As title states, my '01 Mercedes E320 4Matic W210 Sedan is having some intermittent problems where she'll crank and crank but not actually fire up. I'e noticed when this happens I'm not hearing the fuel pump engaging like I normally do. Narrowing down the problem hasn' been easy. Some say could be the Crank Position Sensor while others say it' a Fuel Pump Relay or Fuse. Has anyone else experienced this prob and if so, how'd u fix it?

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Hi leaton62000, 

Sorry to hear about the problem with your W210. Those are very solid cars and often go over 200k with no major issues. How many miles are on yours? 

First, I would like to ask. Does this happened all the time or only when the engine is warmed up? If it is when engine warms up it could be the W210 CPS sensor

If the problem happens all the time, you are correct in suspecting the fuel pump. What I would do is first check for spark. You can remove one of the wires from the coil to the spark plug, use anoter spark plug and ground it to the engine block. Crank the engine and check for spark. 

If you are getting spark, the next thing to check is fuel pressure. In these cars it is very easy to check fuel pressure. There is a schrader valve under the engine cover. On the passenger side of the engine towards the front. Connect a fuel pressure gauge and turn on the ignition. When you turn on the ignition but start the car, you should get a reading of about 50 psi, when the car is running you should get around 58 psi. Here is a guide that will help you with this procedure. 

If you don't get that pressure, the problem is with the fuel pump. If the brushes on the fuel pump are at the end of their usefull life, they will give you intermittent problems like the ones you describe. 

Hope this helps and let us know how it goes. 

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