2001 Mercedes CLK 320 Cuts off

I need to know where to start. After about 15 minutes of driving, vehicle will just cutoff. If you try to restart, starter turns over normal but the engine does not fire. If you wait 5 minutes car starts up fine and
will run about 15 minutes again and then cutoff. I think the cutoff occurrs while idling. Any help appreciated
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This engine's computer stores failure codes. You don't mention whether the "check engine" light is ON or OFF. If ON there will be a code or codes stored. A code reader is needed, some auto supply stores will read the code for free.
The time related aspect suggests an electrical failure but first check the gas cap - that there's no vacuum in the tank that's preventing fuel pickup by the fuel pump. When it quits just open the gas cap for a moment, close it and see if it will start. If no start, that possibility is eliminated.
So then remaining are the electrical possibilities - fuel or spark. When it quits and you try to restart it do you hear the fuel pump run before you crank the engine? If you don't know what this sounds like, pause of a few seconds after turning the key to ON before turning to START and you should hear the fuel pump run for a second or two. Try it in a quiet garage. Anyway, if you don't hear the fuel pump run then the culprit may be the fuel pump's relay or the pump itself.
On the spark side I don't know where to direct you to some central component that would shut down the engine. These engines have several ignition coils so if one failed the others would probably carry on. Beyond that ??
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Try replacing the crank position sensor. It's a black thing that sits at the back of the engine on the bell housing. You can't test it unless have access to an oscilloscope. When it's playing up it can give the symptoms you described.

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It is not the gas cap or the fuel pump relay, I replaced the relay and no change. Vehivle has got worse and now the check engine light come on. The vehicle runs fine for about 15 minutes and then shuts off. Now I have to wait almost 45 minutes to restart and then it will run for 5 minutes. If you leave it for a few hours it will run for 15 minutes repeating the same routine. I have not been able to hear the fuel pump. By the way, where is it in the vehicle? Thanks
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