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Not sure the proper way to do this but wanted to share a vacuum diagnostic check list for finding either a door lock, trunk or fuel filler cover vacuum problem. I found on this forum while doing a search for a related problem. After reading this forum for a few years, seems there is always questions about problems related to the vacuum system. This was the most complete diagnostic I have seen to date for the problems described.

I could not figure out how to reference the old thread so I'm pasting in the diagnostic develop by Sveinn Thorsteinsson, Reykjavik, Iceland. It was originally posted on 11/20/2000. He is replying to someone who has problems with the central locking system. I'm certainly not an expert in this area but thought others could critque and develop this into a ready reference. If this is a bad idea, I'm sure someone will let me know.

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Here is a step-by-step diagnostic that I prepared after fixing a vacuum leak in my own car (W123 230E 84). 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.


Regards, Sveinn Thorsteinsson, Reykjavik, Iceland. http://www.geocities.com/SiliconValley/Pines/6633/w123.html
 
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