1985 K100 fuel leak at sending unit- replaced the rubber gasket and still leaks

[Wheezyryder]

I recently bought a 1985 K100 (produced in July, 1984 according to the VIN) with about 45k miles on it. It's been sitting unused in a garage for the last few years so I'm in the process of getting it roadworthy again. So far new battery, tires, final drive spline lube, rebuilt front master cylinder, new spark plugs and drained/cleaned the fuel tank.

Before I drained the tank I noticed it leaking fuel for the fuel sender unit at the front left side of the tank. Did a bit of searching on this site and found out that this style of sender unit is only on the early model K100s. According to my research, of the fuel seemed to be leaking between the sending unit and the retaining nut and not through the wire penetrations of the sending unit itself, then the likely culprit was the rubber gasket. Mine does not seem to leak through the wire penetrations so I ordered the gasket from Max BMW.

I installed the new gasket, sending unit and retaining nut and it still leaks. Has anyone had the same problem? I disassembled and reinstalled it a couple of times to make sure that the gasket wasn't pinched or out of place, and it didn't seem to be.

A couple of questions:

1) There's an arrow on the sending unit (see first pic). Is there a certain direction that the arrow should point in order to get it to seal? I tried a few different orientations and still got the leak.

2) The flange on the sending unit doesn't seat evenly against the retaining nut (see second pic). The gap between them is larger in some places than others. Should it be like that?

I have the Haynes manual and I downloaded the BMW service manual and they both show procedures for the newer, square sending unit and not the style I have. Djoes anyone have an idea what I'm missing?

I'm really itching to get this leak fixed because I haven't actually been able to ride the bike yet and this is the last thing keeping me from trying it out.

[Wheezyryder]

Uneven spacing

[Wheezyryder]

Sending unit location/style

[Laitch]

Does the O-ring have a flat surface or a curved surface? What is the part number on the package?

[Wheezyryder]

Laitch- the O ring had a flat surface. I've thrown out the package but based on my confirmation email the part number I ordered was 61311459448. It did appear to be the same O ring and the old one that I removed.

[Scott_]

With the uneven spacing you pointed out, something sure doesn't look right, like the sender is either crooked in the hole(which I doubt) or the nut is cross threaded.

[Wheezyryder]

Scott- I thought it didn't look right as well. I have checked and double checked that the retaining nut is not cross-threaded. It didn't feel cross-threaded as I screwed it on, and I don't see any damage to the threads of the retaining nut. I would think that if the plastic nut were cross-threaded onto the aluminum threads, there'd be visible damage. Which leads me to wonder if the sending unit itself is damaged, or if it is supposed to have uneven spacing? The far end of the sending unit is very close to the inside wall of the fuel tank when it is in place, so I thought there's a possibility that it is supposed to be canted toward the center to avoid touching. I wish I could see one of these in place on another bike so I could determine if this is normal or not. I REALLY hope the sending unit isn't damaged, as the part is no longer available from BMW and a quick search of the interwebs did not show a single used one for sale (searched Google, eBay, Beemer Boneyard).

[Wheezyryder]

A bit more searching shows that there may some used ones available internationally, but I sure do hope not to have to go down that rabbit hole. Sounds expensive, slow and painful.

[Laitch]

What is that red substance at the base of the nut? Is is the same as what's on the seam of the tank?

[Wheezyryder]

Laitch-The tank (and the whole bike) were originally a wine red color. It's been painted black at some point. Actually looks like a fairly well done paint job but has since got some minor damage and chips. The red you see is the original paint. The threads on the tank appear to be in good condition and clean, without any sealers out other substances on them.

[Laitch]

Remove the sender. Post a few photos of the tank hole, the reverse side of the flange with the "arrow" and the sender itself. I haven't worked with this sender but if the parts haven't been damaged then there might be an alignment tab you're overlooking.


Maybe somebody with one of these old tanks will chime in. This shouldn't be complicated.

[Wheezyryder]

Laitch- I'll get some pics and part them as soon as I get to the house.

I agree that this is probably something simple I'm overlooking. With as many of these bikes as there are on the road, I can't be the first to have this issue. Hopefully someone can chime in with the solution.

[Laitch]

With as many of these bikes as there are on the road, I can't be the first to have this issue.

Somebody had to be first. Why wouldn't it be you?    You're breaking new ground here.  This style of fuel sender was only used for two years out of a run of maybe 7 years for 2V motos. I couldn't find another example of this problem's needing Internet consultation but I'm still looking.

[Wheezyryder]

Laitch- I was hoping not to be the first guy to pioneer a new problem. Oh well 

Here are some pics

[Wheezyryder]

Sending unit arrow

[Wheezyryder]

tank hole

[Wheezyryder]

More tank hole

[Laitch]

Is the twisted thing at the arrowhead a gasket?


