the Trap Ox replaced with the catalyst. Drove the car off and on after the Catalyst replacement job at the dealer and had normal boost. When summer arrived, began driving the car on a regular basis and finally realized that the turbo boost I had a few months ago was gone.
Last week , while doing maintenance, saw a plume of exhaust shoot up in the engine compartment and found the flex tube/pipe in the exhaust manifold leaking. My thinking was ahhh , there is my lost turbo boost.
Pulled all of the necessary stuff off to replace the pipe.
For good measure I also removed the turbo , replaced the oil supply line gasket and return line gasket and o-rings.
Replaced all of the Turbo riser pipe o-rings, crossover pipe gasket, new exhaust manifold gasket etc....
After completion had no exhaust leaks .

Still no turbo boost.
I have previously removed the EGR and ARV vacuum lines as well as all of the other vacuum garbage on that side of the car.
I have verified that the EGR is closed and not leaking.
I have verified that the turbo waste gate is closed, though there is no spring return pressure on the waste gate. The waste gate is held closed very tightly by the actuator. The turbo spins freely by hand .
This really got me going, so I removed the ARV , plugged the recirculate port and checked for boost - - 1 PSI -- at 2000 RPMs measured at the hose from the compressor housing to the waste gate actuator port.
Obviously the waste gate actuator is disconnected during this test so it can not be opening the waste gate.
Just for kicks , measured boost at 2000 RPM at the intake manifold ... about 1 PSI again.
The last look was at the turbo spinning up with the intake boot removed.

Am I missing something or do I just have a dead turbo ? ? ?

I was fairly concerned when taking the car to the dealer to have the turbo
checked out for damage caused by the Trap Oxidizer. The dealer assured me the turbo was "OK" based upon the 1/10 hour check they did.
So, I can rest on the assurance from the dealer that if I have a problem with the turbo , "they'll take care of me" ...

Thanks
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Last edited by wgilmore; 08-21-2008 at 03:20 AM.
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  #2  
Old 08-21-2008, 02:56 AM
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Is the ALDA hooked up and working properly? Changeover valve isn't sooted up? Can't have boost if you don't have fuel...

-J
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Old 08-21-2008, 03:26 AM
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Quote:
Originally Posted by compu_85 View Post
Is the ALDA hooked up and working properly? Changeover valve isn't sooted up? Can't have boost if you don't have fuel...

-J
Hi J:

Yes the ALDA is hooked up, working , I can't say for sure.
The changeover valve may be sooted up but I can't see why I would not have boost pressure regardless of the condition of the changeover valve.
My understanding of the changeover or boost over pressure regulator is that it reduces the amount of available fuel by decreasing supply at the IP.
If this is the valve you are referring to, then it should only come into play when an overboost condition exists and cause the ALDA to clamp down on the IP.
Since I don't achieve more than about 1 PSI , I don't understand how the changeover valve could cause a loss of 12+ psi of boost.
Thanks
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Old 08-21-2008, 03:35 AM
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are you just revving it in your driveway or is this boost reading from actual driving
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  #5  
Old 08-21-2008, 03:41 AM
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Quote:
Originally Posted by turbobenz View Post
are you just revving it in your driveway or is this boost reading from actual driving
This is just in the garage, not actual driving under load.
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Old 08-21-2008, 04:13 AM
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Turbo Boost Relation to ALDA

