My 84 bmw 733i will start and run but engine dies as soon as i let go of the key

I had this exact symptom on my 1984 533i. I spent months diagnosing everything but the computer. It was the computer that failed. Actually only part of the circuitry failed and is probably repairable. When the car is cranking it would run. I set up a wire harness that allowed me to shut off the starter motor while I was holding the key in the starter motor position. The car ran just fine. The 533, 633 and 733 all use the computer ending with 008. DO NOT slap in any other computer unless you actually know what you are doing. I tried. And you risk damaging good components, like the donor computer and the injectors.