Hi,
I recently purchased a 1982 300D turbo sedan. I am currently chasing down various quirks in my car. One problem I had was with the vacuum system. Below you will find a set of steps someone on another board wrote and posted there. I asked this person if they would mind if I posted it here as I thought people here would benefit from this and he said it was fine. I hope this is of help to you. I also saw some postings on AC diagnosis. If needed let me know and I will atempt to bring these also.


The vacuum comes from the vacuum pump on the front of the engine and is stored in a reservoir in the trunk. The large line coming from the pump to the brake booster, large round thingy connected to your brake master cylinder. Off this large line should be a branch that goes through the firewall on the left side . Check this for leaks and if no leak continue on and follow the steps mentioned. Note, I found it VERY helpful to mark these lines as I identified them to avoid confusion in this process or in the future.

If the lock in your doors only works while the car is running or will only work once after you shut the car off, then you've got a
vacuum leak. Generally, it's one of the following: a leak in one of the vacuum lines, a leak in one of the vacuum-powered door
lock mechanisms or a leak in the reservoir. Most likely it's one of the door lock units.

Here is a step-by-step diagnostic that I prepared after fixing a vacuum leak in my own car. It turned out to
be a failing locking unit for the fuel filler cover in my case.

Step 1. (a) Sit in the car with the doors unlocked and the car running. Stop the engine and wait for a minute or a two. Lock the
driver's door. (b) Do the other doors lock properly? If not you have a leak in the unlocking system (yellow lines with a green
stripe) and should proceed to "Step 3".

Step 2. (a) Sit in the car with the doors locked and the car running. Stop the engine and wait for a minute or a two. Unlock
(open) the driver's door. (b) Do the other doors unlock properly? If not you have a leak in the locking system (yellow lines
with a red stripe).

Step 3. Pull out the driver's floor mats. Underneath them, near the door you will find a black plastic cover that protects the
vacuum lines. Open it and pull out the appropriate lines (green or red striped). Near the seat you will find an "H" shaped rubber
connector. One vacuum line is coming from the engine compartment into the connector, one will go back to the driver's door
and one will go ahead to the rear door behind the driver (left side). The last line will go over to the other side of the car.
Holding the rubber connector, gently pull out the line that is going to the other side of the car and place the plug into the
connector instead. Now you have disconnected the doors on the right side of the car, the trunk and the fuel fill cover. Go back
into the car and close the door and start the engine. Repeat either step 1a or 2a, depending witch of them gave you the right
result before. Is the rear door on the driver's side locking/unlocking properly? Note that the locking of the other doors will not
be working. If the rear door is locking/unlocking properly go to "Step 6" else continue to "Step 4".

Step 4. The rear door is not locking/unlocking and you have a leak in the left side of the car (front and rear door) or in the
engine compartment or in the reservoir. Place the line that you pulled out before again into the rubber connector. Gently pull out
the line that goes to the rear door and place the plug there instead. Go back into the car, close the door and start the engine.
Repeat either step 1a or 2a, depending witch of them gave you the right result before. Do the doors on the right side of the car
lock/unlock properly? If they are locking/unlocking you have a leak in the left rear door and should proceed to "Step 5". If they
are not working you have a leak in the drivers door or in the engine compartment or the reservoir. You will need a vacuum
gauge to continue. *END*

Step 5. You have found the door that is the source of the leak. Now you need to know if it is the vacuum lock unit or the
vacuum lines that are leaking. Remove the door panel and you will find the locking unit in the bottom of the inside of the door.
Pull out the appropriate line (red or green striped) from the unit and place the plug into the line. Go back into the car, close the
doors and start the engine. Repeat either step 1a or 2a like before. Do the doors on the right side of the car lock/unlock
properly? If they are locking/unlocking the leak is in the locking unit, if not it's the vacuum line that is the problem. You can
easily repair the line with a small rubber tube if you find where it is leaking (just cut it where it is leaking and connect the parts
again with the rubber tube). Can't help you with the locking unit. *END*

Step 6. You have a leak in the right side of the car, the trunk or the fuel filler cover. Remove the plug from the rubber connector
at the drivers side and place the line back into it again, close the cover and put the floor mats back in. Pull out the front
passenger floor mats and underneath you will find another set of rubber connectors in the plastic cover on that side (one for the
locking line and one for the unlocking lines like before). These connectors will be "Y" shaped with three ends going back and
one to the front. One of the tree ends is connected to the line that is coming from the other side of the car, one goes to the rear
door, one to the trunk. The only one that goes forward into the other direction is for the front door. We will start by
disconnecting the front passenger door. Gently remove the line from the connector and place the plug instead. Go back into the
car, close the doors and start the engine. Repeat either step 1a or 2a like before. Do the rear doors lock/unlock properly? If
they do, the front door is the source of the leak and you should proceed to "Step 5". If they are not working just proceed to
"Step 7".

Step 7. You have a leak in the right rear door, in the trunk or the fuel filler cover. We will first test the rear door for leaks.
Remove the plug from the connector and reinstall the front door vacuum line. Remove the rear door vacuum line from the
connector and place the plug there instead. Go back into the car, close the doors and start the engine. Repeat either step 1a or
2a like before. Do the left rear door and front passenger door lock/unlock properly? If they do, the rear door on the right side
is the source of the leak and you should proceed to "Step 5". If they are not working just proceed to "Step 8".

Step 8. You have a leak in the trunk or the fuel filler cover. To verify this remove the plug from the connector and reinstall the
rear door vacuum line. Remove the last vacuum line from the connector and place the plug there instead. Go back into the car,
close the doors and start the engine. Repeat either step 1a or 2a like before. Do the all the doors lock/unlock properly? If they
do, you have verified that the leak is in the trunk or the fuel-filler cover and can proceed to "Step 9" after removing the plug and
reinstalling the line. If they don't work, try to repeat the process again if you have missed something. If they still don't work the
leak will probably be in the line that goes from the left side of the car to the right under the front seats (Hmmmm, you might also
have a double leak, but since that is highly unlikely I will not deal with that here). *END*

Step 9. We will first examine the fuel filler cover locking for leaks. Remove the plastic cover from inside of the right rear fender.
You might have to go into the trunk yourself to disconnect the tree-way rubber connector. Disconnect the line that goes to the
filler cover and place the plug instead. Once again repeat either step 1a or 2a like before. Do the all the doors lock/unlock
properly? If they don't proceed to "Step 10", but if they do, the leak is in the fuel filler cover vacuum locking unit. *END*


Step 10. The leak is in the trunk locking part.


I hope this helps. I am sorry for the length. If I am not to put this length on here in the future, please let me know.

Rick