Vacuum Puzzles - PeachParts Mercedes-Benz Forum

kolotour · #1 05-04-2020, 04:34 PM

I decided to begin looking into the vacuum system on my 1981 240D. Car was starting and stopping without a problem. Got my vacuum pump and golf tees ready. Got the ************** PDF. Began with the yellow check valve. Air passes one way and not the other. Fine. Check yellow line. Pressure builds quickly but no action with door locks. Check yellow w/blk. Pressure also builds quickly (maybe too quickly) and holds. Hummm? Connect everything and start car. Engine will not shut off!! Push the red stop button. Engine will not shut off!! Disconnect check valve vacuum line and block hose with golf tee. Engine stops. Play with system for several more hours and conclude that yellow line has the vacuum leak. Check yellow line again with vacuum pump. No leak!!! But by blocking off the two lines from the check valve the engine will start and stop but it seems to take a little longer now to stop running. What happened?

Diesel911 · #2 05-05-2020, 11:05 PM

Quote:

Originally Posted by kolotour

I decided to begin looking into the vacuum system on my 1981 240D. Car was starting and stopping without a problem. Got my vacuum pump and golf tees ready. Got the ************** PDF. Began with the yellow check valve. Air passes one way and not the other. Fine. Check yellow line. Pressure builds quickly but no action with door locks. Check yellow w/blk. Pressure also builds quickly (maybe too quickly) and holds. Hummm? Connect everything and start car. Engine will not shut off!! Push the red stop button. Engine will not shut off!! Disconnect check valve vacuum line and block hose with golf tee. Engine stops. Play with system for several more hours and conclude that yellow line has the vacuum leak. Check yellow line again with vacuum pump. No leak!!! But by blocking off the two lines from the check valve the engine will start and stop but it seems to take a little longer now to stop running. What happened?

I would fix the manual shut off first. If you have an MW Fuel Injection Pump there is spring loaded plunger within the bottom of the idle adjustment screw. I have read of one occasion where that got rusted and would not compress up into the Idle Adjustment Screw and was keeping the Engine from shutting off.

If you linkage segments are not the correct lengths that would also keep the Manual Shut off from working.

Lastly if your Idle Adjustment screw is turned down too far it would keep you from shutting off manually.

Concerning the other vacuum stuff it was too confusing for me and I don't know if my year 300D uses the same colored vacuum lines.
I also believe at the firewall mine only has one check Valve with one nipple for the vacuum source and 2 nipples on the other side for lines that goes to the inside of the Car.

kolotour · #3 05-08-2020, 06:03 PM

Thanks for the advice. i adjusted the linkage rods and the STOP button works!!
Now back to vacuum. LR door and RF & RR door locks check OK. Have vacuum leak in yellow line under hood but if I seal one end of yellow line and check vacuum there's no leak in the line which I think means the leak is in the LF door master vacuum switch. The gas flap door locks but retracts as soon as vacuum is removed. I haven't figured out the trunk locking mech. yet.

kolotour · #4 05-09-2020, 01:38 PM

Aaaah, the trunk lock. Found a vacuum leak. A crack in the external rubber boot on the shaft. Won't lock but will retract. Seems like you have to buy a complete lock assy. for about $50 to fix this 15 cent part...which can't be replaced anyway because of how the shaft and springs are assembled. Maybe a flaw in the "best car ever made" concept.

kolotour · #5 05-09-2020, 10:21 PM

I tried a quick fix to the trunk lock using a piece of bicycle tube, a plumbing washer and lots of glue. We'll see. Also traced the line back to the trunk vacuum tank and found the line plugged with a bolt and...no tank!!!! I begin to see why there is a no vacuum problem. Bought one on eBay. Coming next week. Updates to follow.