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Tech Article

BMW E30 3-Series Fuel pump analysis

Last Updated: 07/15/2019
   
Parts Group: Fuel pump assembly
This article discusses the fuel pump for the E30 with the main focus being on the variations. If you're not here for the technical analysis but would rather
  • Buy a used unit from us, guaranteed to work and fit, then please select the link below.

    The information herein is based on my own cars and used parts that I’ve bought from various junkyards though my personal experience does not yet cover the M3 or the 318is/ic with the M42 engine.

    Up to and including the 1987 model year, there was a low-pressure pre-pump in the fuel tank. It had a screen at the bottom, to filter out sediment and dirt rather than sucking it into the pump and the fuel injection system. From there the fuel was sucked into the actual motor unit, and from there hoses took the fuel to the exit line. A complex bracket structure kept everything in position, including providing a way to attach the assembly to the top of the fuel tank because the unit is installed by lowering it into a hole atop the fuel tank.

    Its function was to lift the fuel out of the fuel tank and to deliver it to the main fuel pump, which was located underneath the car, on the driver side, in front of the rear wheel well.

    This main fuel pump was mounted in a cradle in conjunction with a fuel filter. The fuel flowed from the top of the in-tank fuel pump down to the main fuel pump with filter. From these, a steel line took the fuel forward to the firewall area, where a rubber hose took it to the fuel rail.

    With this set-up, even if the pre-pump in the tank failed, the main fuel pump could still suck the fuel out of the tank, through the pre-pump, and get you home.

    The problem is that operating the system without a pre-pump puts extra stress on the main fuel pump and shortens its lifespan. More dramatically, a benefit of the pre-pump is that it prevents vapor lock. Without a working pre-pump, the car can stop running in hot weather, and then the remedy is to wait until the fuel cools down to the point where it's no longer fuel vapor but liquid again.

    I gather it that before the 1988 model year, the technology wasn't mature enough to put a main fuel pump inside the tank. That changed for the 1988 model year. The first improvement was to delete the main fuel pump from underneath the vehicle, and to replace the pumping unit of the pre pump with a much stronger pump unit, so as to make the in-tank unit the main fuel pump.

    The unit used was made by Bosch with part number 0580314070. This unit had a square plug that took power via red and black wires to the external connection at the top of the contraption. As I understand things, the equivalent BMW part number for this was 16141179711. This variation used a black rubber hose to bring the fluid from the main pump up to the exit line.

    As I understand the part number data, this design was used only for a few months at the very beginning of the 1988 production run. After that, the general setup remained the same but the motor unit of the main fuel pump was upgraded to a larger and heavier version. This new unit had two individual spade connectors, one with a brown wire and one with a red wire, connecting the motor unit to the external attachment point. For this design, the fuel was fed upward to the exit line using a more-complex, integrated plastic setup as opposed to using a simple black rubber hose.

    This seems like a more robust design than the earlier variation. As far as I can tell, that variation was used through the end of the E30 production run.

    The early design used in the first few months of the 1988 production run might have been a stop-gap measure or otherwise found inadequate. Regardless, the design was superseded by BMW part numbers 16141179992 or 16141184022.

    As to how each of these two variations connects to the outside world, the fuel line and electrical connections are identical; just the internals are subtly different, so they should be interchangeable.

    Throughout the E30 production run, whether the pump inside the tank was a pre-pump or the main fuel pump, there is an oval hole at the top of the bracket into which slides the fuel level sending unit. Some 8 mm nuts attach the fuel level sending unit to some studs protruding from the top of the bracket. Other 8 mm fasteners attach the bracket to the top of the fuel tank. An o-ring is wedged between the bracket and the top of the fuel tank, to prevent fuel vapor from entering the passenger compartment.

    After 30+ years, the pre-pump in pre-1988 models tends to fail and can be expensive to replace. The temptation to remove both the pre-pump and the external main fuel pump and replace them with the main fuel pump of a 1988-or-later model is justified. Assuming it’s done with attention to detail, this upgrade should be viable.
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