Is this arrow pointing at a gasket?
[

[Wheezyryder]

In the top pic that's just a part of the tank. The bottom pic is pointing toward the new o ring.

I've been thinking about this. The inner part of that o ring seals between the cylindrical body of the sending unit. The outer part of the o ring seals against the threads of retaining nut (which seems like a bad idea). My suspicion is that the fuel is leaking at the threads. I went on the webs and found some PTFE thread sealant tape that's fuel resistant. I'm thinking I may order that and try some. If it doesn't work I can just remove it.

[Laitch]

[size=78%]The gasket should be contacting the upper inside surface of the nut, the sides of the sender body and the opening of the threaded port. If this were my problem I'd clean the corrosion from the inside of the hole and the tank port threads with a Scotchbrite pad, wipe clean the sender body, use a [/size][size=78%]scant[/size][size=78%] coating of petroleum jelly on the sender body and gasket, shove the sender down into the passage then screw down the nut. You could orient what you're calling "the arrow" so it points toward the center of the tank as long as doing that doesn't necessitate twisting the electrical plug's wires severely to insert it.[/size]

Teflon paste or other sealing concoctions could come later if that doesn't work.

[Martin]

Check the underside of the tank for a corresponding mark that you might have to align the arrow to.
Regards Martin.

[The Mighty Gryphon]

What is the condition of the flange on the sending unit that the gasket seals against?  Any cracks in it or missing chunks?

Per Laitch's advice, clean the top of the threaded spigot on the tank and make sure there aren't any pits or dings in the sealing surface. 

I wonder if a good cleaning of the sealing surfaces followed by a little silicone sealer before assembly would do the trick.

Finally, is it possible that the leak is along the wires?

[daveson]

Remove the filler cap assembly for a look see. Is there a pipe or something pushing the sender unit off centre?

[Wheezyryder]

Laitch- I did everything you said, except that I used WD40 to lube the o-ring before I put it in. I used a scotch brite pad on the inside of the hole in the tank and a Dremel with a brass brush to clean the threads. There's still a bit of discoloration but nothing that you can feel. Feels completely smooth/clean when you run your fingertip along it. Unfortunately, still leaking.

Martin- I looked for a mark or indentation on the tank that would indicate where the arrow should point. I don't see anything. I don't know if a mark was covered up or removed when it was repainted.

Mighty Gryphon- I inspected the sending unit, retaining nut and flange very closely with a light, and don't' see any cracks, holes or chunks missing. I also checked the sealing surface on the on the threaded spigot and I don't see any imperfections.

Daveson- Good thought. I removed the filler cap. The top of the sending unit is within a 1/4 inch of touching the wall of the fuel tank, but it doesn't touch. I spun it around 360 degrees and it doesn't touch anything no matter which orientation.

All- After acting on everyone's suggestions and disassembling/reassembling this thing several more times I still have the leak. I think I'm going to start trying some other things:

1)There's a good possibility that it's leaking through the threads of the retaining nut/spigot. I found a product on the web called Petro Tape. It's a PTFE/teflon thread sealing tape that's impervious to fuel. I can try that and if it doesn't work it won't hurt anything.

2) After putting together/taking apart so many times, the new O ring now has some imperfections. I'm gonna order a few more. I may also try doubling up the O ring. Nothing else I've tried worked and it can't hurt.

3) I really suspect that my sending unit and/or retaining nut is damaged in some way that is causing them to seat together crooked. Being crooked may twist the O ring just enough to not seal. If none of the other methods works, I may try sealing the retaining nut/sending unit together. I found a couple of RTV sealants that are impervious to fuel (Permatex MotoSeal is one of them). Then I could screw the whole thing in as a single unit. The only disadvantage of that plan is that I'd have to disconnect the wires from the fuel pump any time I need to remove the sending unit. No big deal.


I have to work 12 hour shifts the next few days (I'm an ICU nurse in the COVID unit of an Atlanta hospital) so I won't be able to work on it for a bit. Plus I need to order some things and wait for them to come in, Thanks for everyone's help so far. If anyone has a better idea in the meantime please let me know.

Regards,
Mike

[Laitch]

Laitch- I did everything you said, except that I used WD40 to lube the o-ring before I put it in. I used a scotch brite pad on the inside of the hole in the tank and a Dremel with a brass brush to clean the threads. There's still a bit of discoloration but nothing that you can feel. Feels completely smooth/clean when you run your fingertip along it. Unfortunately, still leaking.

Where's it leaking—bottom of the nut, top of the nut, somewhere else? If you need more excitement than an ICU—where I spent 10 luxurious days this April listen to the revelry at afternoon report—you might try tightening that nut a leeetle more.