I have a 1979 280SE 116 with an inline 6. A couple of months ago the door locks would only keep vacuum pressure for a few minutes, but would work again after the engine was started. Also, there is a lot of engine "ping" upon acceleration and after each gear up shift. A person I spoke with said there is a diaphragm that will break after it gets old. Where is this located?

I am the second owner and the car has 98,500 original miles, no damage to the wood and seats have always been covered with sheepskin. The spare and rim have never been used. As an aside can anyone tell me what it would sell for. Everything is original. I really would like to stop the engine "ping".

mine don't hold pressure long, I think they will stay locked if you leave them but I am not sure I've never tried it. I figured it didn't hold much air, not to mention the age of the system. I tend to lock everything in the glove compartment and trunk because I can trust that.

Pinging is a result of fuel octane too low. Used to be correctable with fine tuning ignition timing on points distributors. Maybe an octane booster additive can help or resetting the ignition timing.

Each door lock has its own rubber diaphragm prone to crack and leak, assuming it aint the engine vac pump diaphragm thats shot. Does one door lock move slower then the others? On my 123 it got to the point where i had to rev the engine to lock the doors and then lock the drivers door manually after allowing 15 seconds leak-down time for the rest to stay locked. Not many people actually expect this system to work 100% after so many years.

This is a lousy time of year to sell and depending on location I'd say it should bring at least $2000 if everything else works, electric windows, heat controls etc. The twincam 6 cyl engine is considered one of MB's best. Unfortunately to many its just an "old car" that will scare people away.