My sister has a 1993 190E. I chose it for her years ago when she gratuate. It's kind of a memory car for both of us.

around 2 years ago, she bought a new car and left this car sat since then. Last weekends, I went to help clean it up to sell it for her and found a problem. After jumped the battery, it started right away and idle a little low, around 700rpm, then it came down to 500rpm (in several seconds) and stalled. Restart again, same thing.

Today, I went to see if I can fix it. After cleaning up the cap, rotor, spark plugs, ect, no help. It started right up and idle nice for several seconds, and stalled. If I keep my foot on the gas, or tab it when the rpm comes down, it won't stall. I leave the pedal alone, rpm will come down to 500rpm and stall.
I even open open up the air box lit and let it idle, same problem. Then I tried to tab the gas pedal and shift, it stalled too. When I kept my feet on the gas pedal to have the rppm above 1000 while shiping, sometimes it will stay on but it shake hard when I shift, sometimes it stalled too.

Anyone know what the problem is?
Thanks in advance.