honey, what's that gas smell in the garage? ... leaking fuel tank

[DavidATL]

About 24 hours after I parked the K75 from Monday's ride, it developed a sudden and large fuel tank leak. I put a paper towel under the crankcase each time I park it to look for leaks. It went from one oil drop a quarter to 1/2 gallon of fuel in 24 hours. Of course, the tank was full. Yay me.


Can't have that kind of leak in the house so... I roll it outside, head to Advance Auto for a 5 gallon can and siphon. I get back and the rain is starting. Any by starting, I mean biblical downpour. Lovely. I got about 4-4/12 gallons out of the tank. Probably got a little water inside but it's all coming apart anyway.


I see two likely causes: fuel tank corrosion or level sender o-ring failure. I'm guessing o-ring before I pull it apart (sudden massive failure, fuel dripping from the insulation under the tank one drop every 2-3 seconds). Bike has 20k on it and has been inside and well cared for it's entire life so corrosion seems less likely. Fingers crossed.


Not too depressing as I was going to change the vibration damper this fall anyway. And it's oil change time too.


I have 10 O-rings on on order. Anybody want to buy up to 8 of these? $1 each plus SASE. I may post in the FS section once I have them.


I'll know more this weekend about the leak.

[White Dog]

My '95 K75 suddenly developed a leak similar to what you describe.  Traced the leak to a cracked O ring under the fuel sender unit.  Not hard to fix, but sender unit has to be removed to position O ring in proper groove.

[BrickMW]

I will take an o-ring or two.. planning to replace my sender soon. Plus, spares are always welcome!  :2thumbup:  PM sent for address.

[Laitch]

Maybe all that is needed is to tighten the mounting plate—if that's where it's leaking.  :yes

[The Mighty Gryphon]

If not the O-ring(I hope that is the problem), the tanks on K-bikes are prone to corrosion at the joint between the top and bottom of the tank.  The corrosion is visible as blisters in the paint at the lowest point of the tank as it is mounted on the bike.

The corrosion is the result of water collecting and settling out in the tank.  It is a fairly frequent problem on these bikes, especially bikes that have been sitting unused for long periods.  It seems to appear more often in low mileage bikes in places with a lot of rain and/or humidity.  It seems to be a fairly common problem in the U.K..

While the best fix is to have the tank rewelded which will require repair to the paint, the preferred method is to use a fuel resistant epoxy to form a patch on the damaged area.  These epoxies are sold in auto parts stores specifically for fuel tank repair.  This job can be done in an hour or two and doesn't effect the paint anywhere away from the repair.

[White Dog]

When the O ring failed on my K75, I was down for several days as local BMW did not stock the fuel sender unit O ring.  Only cost a couple bucks but had to wait several days.
 
I just installed a new sender unit I bought and it came with the O ring.

[DavidATL]

Got the tank off and pulled the sender. The insulation on the underside was still soaked with fuel. I noticed this was confined to the section that surrounds the sender. I pulled it up (it was no longer bonded to the tank) and what I found was really nasty - lots of bubbled paint.


I pulled off all of the damaged paint. It was bubbled only under the one section of insulation that surrounds and covers the sender plate, and it was soaked with fuel. On initial inspection, I can't find an area of corrosion.


The O-ring was in place with a few cracks but nothing that was all the away through. I assume it was round when new and is now permanently squished.

[White Dog]

Throw it away.  The gas is seeping thru those cracks.

[DavidATL]

underside of the tank (yuck):

[DavidATL]

and after I peeled off 99% of the loose paint:

[Laitch]

and after I peeled off 99% of the loose paint:

I think removing the other 1% will give your bike just the right street-cred look.

[DavidATL]

I found several pieces of rubber edging(?) in the fuel tank on and around the fuel pump mount bracket. Other than protecting a mechanic from cuts, I can't see that they have any use. The ones in my bike are quite firm much like the fuel damper rubber is.

Should I replace these? If so, anybody have a source? I couldn't find a mention on beemerboneyard or elsewhere on the net. 

Should I remove these? I am concerned about degrading just as with the fuel pump damper. (already being replaced).


btw- I'll mention the need for fuel line with specific rating for in tank use. I didn't consider this and purchased regular FI line but have since found out that specific line rated to SAE 30R10 is required for intake / submerged use. $13 + shipping from beemerboneyard  (item Bbsubhosekit). I am ordering that today.


STILL WAITING ON THE SENDER PLATE O-RING SEAL. I keep finding more things to fix though…

[Martin]

Apparently the rubbers were installed as an anti vibration device.  :dunno
Regards Martin.

[DavidATL]

New and old sender plate O-ring for comparison.  One pair spoken for, 7 left. Let me know if you want one or two!

[jaxon]

Heard a story of a guy that went sport parachuting, the course was really a poor, bargain basement class. He was instructed to pull his ripcord when his altimeter read 1,500 feet.  The instructor says "If the main doesn't open, pull the reserve. They'll be a truck waiting to pick  you up on the drop zone."


He donned his chute, got on the bird. Reached jump altitude and exited the aircraft. After reaching altitude, he dutifully pulled his ripcord. Nothing happened. He quickly recalled the instruction to deploy his reserve chute. Nothing happened. Moments later he sees a man flying up past him. He yells out "Hey buddy! Know anything about parachutes!" The guy flying past him says "Sorry no! Happen to know anything about BMW K-Bike fuel systems!"


Someone on the Drop Zone heard the  new sky diver yell out "I bet the instructor lied about the fucking truck too."

[Chaos]

I'll take one of the o-rings.  Where do I send the dollah?

[DavidATL]

Went on a 90 minute test drive this morning and everything was fine!!! Yay! still some good riding weather left.


at this time, I have 5 O-rings unspoken for. Still $1 + SASE to return them in. PM me for address.


Seriously, if they haven't already, everyone should replace this part. Mine gave out after 25 years and gas leaking down onto the engine and the wiring harness was no laughing matter.