United Bimmer Do-It-Yourself Disclaimer: The following tutorial is meant as a guide and is not guaranteed to be complete or 100% accurate. By following this DIY, you understand any work done on your car is at your own risk and we hold no responsibility if you break something. If you feel uneasy with this risk, we recommend you take your car to a professional mechanic to have the work done. Otherwise, enjoy yourself and good luck!
The fuel pump can fail either from old age, and inadequate fuel supply. Driving with less than 1/4 tank of gas can overheat the pump; the fuel actually keeps it running cool, and debris or sediment from the bottom of the tank can block the pump's filter, causing it to get overworked. If you suspect a bad pump, there are a few things to check before pulling it out.
DIAGNOSIS
The order of electrical flow through the fuel delivery system is fairly simple: The DME (ECU) sends a signal current to the main DME relay, which in turn feeds the signal current to the fuel pump relay.* From there, the current flows through the fuel pump fuse before heading down to the fuel pump. This whole circuit is complete and live whenever the ignition is cranking or the engine is running. To summarize:
DME
--> DME relay
--> Fuel pump relay
--> Fuse #18
--> Fuel pump.
*Technically, the (+) leg of the DME relay is already hot, and it's the (-) leg that is grounded to complete the circuits.
Testing the relays can be a painstaking process, so you'll want to leave that till later and do the easy stuff first. The most obvious is the pump fuse, which is #18 in the fusebox. If that checks out okay, then remove the back seat bottom by pulling up at the locations indicated by the arrows:
There may be other stuff you have to remove first, like the side bolsters on coupes. But in my car they're integral to the seatback, so I'm not sure about that. But once the seat bottom is off, look on the right side of the vehicle, you'll see a section of padding that is cut into a circle. Pull it up to reveal the metal cover, and remove the 4 Phillips screws to open it. Careful- there's a flimsy foam gasket underneath it. Here's what you'll see:
As you can see in the photo, mine had a ton of crud on it; the clamp on the return hose wasn't tight enough. Yours probably won't look as dirty. At this point, have someone crank the engine while you listen for the pump.** The sound is unmistakable; a medium to high pitched buzzing, and you only have to get as close as a few inches to hear it. If it's buzzing, then the fuel pump is not your problem. If there is no sound, disconnect the pump wiring harness and test the leads for current while the engine is cranking. The green/violet wire is the positive, and the remaining wire is the ground (ground wire can be either brown or brown/green). You should see +12v while the engine is cranking.** If not, then move on to testing the relays & sockets.
**Alternatively, you can make the circuit go live by removing the main DME relay and installing a fused (15A) jumper wire between sockets 30 & 87 (see below).
TESTING THE RELAYS & THEIR SOCKETS
When troubleshooting the relays & sockets, you can refer to the photo below (relays removed). I don't see how the socket and pin locations could be different, but verify your individual application anyway by matching the socket to the pin numbers on the relays. What will not change is the pin numbers and their functions; these are standard Bosch type 12v relays. Note that the DME relay has two 87's: one feeds the fuel pump relay, the other feeds the fuel injector valves. This is known as a Double Pole Single Throw (DPST) relay, as opposed to the fuel pump relay, which is a Single Pole Single Throw (SPST).
You will need a multimeter, +12v power source & ground to test each relay. Connect relay pin 85 to ground. Set your meter for a continuity test, and attach the meter probes to pins 30 & 87. When you apply +12v to pin 86, you should hear an audible "click", and the meter should indicate that 30 & 87 have been bridged (continuity confirmed). If not, then the relay is bad.
If the relays check out okay, there may be a break or short in the wires leading to the pump. Move on to the relay sockets. Test for continuity between the fuel pump relay socket 87, and the green/violet wire at the pump (remember that fuse #18 lies in between these two points). Then test for continuity between the other wire at the pump, and a ground source. If those check out, then the wiring is okay.
On both relay sockets, #30 should be reading +12v at all times. At this point, a fault at the socket or the wiring harnesses is beyond the scope of this writeup, and this is where the setup between the different model E36's will begin to vary. Furthermore, if you haven't found anything wrong by this juncture, then it's no longer a fuel delivery issue, and you should be looking elsewhere for the culprit of your problem.
REPLACING THE FUEL PUMP
If you're positive that the fuel pump has failed, then the rest is a cakewalk as long as you follow a few simple but important rules:
--Don't smoke . . . duh!
--Have your radio code handy (if applicable), then disconnect the (-) battery cable.
--No cellphones, portable lights, or anything electronic/electrical in the vicinity.
--Do not use battery or electrically operated tools- hand tools only.
--Do not perform this in a closed garage- ensure adequate ventilation.
--Remove the gas cap to relieve pressure in the tank & lines.
--Wear goggles and rubber gloves. Seriously.
--Gas tank should be no more than about 3/4 full, or it'll get really messy.
When ordering your new pump, make sure to also buy the seal ring, shown in photo below. Notice that the fuel level sender is integral to the pump assembly. You can also use the same identical procedure to replace the fuel level sender on the left side of the car. The white rectangular thing is the aforementioned filter that gets clogged up.
Begin by disconnecting the wiring harnesses, and let the car sit for about 10-15min to dissipate any possible remaining current. Then disconnect the fuel hoses. Have some paper towels handy, as some fuel will spill out. Go easy when pulling off the hoses; they may still be under some pressure- this is why you want to wear goggles. Unscrew the collar ring counter-clockwise. BMW makes a special tool for this, but it's not necessary. Use a flathead screwdriver and tap gently with a hammer to get it going, as shown in photo below. Then turn with your hand the rest of the way.
Make a note of how the pump is oriented in the tank- you'll be putting the new one on in the same way . . the harness receptacles should face towards the front of the car. Once the collar ring is removed, pull up on the whole pump housing. Easy does it- take your time so you don't make a mess of the fuel. You'll have to do some slight finagling to get the unit out, but it's not difficult at all. The seal ring should come off along with it; if not, pull it off the rim opening.
Installation is the reverse of removal. Don't forget, the seal ring goes on first before you drop the pump in. A little past hand tight on the collar ring should be sufficient- you don't want to crush and deform the seal ring. Hook up the hoses first before the harnesses, in case of any stray arcing (not likely). New pump installed:
Reconnect the (-) battery cable and start her up. Reset your clock, radio, & power windows***, and you're good to go
***To re-initialize your power windows, close all doors and turn ignition on. Fully open and close window, holding the button in for at least 5sec in the closed position. Repeat for other windows.