Dies when Hot

[MarcoK75c]

I have a 1986 k75c that is getting very frustrating to diagnose. After about 30-45min of running it will shut it self off and die. I can crank and crank and not get it to fire. After it cools down it’s good for another 30 minutes.

When this happens I can see a spark.
I have recently replaced the fuel pump.
The fuel pump relay works at room temperature
The pickup coils have been replaced with used ones.
The connection to the fuel pump through the level gauge is good
The connector below the tank has been cleaned

I cannot replicate this in the garage… and this is the most frustrating part. I have put a hair dryer on the ignition module and the pickup coils without any effect…

Looking for some suggestions.
Thanks
Marco

[bast1472]

This is probably a long shot, but I had these exact same symptoms when I first got my '95 a few years ago. The culprit ended up being the leads that connect to the fuel pump. One of them had come loose, and for some reason it would run fine until the bike got hot, then the fuel pump would randomly cut out. Check that those connectors are crimped tight and attached securely to the blades on the fuel pump. When I looked at mine it was black from arcing inside the tank 😱

[MarcoK75c]

The fuel pump connections have been re-crimped, when the problem first arose I found that I could not hear the fuel pump when cranking so this is where I went first and found power to the pump was just a few copper strands, fixed it, went back out, got stranded again....

[sooprvylyn]

  Have you checked your thermostat to make sure its working? Is your oil/water pump working properly? the impeller on that pump tends to rust over time and could potentially stop working.

Your motronic unit is getting readings from your water and oil temp sensors.....im guessing it might cut the engine if there is a problem with either of those....which a bad thermostat or bad oil pump would cause. Not sure if it does cur the engine on those, but would make sense that it would in order to protect the engine.

Thermostat is easy to check....drain your coolant, pull out the thermostat, check the rubber seal is in good condition, then slick the thermostat in a pot of boiling water to see if it opens up.   If it doesnt, replace it.  If it does, might need to check that oil/water pump out for wear.

[frankenduck]

Your motronic unit is getting readings from your water and oil temp sensors.

The above is misinformation:

1) A K75 uses the analog L-Jetronic, not the digital Motronic for fuel management. They are very different.

2) There is no oil temp sensor on a K75, K100 or K1100. The only thing that's monitored for oil is the oil pressure via a binary switch in the oil pump.

[MarcoK75c]

I did check my water temperature sensor resistance cold and will get the engine warm tonight and check it again at temperature, believe I read somewhere that if it senses an over heating engine, it sends a signal to the L-Jet ECU to shut off the fuel pump. I also plan to take my fuel level sensor off the tank and look at the resistance of the wires powering the fuel pump.

[Laitch]

. . . believe I read somewhere that if it senses an over heating engine, it sends a signal to the L-Jet ECU to shut off the fuel pump.

That feature is not cited in BMW's LE Jetronic Training Manual, so don't count on it. The pump shuts off when the crankshaft stops turning, for example, in a crash. At overrun, >8900rpm, the injectors stop injecting fuel. If the engine fan is running and the temperature light in the instrument cluster is lit, let the engine cool some before proceeding. Check the coolant level in the reservoir and oil level in the sight glass.

[MarcoK75c]

Few more checks off the bike tonight:

• Removed fuel level sender, hooked to an Ohm meter wiggled the wires heated it.. never stopped working so no problem here. Everything looks really clean, original VDO
• Tested fuel pump relay with 9V battery, closes as it should, resistance on switched circuit goes from 2Ohm to 4Ohm when heated but that should not indicates failure.
• Swapped ignition module with another used one. Bike starts and idles from cold
• Checked resistance value of water temp sensor cold and hot, these check out at the values they should.

Tomorrow i will remove the pickup cover and get out the hair dryer… and will heat up the ignition coils too for good measure….

[Laitch]

Tomorrow i will remove the pickup cover and get out the hair dryer…

When you write pickup cover, are you referring to what many of us call the Hall Effect Sensor cover? If you are, then I believe it is likely the engine will stall if you heat that sufficiently. Just don't barbecue it. You also might need to replace the seal. They can be reused if not too old, or damaged.
 

[MarcoK75c]

One more test this morning: Hall Effect sensor cover off, warmed engine and then 15min hair dryer heat. Bike runs just fine…..

Tomorrow when the rain stops, i will road test it again. I will pack lunch, water, multi meter and a spray bottle to cool electrical components down… just in case….

[frankenduck]

Not necessarily heat or time related but another potential cause of SKDS (Sudden K Death Syndrome) is a dirty ignition switch.

Clean Ignition Switch

[MarcoK75c]

Well my test drive ended a 1/4 mile from home and I got a good workout.

This time I had my multi-meter with me: 12V at the connector below the tank when cranking, no power at the new fuel pump. Got back, disassembled the float level sender for the second time and the positive connector at the pump was no longer connected to the wire. I started pealing back the insulation to find both wires highly degraded and decided to replace them. But my soldering skills suck and after 2 hours I realized the connection through the fuel level base is also intermittent...gave up and purchased a new fuel level sender. I need this bike running in two weeks and I am starting to get fed up....

Hopefully this cures my problems....

 

[MarcoK75c]

Well my 30 mile run went very well. The new fuel sender fixed my issue (but dented my pocket book). Apparently when electricity runs through just a few strands of copper it can do strange things and wires sitting in gasoline for 35 years degrade quite a bit.

[bast1472]

Nice! My initial hunch wasn't too far off :D

[Laitch]

. . . The connection to the fuel pump through the level gauge is good . . .

Just not good enough.  Congratulations for being persistent. 


bast1472 wins the coconut!