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DIY: Do It Yourself
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Old 02-08-2015, 04:21 PM   #1
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Join Date: May 2005
Location: UCLA/Bay Area
Posts: 47
My Ride: carbon black e46 m3
DIY: Quick check to see if fuel pump relay priming properly


If you are having problems where you have to crank the car (E46 M3, but may apply to other models) at least twice or three times before it starts, then the problem could be the "black fuel pump relay in the trunk." The black fuel pump relay turns on the fuel pump to let it prime for about three seconds when the key is turned to the second click position. This allows the car's fuel rail to build fuel pressure, so when you ignite the engine, then the fuel pressure is sufficient to start the engine.

Most people who are having starting problems where they have to crank multiple times are unknowingly "manually" priming the fuel pump by having the car start and die multiple times. This is actually building fuel pressure slowly, with eventually enough failed cranks to build fuel pressure to finally start the engine.



This quick check will allow you to see if you need to replace the black fuel pump relay.

1) Move your passenger side seat all the way to the front and have it leans towards the front.

2) Squat on the floor behind the passenger side seat. Have your entire body facing the left side of the vehicle.

3) Put your left/right ear where the center of the rear bench seat is located.

4) reach with your right arm and stick the key into the ignition. Turn the key two clicks.

If your fuel pump relay is working correctly, then you should hear a "cell phone vibrating sound" where your ear is listening. The sound should only be for about three seconds. This is your fuel pump properly receiving a signal from the black fuel pump relay box, so your fuel pump is priming the fuel lines to build fuel pressure.

I hope this helps everybody who needs to investigate their fuel pump problems.

All the best.


Last edited by mrpinstripsuits; 02-08-2015 at 04:25 PM.
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