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Q: Electrical & lights on 1991 Mercedes-Benz 560SEL

Rookie cbe0621eac06868b3efe0d8d1d3611e23c60d3114864ea2ec19a68cfbd3eebab
My car shut off when the engin is hot.The problem occurs on the highway or in the city.What can it be ,the fuel pump,the relay,the distributor.....
Thank you for your time.
No car image 94a1663db56199c5353592009e34aaa51078a2469bed068bb8d6f0ba43accf97
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An engine needs three things to run, mechanical integrity (which it must have or it would not restart), fuel and spark. To find an intermittent problem like this I "tee into" the fuel line and tape a fuel pressure gauge to the windscreen so I can monitor fuel pressure when the problem occurs, if fuel pressure falls off you then start tracing the fuel pump, fuel pump relay and sensors that allow fuel operation. I also drive around with a timing light attached to one of the spark plug wires, when the vehicle falters I pull the trigger on the timing light and see if I have spark if not I check the ignition system and wiring and sensors associated with the ignition system. Intermittent problems are hard to find and your vehicle has limited diagnostic capabilities. Understanding how each system works, having a good wiring diagram and volt/ohm meter and patience are required to trouble shoot this kind of problem.
There is a fuel accumulator in the rear which is supposed to apply back pressure to the fuel line. If it wears out, the heat of the engine causes expansion and not enough fuel gets to the engine.
I had problems with my 1991 560 stalling only in 104 degree weather. Changed the fuel pump relay and ignition module ($1700) and still had the problem . Engine always worked at the dealer. Small town mechanic let it idle until it stalled after an hour and found that the separate fuel system power take off from the positive battery terminal was loose. Problem fixed. We surmise that when the battery was changed the wrong nut was initially loosened in mistake for the real terminal nut and not tightened afterwards. It is often a very simple problem.
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