Intermittent fuel cutoff and then full recovery-baffled

[BrickFlyer]

My 85 K100RS runs  super smooth most of the time.  On a typical 3 hour or so ride I have 179 minutes and 20 seconds of pure motobrick bliss.  However, at seemingly random intervals it will feel like someone choked off my fuel line. I could be cruising along at 60 or 80 mph and all the sudden it just starts to die. No warning lights or anything that would suggest a electrical fault.  Rolling throttle does nothing. So I look to the right hand side of the road so I can pull over if needed and then whoa ----it roars back to life and I can keep running along smoothly for another random interval.  The entire event probably lasts 2-5 seconds, or perhaps even less.  But if it happens on a 6-lane slab in Atlanta it gets real interesting real quick.

The fact that it returns to perfectly smooth operation so quickly baffles me.  It might be easier to figure out if the problem were not a random intermittent. My fuel filter is pretty new (replaced in July),  I synced my throttle bodies, the HES has been replaced and the timing is at the stock settings. All the fuel lines, vacuum hoses and the breather hose were also replaced this past summer.  The only thing I haven't done is replace the throttle body rubber boots. The electrical connector that runs to the bottom of the gas tank was cleaned with contact cleaner and a little dielectric grease was added. The connector doesn't make a really strong mechanical connection, but it seems to stay seated. I checked the EFI brain connection and it seems to be seated well and locked by the tabs on the sides.

I'd appreciate any ideas of things that might cause these symptoms. Thanks!

[johnny]

the bugeye motobrick would do the same thing... i replaced the 4 pin electrical connector that connects the gas tank to the wiring harness and all was well... f ed with it for 2 years of cutting out... then installed a 4 pin trailer connector and it never cut out again...

j o

[frankenduck]

Ignition switch.  (Had a guy bring by a K75 last summer. Same symptoms.)

[Inge K.]

Would probably be either one of the above suggestions.

If it`s the ignition switch you would loose the gear indicator, and instrument needles
go rather qiuck to zero.
If it`s the fuel pump connector, the needles and gear indicator would stay.

Here is a "how to" dismantle the ignition switch.

Inge K.

[mystic red]

Would probably be either one of the above suggestions.

If it`s the ignition switch you would loose the gear indicator, and instrument needles
go rather qiuck to zero.
If it`s the fuel pump connector, the needles and gear indicator would stay.

Here is a "how to" dismantle the ignition switch.

Inge K.

In German.

[Inge K.]

In German.

The pictures is in a international language, use google translate on the remaining.

Inge K.

[mystic red]

In German.

The pictures is in a international language, use google translate on the remaining.

Inge K.

 :lol:

[BrickFlyer]

Would probably be either one of the above suggestions.

If it`s the ignition switch you would loose the gear indicator, and instrument needles
go rather qiuck to zero.
If it`s the fuel pump connector, the needles and gear indicator would stay.

Here is a "how to" dismantle the ignition switch.

Inge K.

Actually, I forgot to mention that I had also disassembled and cleaned the ignition switch this past summer.  Plus my instrument needles do not drop off when it happens.  So that helps narrow it down.  Therefore it sounds like the fuel pump connector is the likely bad actor. As I mentioned it's not a strong mechanical connection. I can pull the plug out very easily. So I'll have to replace it with a solid weatherproof set of connections.

I guess my next question would be, what's the best repair or replacement option? If I recall correctly, the part that goes into the tank has 4 male pins and the part that is at the terminal end of the harness has 4 female pins inside the white plastic shell.  So I'd either have to A) find an entire mating piece with the female pins in it,  B) push out the pins and then crimp/solder new pins onto the harness wires and push them back into the shell, or C)  bypass the whole thing entirely with new wiring such as the trailer connector suggestion.

As far as the trailer connector goes,  how do you make a connection to the fuel pump end? Does the connector come out easily to make a direct hookup to the fuel pump?

[frankenduck]

Replacing the connector for the fuel level sender

[Rick G]

I had to replace the connector with a 4 pin multi purpose connector from the local auto shop.
I have also had similar symptoms when a fuel pump was in its dying throes, I reckoned it was a connection fault from the way it behaved but it finally died and was a pump. that was when I found out about the infamous BMW fuel pumps and just how to avoid the expense.

[kenray]

 I had a similar problem with my 89 k100rs. The problem existed for some time, and would come and go. Drove me crazy. Finally got really bad. I replaced the ignition switch when it started to act really strange. I have had no problems since. Don't know if this could be your problem but thought it might be worth mentioning. kenray in Katy TX.

[Scott_]

Another possibility would be the fuel level sending unit mounting(underside of the tank where the wires enter into the tank). It's possible that the wire could be broken where is passes thru this fitting. Has happened to some. This is not repairable, only option is to replace the whole assy.

[johnny]

i used 1 of these... 4 pin flat...

j o

[frankenduck]

i used 1 of these... 4 pin flat...

j o

You left out the half roll of electrical tape that you used for "soldering."  :giggles

[BrickFlyer]

Another possibility would be the fuel level sending unit mounting(underside of the tank where the wires enter into the tank). It's possible that the wire could be broken where is passes thru this fitting. Has happened to some. This is not repairable, only option is to replace the whole assy.

Yes. that's the connection I suspect is my problem. I was trying to find out how to replace or bypass both mating sections. The top part of the connection (that goes into the tank) looks ok. The pins are straight and fixed very firm but that isn't proof that the connection is good.

