I am trying to locate a vacuum leak on my 85 300D. I've isolated it to
one of three hoses that go into the firewall. Two are yellow and attach
to a yellow plastic piece just before the wall. the other is red and
looks a lot like the one that comes out of the firewall and goes to the
shut off thingy. All three of these will nto hold a vacuum, or even
create on. when i hook a tester up, I can pump and pump and not create
a vacuum. one of the yellow ones will hold a pressure though. very
My question is what are these for? I am assuming that one goes to the
shut off circuit within the dash. The yellow ones I have no idea about.
do I need them? can I cap them off? I am doing all this to try to get
the engine to shut off when I turn off the car. So I don't think I can
just cap off the red one. Any insight would be appreciated.
Have a great day,
Find and fix the vacuum leak, don't speculatively change the car's design.
You should also know that the "shut down device," the vacuum powered
bellows inside the aft end on the injection pump, can fail.
Suggest you first determine if the shut down is OK by seeing if
pulling a vacuum on it shuts down the engine. If so, it's time to look
for the leak.
Most vacuum leaks are in the door locks.
Look closely at the yellow lines. One is all yellow, the other is yellow with a
Yellow goes to the vacuum switch in the drivers door.
The switch itself rarely goes bad. Usually it is one of the other 3 doors, the
gas flap, or the
trunk element that leaks.
Yellow w/black stripe goes to the vacuum reservoir that sits on top of the fuel
tank. It holds
vacuum in reserve so tomorrow morning when you unlock the car, all the doors
unlock. If you have no
You need to pull a lot of vacuum to see if it leaks, Very rare to leak.
Red and green go to the automatic ACC.
Best to test the ACC system with the key on and the center [arrow up] button
pushed. With the key
off, the switch over valves bleed off the vacuum in the dash.
BTW, the brown line goes to the vacuum engine shut-off.
I recently had this problem twice on my 116 300SD. The vacuum line for
the shutoff was broken exactly at the point where it goes through the
rubber grommet in the firewall. While probing around, it broke off in
my hand. Six months later, same thing with the line for the door
locks. So, I'd take a good look there. Now as to why it happens,
that is a real mystery. The lines didn't make a sharp bend or
anything. I'm guessing it might be due to vibration or temp change.
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