Fuel pump relief valve pressure:
Some where in the 65-80 psi range
Regulated fuel pressure: 2.5 bar
Despite rumors to the contrary,
ethanol will not harm the fuel pump, hoses or vibration
dampener. BMW has issued a TSB (technical service
bulletin) indicating that up to 10% ethanol is fine in K
Fuel Pump Filtering
The main fuel filter on K bikes
is after the fuel comes out of the pump to keep contaminates
from entering the fuel system. However, both types of fuel
pumps have pre-pump filters. For the 52mm fuel pump it is a
screen cup over the bottom of the fuel filter - part
For the 43 mm fuel pump it is a
sock with a spring in it at the bottom of the fuel
pump - part 16141341233.
Fuel Pump Failure
Although typically a K bike fuel
pump should last well over 100,000 miles, sometimes they just
wear out. More common is that a K bike is left to sit for an
extended period of time (years, not months) and water from
the atmosphere builds up in the gas. This causes one of two
problems: 1) the fuel pump vibration dampener (rubber collar
that holds the fuel pump in place) decays, bits of it get
past a damaged screen or sock and these bits jam up the fuel
pump or 2) water causes corrosion inside of the fuel pump.
Replacing The Fuel Pump With New OEM Parts
The BMW OEM 52 and 43mm fuel
pumps by themselves cost $409 and $374 respectively. A
complete 43 mm assembly costs $435 and includes all of the
parts you need for replacing everything. This will fit both
early and late K bikes. In my opinion, if you choose
to buy new OEM parts then it makes sense to buy the complete
43 mm assembly - #1 in the diagram below. (2013 prices)
Replacing The Fuel Pump With An Aftermarket Fuel Pump
My test to see if a fuel pump is
repairable is to remove it from the tank, hook it up to 12V
(the smaller terminal is positive) and see if I hear a small
clunk when power is applied. If I hear that then I
know that the electrical motor is "trying" but that the fuel
pump is jammed, either by small bits of foreign matter or
Try This First
If it's just minor corrosion in
the fuel pump then sometimes all you need to is turn it
over, fill the bottom of it with a fuel system solvent (Sea
Foam or Techron Concentrate for example) and let it soak for
a few days. Then try applying power to see if the fuel
pump has freed up and whirs as it should.
Fuel Pump Disassembly
If the fuel pump is obstructed by
foreign objects or larger bits of corrosion then you can
disassemble the fuel pump, clean it and re-assemble it.
This is a fair amount of work but given that fuel pumps cost
about $400 from BMW these days it is worth trying.
The following are pictures of a
fuel pump from a 1985 fuel pump that sat in some wet gas for
a couple years. When power was applied I could hear a
clunk, indicating that there was some foreign matter jamming
the fuel pump.
The first step was to bend the
top lip back so that the fuel pump assembly could be removed
from its outer aluminum housing.
I then tapped it out from the
bottom using a small hammer until the top fell out. Then I
sprayed some penetrating oil in to help the rest of it come
Then I placed the top in a bench
vise to keep it from turning. Note that I did not
tighten the vice to avoid deforming the inner housing. I
just closed the vice enough so that it would keep the inner
housing from turning while I twisted the outer housing up
Here it is after I removed it
from the outer housing.
The inside of the inner housing
was pretty dirty. (I took this picture after I'd cleaned the
outside of the inner housing with some steel wool so the
silver strands you see are steel wool bits picked up by the
Then I removed the armature from
the shaft by removing the C-clip and two washers at the top
of it. (This step is probably optional.)
The fuel pump below the motor is
housed in three plates held together by four Torx 20 machine
screws. Before disassembling it I put a mark on each
of the three pieces with a small grinding bit to make sure I
re-assembled it correctly.
Here's a close-up of the nylon
driver that transmits power from the electric motor to the
fuel pump mechanism at the base.
Here's a diagram that shows how
the fuel pump works.
Here's the fuel pump
When I took it apart these little
chunks of corrosion fell out. I suspect they were what
was jamming the fuel pump.
The middle part of the fuel pump
has a Teflon ring inside of it. I used rubbing alcohol
and a paper towel to clean that and used steel wool to clean
up the metal parts.
When putting the fuel pump back
together I determined that if I screwed the Torx machine
screws in too tightly that the fuel pump would not spin.
Therefore I only got them finger tight and used medium
Loctite on the threads to make sure that they stayed in
Before installing the pump and
armature in the inner housing I tapped the shaft into the
base with a small hammer.
I used some small wires to pull
the brush springs back to in order keep the brushes far
enough apart so that I could install the top of the armature
in the brushes.
When re-assembling it be sure the
O-ring at the base of the outer housing is in place.
Once I had it back together I
used a hammer and nail set to bend the aluminum to hold the
top of it back in place.
© 2013 Drake Smith - Please do
not use or reproduce this elsewhere. Feel free to link
to it though.