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My 2003- 525 Diesel E39 model runs very well without any major problems. This morning started it, SWITCHED it off to get back in the house and IT WOULD CRANK but NOT START.
Called AA home Start, who diagnosed it having "Low fuel Pressure" and asked me get a further check by a garage. Suggested it could be the Fuel Pump.

Any advise on this as i have google to find out there are 2x fuel pumps in E39. ONE in the fuel tank with the Sender Unit and 2ND one under the Floor.
 

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The under the front passenger seat one is about £100 . DO NOT BUY the shit italian ones. They make a lot of noise ! .I fitted a ebay/BMW one from a low milage car and its been fine . If its the other pump ...££££££
 

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In-tank fuel pump will only affect if tank is below ~1/3. Will show as air bubbles in the fuel line.

Inline fuel pump under floor. Pierburg 7.50051.60.0 = OEM.

Both pumps are activated when ignition turned to pos. 2 so you should hear them running.
 

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E39 525TD poor acceleration

I had the exact same issue on my 2003 E39 525TD and the AA lad said it was low fuel pressure. He got it started and on the drive home it would not rev above 2500rpm. I put it into a garage, they came back and said it is likely to be 1 of 2 pumps but they can't identify which one it is. If they have to end up replacing both, it would cost over £1000. I decided not to leave the car with them. It is now starting okay but the poor acceleration is still present. I've now been doing my own research and trying a couple of remedies. So far I've replaced the air and fuel filters and cleaned the Mass Air flow sensor with the correct cleaner. The fuel filter did improve it a little, suggesting to me that it is a pressure issue. I bought a replacement pump for under passenger seat and got set up to change it yesterday. Before stripping it out, I decided to check the fuel pressure first. I connected the in feed fuel pipe to a pressure gauge and turned the key to pos 2. The gauge was flipping about mid way between 80 & 90 psi. With that being the case, I thought there was little point in changing the pump before doing some more research.

Last night I read a post on here about a similar problem being caused by the pressure sensor on top of the fuel filter. This morning I took the car for a drive, once it was warmed up, I pulled over and disconnected the electrical supply to the sensor and set off again. The engine sounded quite rough, but it accelerated better and the rev counter went to the red when I had it in sports mode. Lots of blue smoke was being blasted from exhaust. I drove it home and connected it up again. I've now ordered a pressure sensor and will fit tomorrow> I'll update this post after I've taken it for a test drive.
 

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You should hear the pump under the passenger seat prime when you turn ignition on.

I had the same issue no power over 2500 rpms, Home INPA diagnostic reviled low pressure. Replaced with a Pierburg pump and works fine.

I have posted about this before if you can find my thread. has the correct pressures etc in that.
 
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