This is a common thread but perhaps I have a unique problem. It all started after I got it back from lending it to my brother for 2 days. My engine seemed like it took a little longer than usual to come to a complete stop...usually it stops immediately but it started to delay a little bit. Today, as I was going into a grocery store, I turned off the key and it kept running. It seemed like it wasnt idling pefectly...but still idling. So, I revved it up a bit with the ignition off...as it came back down from 2000 RPM it finally shut off (rather reluctantly). Looking at these symptoms (gradual delay turning off to not at all) what would YOU look for?

Putty

Common symptom of a vacuum leak. Seeing as the W126 has a seperate vacuum system for the locks the leak is somewhere in the engine compartment..

Invest in a Mity Vac vacuum tester and test the shut off valve as well as tell us if the vacuum pump is original or not

Yes, original pump...with less than 140k original miles though

After I tried the ALDA removal I accidentaly cut the vacuum stop thingy (yellow cap with vac line). Everytime I tried to cut the engine off, it kept running. Scrambling to find a solution, I decided to take the straw from a WD-40 can and cut just enough to stick in the vac. shutoff and attach the hose. Superglued around it for safety and now it works good as new and hangs in there with the way I drive.