The following page is a backup which was originally created by Richard Easley and Stu Ritter, all credit should be given as such. Most links on these pages are unfunctional.
Checking Boost Sensing Lines And Overboost Sensing Switch
for 617 Engines
Introduction
The technical material for this FAQ (Frequently Asked Question) was provided
by Stu Ritter of Stu Ritter Mercedes-Benz Technician, Inc. of Denver, Colorado,
was edited by Richard Easley of Baylor University, and is provided as a service
to the subscribers of the Mercedes-Benz Discussion List.
To receive similar quality tips as described below on a daily basis, consider
subscribing to the Mercedes-Benz Discussion List, which is located at the
following site:
Using the categories of mechanical ability from the Mercedes-Benz Discussion
List <
http://hsb.baylor.edu/html/easley/mercedes/subscribe.html>, you need to be at the level of "Light
Do-It-Yourselfer" at minimum, to check/clean the boost sensing lines. If you are
below that level, you may want to provide these instructions for someone who is
at the light level or beyond.
Using the categories of mechanical ability from the Mercedes-Benz Discussion
List <http://hsb.baylor.edu/html/easley/mercedes/subscribe.html>, you need to be at the level of "Medium
Do-It-Yourselfer" at minimum, to check the boost pressure. If you are below that
level, you may want to provide these instructions for someone who is at the
medium level or beyond.
Tools
needed for checking boost sensing lines and overboost sensing switch
High quality vacuum/pressure gauge (needed for checking boost pressure
only)
"T" fitting for vacuum gauge and 6 - 8 foot length of vacuum hose for
monitoring boost pressure while driving vehicle (needed for checking boost
pressure only)
Phillips screwdriver
12 mm box end wrench for banjo fitting
Cutting pliers or knife for cutting fuel line and vacuum hose
10 mm socket and 1/4" drive ratchet and extension for removing air horn
Pliers for removing sensor hose clamps at trubocharger
Checking/Cleaning Boost Sensing Lines
Make sure that boost line going to the overboost sensing switch (from the
banjo fitting at the back of the intake manifold to the switch on the
firewall) is clean and free of any debris.
Remove banjo fitting at rear of intake manifold (boost line attaches to
this), save sealing washers on both sides of banjo fitting, clean banjo
fitting and hollow bolt, remove debris from immediately inside of opening with
cloth and small screwdriver, and re-install. Please note: careful with torque
on banjo fitting bolt. Snug will do it – breaking this bolt would most likely
mean R&R of intake manifold and special order of this hollow bolt.
Check the "sensor" hose that goes from the intake manifold to the
wastegate valve. This is a rubber fuel line-quality hose that can be seen
after removing the u-shaped air intake horn between the air cleaner and the
turbocharger. If this hose is leaking or loose, it is going to "leak" boost,
as is the wastegate cover if it is not firmly attached or the rubber gasket
behind the cover is leaking. Also, the switchover valve could be leaking,
which would cause "leaking" of boost. In short, anything attached, directly or
indirectly to the intake manifold is a potential cause of turbo boost
leakage.
Checking Boost Pressure
Attach vacuum/pressure gauge with "T" at overboost sensor switch. You’ll
want to test on both sides of the switch to make sure that switch is not
impeding flow of pressure. The overboost sensor switch works in the following
way: at approx. 1.1 bar it will close to ground and cause boost pressure to
vent from the atmospheric correction/boost enrichment device. The lack of
boost will reduce fuel flow sharply.
An important point to remember regarding the overboost sensor switch is
that there is a "reverse" release or venting that takes place which is
important. When the car is decelerating, you still have a lot of boost which
is not needed. During this time, you don't need the accompanying fuel that
would normally be injected at a given boost level [due to receiving "false"
information on boost "needs"]. The overboost sensor switch vents the boost at
this point so that the injection pump is not getting the signal to continue
fuel flow at that particular boost level. This is specifically noted here
since it has been noted that these switches are bypassed by unknowing
do-it-yourselfers who, in addition to causing the engine to consume excess
fuel during deceleration, put their engines in jeopardy by removing this
catastrophic failure device.
You can check the switch by removing the line from the aneroid compensator
(ALDA) and the switchover valve (coming from the intake manifold)and blowing
through the switchover valve. You should have a free and clear connection to
the line on the ALDA line side. Next, switch on the ignition and ground the
lead connecting to the overboost sensor switch. Instead of having a clear
connection to the ALDA line side, the line should now vent to outside air.
While you are working on your engine, may as well make sure that the
rubber vacuum (pressure) hoses at the overboost sensor switch are in good
condition --- don't want to waste that precious boost!
Finally, let me know if you complete this procedure successfully; it took a
while to type this, and I'd appreciate knowing when each person has completed
the repair! Please e-mail me at richard_easley@baylor.edu
Note: To receive similar quality tips as described above on a daily basis,
consider subscribing to the Mercedes-Benz Discussion List, which is located at
the following site:
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