Q: I have a Mercedes 280 SL, 1985, WDB1070421A020760, many times it just stop while driving or at a stop. I have changed the fuel pump

Wow! A 280 SL with a manual gearbox is a rare machine indeed! It looks like you’ve eliminated a few of the prime suspects. The fuel pump relay would have been my first guess. But it looks like you’ve already been there.

One thing you haven’t mentioned that is a problem with the older MB’s is the fuel tank screen. Rust accumulating at the bottom of the tank can restrict the fuel flow to the pump and cause a drop in fuel pressure. when the car shuts off, the rust settles back off the screen so when you crank it back up, it can make enough pressure to restart.

Another big issue with the older cars is intake air leaks. As the rubber air hoses and boots get old and hard, they leak around the edges causing the fuel system to go lean. This can make it cut off when it’s idling, and it might make it stall when you first try to accelerate. Give the air hoses around the area of the intake a good look.

That car is equipped with the Bosch K jetronic injection system. A fuel pressure test is an important diagnostic procedure for a car that is stalling and requires a specific gauge and valve arrangement. I would have a local expert, like one from YourMechanic, inspect the stalling issue in person in order to have this properly corrected.