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Hey guys I've got a high mileage 08 S550 that has developed a starting problem when cold. The car will not start until after three or four attempts. I put a screw-on type pressure gauge on the fuel rail to monitor the pressure and sure enough overnight the pressure drops to zero. As soon as you press the start button, it immediately shoots up to 55,* but doesn't always start until three or four attempts. Then it is fine for the rest of the day. So my question is, does the fuel pump itself have a check which maintains pressure or is there a check valve somewhere in line that I should be looking for? I'm considering just replacing the fuel pump on Principle as the car does have 160,000 miles. Clearly I don't think this is a problem with the pump not working or the relay for the ECM or the DAM as the pump is kicking on when the start button is pushed. Again,* the problem seems to be it is losing pressure overnight. Any help greatly appreciated.
 

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I can't answer concerning the pressure drop, but I would be more concerned about a Camshaft Position Sensor or a Crankshaft Position Sensor problem being the direct cause of failure to start. See the Engine section of our DIY & FAQs in the stickies (the blue backgrounded section at the top of the first forum page). Read the "Common Sources of Engine Problems" and the following CPS writeup. Both sensors operate together, and problems with one can cause trouble with both.

However, you will be money ahead having the problem properly diagnosed rather than throwing parts at it; and you'll want to know if the drop in fuel pressure is normal, or indicates a leak.
 
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