Wanted to add a note regarding load - no load and ALDA.
It may be my ignorance of the system but I have read a post originated by
" Mr. FancyPants " covering this same topic.
The thread referred to load and ALDA a number of times.
Again this may be my ignorance , but - I don't see how the ALDA or load can have much influence on boost under no load.
If the volume of air into the turbo compressor at 2000 RPMs is - as an example - 1000 liters per minute, then the volume of exhaust would be 1000 liters per minute. If the IP is delivering slightly more or less fuel per RPM and is under load or no load seems to me to have little affect on the volume of gas that flows through the closed system of intake and exhaust. The speed at which the gas flows through the turbo system is dependent only on the speed of the engine and as well the engine pumps the same volume of air / exhaust gas constantly.
I can see that after the reaction of combustion, change in composition of gas occurs and so some change in volume. The exhaust has CO2, CO, NO ...etc, but is the exhaust volume so dependent on the load and the ALDA?
Am I just missing the obvious?
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Old 08-21-2008, 06:41 AM
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You will not see appreciable boost unless there is a load present. You must drive the thing in order to get boost and make real measurements.
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Old 08-21-2008, 06:56 AM
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Quote:
Originally Posted by Bajaman View Post
You will not see appreciable boost unless there is a load present. You must drive the thing in order to get boost and make real measurements.
Hi:
Thanks for your reply.
Would you explain why there would be no boost at 2000 RPM's if the vehicle is stationary or moving .
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Old 08-21-2008, 07:43 AM
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The amount of fuel burned to turn the engine at 2000 RPM under no load is very minuscule (slightly more than idle). Thus there is little energy (heat) in the exhaust gas entering the turbine= no boost. Have you tried to make a measurement under load?
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Old 08-21-2008, 08:29 AM
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You may see some boost when stationary, if you're holding it up around 5k...

Where are you teeing in for your boost gauge?

Also check all other assorted vacuum lines. I had one pop off the underside of the vacuum amplifier that robbed me of boost before.
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Old 08-21-2008, 09:58 AM
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I like to think of it like this:
boost is pressure that the motor is blocking from flowing through the system.
at idle, the pistons are spinning freely, and there is nothing pushing against them.
under load, the pistons are pushing against the weight of the car and actually working, so the turbo needs to push harder and harder to keep the car moving.
this is an oversimplification, but it gets the point across of why no boost is present in park.
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Old 08-21-2008, 10:37 AM
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Quote:
Originally Posted by vstech View Post
I like to think of it like this:
boost is pressure that the motor is blocking from flowing through the system.
at idle, the pistons are spinning freely, and there is nothing pushing against them.
under load, the pistons are pushing against the weight of the car and actually working, so the turbo needs to push harder and harder to keep the car moving.
this is an oversimplification, but it gets the point across of why no boost is present in park.
I like to think of it like this:

The turbo requires a certain speed to produce boost. At low exhaust volume, the turbo will spin about 30K rpm.........which is not sufficient to produce any significant airflow to the engine to pressurize it.

As more exhaust gas is produced..........under greater engine load.........the turbo spins faster.........above 50K.........and starts to produce enough airflow to slightly pressurize the manifold.

Finally, at maximum exhaust velocity at high engine loading, the turbo can reach full speed..........about 120K........and push a significant amount of air to the manifold.........thereby pressurizing the manifold to more than 15 psi. The wastegate opens at this point and limits the manifold pressure to 15 psi.

In summary, you will never get any boost when the engine is in neutral..........insignificant exhaust volume and velocity to spin the turbo at high speed.
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  #13  
Old 08-21-2008, 11:39 AM
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My version-The Turbo is turned by the Exhaust Gas. Exhaust gas is created by burning fuel.
The Car burns more fuel under load and more fuel burned=more exhaust gas volume and increased exhaust gas pressure=Turbo turning faster=turbo pumps in more air into the manifold at increased pressure.
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Old 08-21-2008, 11:41 AM
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Sounds like your not getting fuel instead of boost.
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  #15  
Old 08-24-2008, 01:16 AM
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Thanks everyone for the replies regarding boost in park.
I get it now after reading your responses and reading up on the design principles
of turbos.
Of course , have now been struck again with another SDL demon - -
this time the head gasket, the portion that seals the top of the timing case to the front of the head just decided to break ( probably months ago) and vomit oil all over the front of the engine. Fun -fun - fun.
Get to pull a #14 head , can't wait to see what needs to be replaced.
If the head is good, now would be the time to do valves, guides and seals as well as true up the head. I really hate to see what this looks like when I get it torn down.
Thanks again.
Chuck
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