e46 Fuel Pump Failure

[Manolito]

Based on my micro sample of four e46s in my family and immediate circle of friends, I have learned that e46 fuel pumps will fail some time between 70k miles and 140k miles. You accelerate the failure date if you run it completely out of fuel a few times.

Like the e36, the e46 has a saddle-shaped fuel tank that straddles the driveshaft. It has only one electric fuel pump that delivers fuel to the fuel filter and engine, so it uses a jet pump (aka "siphon pump") to move the fuel from the (US) driver side lobe of the fuel tank over to the (US) passenger side where the electric fuel pump is located.

There is a fuel level sensor float on each side. The gas gauge averages the two readings to tell you how much fuel you have. If you suspect that the jet pump is not moving fuel from the left lobe to the right lobe (you run out of fuel when the gauge still reports 1/4 tank) you can snap up the rear seat and unplug the sensors and measure the resistance with a multimeter. You should see roughly the same resistance on both sides.
70 ohms either side = empty
310 ohms on driver side = full
395 ohms on passenger side = full

The pictures below show what it looks like with the rear seat bottom pulled up.



To unplug the fuel pump on the passenger side, you have to slide the latch horizontally to release the plug.



My '04 325iT had been acting up for some time, particularly when at 1/4 tank or less, and when making a high demand for fuel, like an uphill merge onto the freeway. The ohm meter told me that I had a lot more fuel on the driver side than I had on the passenger side, so I concluded that my jet pump had failed. Because I had fuel starved the electric pump several times, and my experience with three other cars told me that at 140k miles, I was due for a fuel pump anyway, I purchased both the jet pump and the electric fuel pump.



In retrospect, the jet pump was probably unnecessary, so I wasted $80. The jet pump that I pulled out looked perfect, with no sign of cracks or other leaks.

The jet pump works like the pump you attach to your faucet to suck the water out of your aquarium. It runs a fast-moving stream of liquid through a carefully-shaped 'Y' and that stream draws fluid from the third leg of the 'Y' into itself. If you turn the faucet down to a trickle, it won't draw the water from your aquarium. If the delivery rate from your failing electric fuel pump is too low, the return stream coming back from the fuel filter will not be strong enough to make the jet pump move fuel from the driver side lobe to the passenger side lobe of the fuel tank.

The remaining photos show how to replace the fuel pump and jet pump. NOTE: This is probably the most dangerous job you can do on a car other than working beneath a poorly-supported car. Liquid gasoline does not burn. Gasoline vapor burns, but only when it is in the correct ratio with air. That correct radio of vapor and air will be all over the back seat of your car while the tank is open, and even a static spark too small to see can blow up you and your car. I had a fire extinguisher handy, but the fact of the matter is, if those vapors had lit, there would have been no one left alive to use the fire extinguisher.

If the car has been running in the past few hours, there will still be fuel pressure in the system. To avoid spraying fuel in your face or on your leather, pull the fuel pump fuse and crank the engine to bleed down the fuel pressure. You will still get a trickle when you pull the hoses, but it won't spray. This picture shows the fuse tool grabbing the fuel pump fuse on an '04 325iT.



Here is my fire extinguisher (as if I would still be around to use it).



Before you begin, disconnect the battery. Even with the key off, there are still a lot if live circuits and switches (like door jam switches) in the car. Besides, my central locking has worked better since I disconnected the battery and used steel wool to clean up both posts and clamps. Apparently central locking really likes getting its full 12.6 volts.

You have to carefully pry open the clamp-on hose clamps. You will (of course) need new hose clamps. Be sure to buy the big rubber gasket that seals the fuel pump. The old one will still look OK, but it will be much harder than a new one. If you buy a jet pump, those come only from BMW and a new gasket comes with it.



A pile of rags nearby to quickly stop those drips. Also, have a couple of golf Tees or other suitable plugs to plug the ends of the hoses while they are disconnected.



And now it starts getting messy. Pull off the hose and quickly plug the end.



There is a BMW tool for this job. You might be able to loosen the ring with a hammer and punch. The tank is not open yet, so the hammer and punch are still allowed in the area, but I found that my giant Channel Locks spanned the top and made it easy to spin the ring right off. I did not grip it. I used the tips of the jaws to push sideways on the lumps in the ring. You have to twist and jiggle the pump a bit, and you may need to pull the float toward the pump to get it to clear the hole.



There are no pictures taken while the tank is open. No way was I taking a camera anywhere near the inside of the car. The car at this point was wide open to keep the fuel vapors inside to a minimum.



If you want to replace your jet pump, you will need to open the driver side of the tank to access it. Leave the electric fuel pump out until the jet pump is installed. The long discharge hose from the jet pump plugs into a hole that is right next to the electric pump on the passenger side.