Yesterday I was driving my mum's 1990 740 and the engine quit twice. The engine was at operating temp and there was no warning or indication that anything was wrong. As I turned the corner into my street the engine quit. I walked the last 300 metres home and grabbed my spare fuel pump relay and radio suppression relay. Swapped the fuel pump relay and the engine fired up. Beauty! Or so I thought.
Got about 100 metres down the road and it quit again but this time I saw all the warning lights randomly flashing a few times until they finally stayed lit. I swapped in the spare radio suppression relay but no go. I jumped the fuel pump relay pins in the board and got the fuel pump running but it still wouldn't start.
I checked for fault codes and the Fuel Injection side displayed code 311 which is something to do with the speedo signal being absent. My research indicates that this would lead to an idling problem but wouldn't cause the engine to quit. And i hadn't noticed whether or not the speedo was working at the time.
I tried cranking it again and the engine started so I got it home and left it idling for a bit but it didn't quit again.
I am wondering if this might be the crank position sensor. Can they go randomly faulty without causing a code to be logged? I know they can become heat soaked and fail. As far as I know the sensor is original so, being a 1990 model, it's 34 years old. I checked the plug and the wiring for it but didn't find any obvious problems but I didn't remove it so at the moment I can't be sure there is nothing wrong with it. I am going to try swapping it with a new spare one tonight and see how it goes.
Is there anything else I should be looking for? Thanks guys, I appreciate any help.