On the other hand, the lower part of the connection (which is the terminal end of the wiring harness that plugs into the recess under the tank) has hollow pins that look slightly split and do not form a tight mechanical connection.

It looks like the folks who have put trailer connections on are splicing them into the wire harness to provide a secondary  detachment point below the round fuel tank connection. That won't do me any good if I do not have a solid connection to the tank fitting.

Has anyone found a source for the round lower (harness) part of the connection? It looks similar to an unused 4-pin white cylindrical connector I saw elsewhere on the bike (maybe an alarm connection or something?). Could I repurpose that connector or find one online somewhere?

[frankenduck]

Why not just splice in a trailer wiring connector and have something reliable?

The gear position indicator and early ABS modulators use a similar connector but why replace it with something that is equally prone to failure?

[BrickFlyer]

Apparently I'm either not descriibing this well or I am missing something really obvious :-(   When you install the trailer connection, do you still have the white cylindrical 4-pin socket connection to the tank? If not  how do you work aroud that?

[frankenduck]

If you think that the connector at the base of the fuel level sender is the issue then you need to replace the fuel level sender.  Those have been known to go bad as welll.

[BrickFlyer]

Here's a picture to clear it up.  This is the only electrical connection to the fuel tank, and I understand that it provides power to the fuel pump, and picks up the signal from the level sender inside the tank.  If you look closely you can see what I mean about the splayed connector pin inside. I think the ones at 9 O'clock and 12:00 are getting about 50-75% contact. That's dielectric grease all around the pins. 

My fuel level indicator is actually pretty reliable and when it comes on I need around 3.5 gals of fuel. Usually between 120-140 miles since last fillup. Probably works better than most K100's from what I hear.

So I was wondering if I can just buy a new connector like this, or maybe remove the pins and replace them, or repurpose a similar connector on the bike.   I do not think the mating half of this connection is the issue.

I am a bit concerned about the other comment that maybe this is the early signs of a fuel pump failure. But based on the looks of the connection I would like to eliminate that option first as it would seem cheapest.

[BrickFlyer]

BTW -I'm going to check a local auto parts store. Just though someone who replaced their connector might happen to have a Bosch part number, or a similar BMW car connection or something. COuldn't find anything on RealOEM other than a replacement harness.

[Inge K.]

The connector shown in your picture is used on the -83-85 year models.
The suggestions with trailer connectors is valid to -86 on models.
(which have piece of wire between the tank and the connector).


Maybe you could fit some thin pieces of plastic sheet (or whatever) between
the single terminals and the connector housing, and that way getting better
tension on the female terminals?

Inge K.

[BrickFlyer]

The connector shown in your picture is used on the -83-85 year models.
The suggestions with trailer connectors is valid to -86 on models.
(which have piece of wire between the tank and the connector).


Maybe you could fit some thin pieces of plastic sheet (or whatever) between
the single terminals and the connector housing, and that way getting better
tension on the female terminals?

Inge K.

Aha. Now I understand. Thanks for pointing that out! I couldn't figure out how a trailer connector would help in my situation. We were comparing apples and oranges.  The 85 K100RS has a lot of unique bits and pieces.

I like the idea of putting some sort of insulating material like plastic inside the gap.  Or maybe I could fill the female pin with grease and pour some epoxy around the outside. The grease would keep epoxy out of the pin hollow and then the male  pin would just push the grease out the bottom after the epoxy sets.  Might be a good workaround if I cant find a replacement connector.

Of course I took the bike out for a 150 mile ride and it didn't show any symptoms. Possibly that was because I removed the connector to take the picture and reseated it again before the ride. SO I think I'm getting close to root cause.

[Inge K.]

I like the idea of putting some sort of insulating material like plastic inside the gap.  Or maybe I could fill the female pin with grease and pour some epoxy around the outside. The grease would keep epoxy out of the pin hollow and then the male  pin would just push the grease out the bottom after the epoxy sets.  Might be a good workaround if I cant find a replacement connector.

If you use some thin sheet of plastic (or similar), and it don`t turn out as expected the first
time...you can pick it out and replace it with something that is a hair thicker.
If you fill the area between with epoxy......you`re kind of stuck.

Inge K.

[BrickFlyer]

Good point. I will try the shimming method first.  :2thumbup:

[Milton]

My experience. Bike would run fine for hours on end then all of a sudden it would just quit, usually on deceleration. I would pull over to the side of the road and wait a couple of minutes and it would fire right back up. Might cut out a few more times right after the first one and then would run fine for hours. This was giving me fits.

Fixes I tried.

1: Jumpered the side stand switch. Didn't work but now I can start it on the side stand. 
2: Fuel filter. Didn't work but it did seem a bit blocked; could still blow through it but with resistance.
3: Fuel pump. Didn't work, but the bike seemed to run better after replacing the fuel filter so I thought that maybe the fuel pump was getting weak.
4: Fuel tank electrical connector. Fixed it.  :clap: :riding: :clap: :riding:

Got a 4 pin trailer style connector from Napa and soldered in the wires (even had the same colored wire pattern) with shrink wrap on each joint then wrapped all together with electrical tape. Works a charm.

Left to my own diagnostic skills I don't think I would have ever considered a wiring connector to cause such a problem.

Thanks a bunch.