1985 BMW K100 - breaking down under load when hot

[PeterA]

Hello Everyone,

Posting on behalf of my son who has a "new to him" 1985 K100 - 114,000kms.

"Hey everyone, need some help sorting out a (maybe some) gremlin. Rode my k100 into city (35kms - no issue) but coming home today (hotter day), I noticed that it started to splutter and have poor acceleration when twisting the throttle at speed when it was hot. Was wanting to stall when pulling up at stops unless I kept the revs up. Battery light was coming on at stops. Pulled up in the driveway and it died. However, it was still turning on. Left it for an hour, turned on fine, road fine around the block. Obviously trying to diagnose the issue, has anyone run into something similar? Edit: Fuel filter, fuel pump hose in tank were replaced relatively recently by previous owner with new parts."

Thoughts and experiences on possible issues would be greatly appreciated.

- Fuel Starvation, vapor lock?
- Fuel Pump loosing power - poor connection, cracked solder (heat related)
- Hall effect sensor playing up when hot?
- Coils playing up when hot?

Many thanks
Peter

[daveson]

If it plays up when hot, it's often the hall sensors.

It's struggling to survive the heat, the revs dropped, that's possibly why the battery light came on. Understandable, I'm struggling to survive the heat today too.

In the morning with the engine cold, blow some heat on the hall sensor cover, if it runs crappy, that's a pretty good clue. Then spray some water on it, if good again, that's another good clue. I spose you want to be a little bit careful about it.

[PeterA]

Thanks Daveson - good tests - will give that a go.

[Laitch]

If this is the same Brick with the undetermined "noise" that you started with at this site, Peter, then an orderly approach is needed. Sure, Hall sensors can fail but they're a fairly sturdy assembly. The most likely failures on an '85 Brick would be the z-hose or the coils, that is, if the main ground connections attached to the frame backbone have been inspected and cleaned. Heat can cause tenuous electrical connections. The coils on the '85 became more durable in later models. A torn z-hose connecting the crankcase to the airbox can introduce excessive unmetered air into the mix when its tear expands as hot vapor acts upon it. That can cause hard starting. Dirty electrical connections should always be considered when certain failures occur.
A ride around the block doesn't prove much, but it is encouraging. You should also have the battery load tested—even if it's new—to verify that it can hold a charge.

Anyway, the attached photo shows the location of the z-hose. Rubber intake components connecting the airbox to the throttle bodies can be tested for leaks while the engine is idling. Spray a thin stream of propane or Mapp gas onto them. If the idle changes, that indicates leakage.


This image of the z-hose is from Euro Motoelectrics, a reliable vendor of Brick parts.

Pulled up in the driveway and it died. However, it was still turning on.

The starter can crank the engine but not start it running. By still turning on, am I to understand that is when the engine could still be  cranked but wouldn't start?

[PeterA]

Thanks Laitch,

Yep same bike - will check tank connections as previous owner said he replaced pump and filter but I suspect electrical fault here would lead to motor completely cutting out from no fuel - makes me wonder if he was chasing the same intermittent issue.

Interesting comment on grounding as battery seems not to have enough amps to turn over motor on first start button push but second push it starts straight away - possible culprit.

I'm waiting on delivery of good (by resistance measure) coil so I can test swap out (seller only has one so will have to have a couple of attempts) and also a Hall Effect Sensor from a known working bike.

Re: Hall Sensor - if bike starts to play up out on open road would cooling the area with water cool enough to have a positive effect? Can try the hairdryer test in shed as well.

Re: Output Shaft - I've come across a 12 rivet shaft in an 1100 block so I'll snap that up and put in shed for future if required, I've also got Alternator rubbers on order so hope that stops the grumble.

Many thanks for your support.

Regards
Peter

[Laitch]

Re: Output Shaft - I've come across a 12 rivet shaft in an 1100 block so I'll snap that up and put in shed for future if required, I've also got Alternator rubbers on order so hope that stops the grumble.

An old bike is likely to grumble; yours doesn't seem to be experiencing agonal breathing.    A full tuneup would establish a baseline for future diagnostics. I don't know if throwing cold water on a Hall sensor cover will dissipate enough heat to restore the function of faulty sensors.

I suggest doing the simplest tasks first—checking rubber intake parts for air leaks using the method previously suggested, cleaning the central ground connections on the frame backbone beneath the tank. DeoxIT D5 has been an effective electrical contact cleaner for many of us. It's excellent for cleaning oxidized connector components.

In conclusion:

Assuming and suspecting don't function well without thorough knowledge. Download BMW's LE Jetronic training manual to learn how the machine operates. You'll find the link within this list. You'll find a link to a diagnostic guide—the K-Bike Flow Chart—at the bottom of this page.

[frankenduck]

In my experience, stumbling tends to be coils failing (especially with the early oils) while a failing HES causes a total shutdown.

[PeterA]

Hello Everyone,

My son took the K100 out for a ride today (40 degrees Celius) and rode for 35-40 mins and bike started to play up coming off freeway near home. Tank excessively hot, fumy petrol smell, fuel pump making much louder sound than normal, 4L engine fuel light on even though tank is approximately 1/2 full - 4L light is heat driven so when fuel is low sensor would get hot indicating level but if fuel is hot then sensor will falsely trigger - that's our thinking anyway. Tank still at 45 degrees Celius when measuring outside of tank with laser temp gauge after an hour of being parked. Suspect boiling fuel.

Previous owner has replaced fuel pump, fuel filter and fuel lines. Upon inspecting fuel line from front of tank to fuel rail it seems 20-30mm too long so it kinks against top radiator hose and touches actual radiator tank on left side under the top hose which will be radiating heat straight into hose and then heating fuel as its circulating. We will cut the fuel hose to ensure no touching and insulate with some Aeroflex pipe insulation (-53 to 125 degree C rated). Will do a small test piece to ensure Aeroflex itself doesn't melt before treating fuel lines. Will also look to use some 6mm Aeroflex sheet under the tank to help with insulation (factory piece is still there but will inspect).

Also will remove fork Gators to get some airflow up around tank and cockpit. If anyone has the fin replacement pieces (instead of gator) I'd be happy to purchase them.

Will report further on next hot day and will monitor tank temp with laser thermometer before it gets near boiling (and very dangerous)!!


Regards
Peter

[Laitch]

Does the fuel tank cap have a push button lock or keyed lock? Are hoses connected to the two ports beneath the tank or do they empty into a plastic cup-like device mounted to the frame?

[PeterA]

Hi Laitch
Tank has a key lock. Haven’t removed tank yet so not sure about connecting to a cup like device. Will look on the weekend.

Regards
Peter

[daveson]

It's handy if the hose is slightly longer. Maybe re-route it so it can't kink, or you can get external coils to slide over the hose to form a bend that prevents kinking. It's rare but I've seen on cars that a hose can kink when it gets hot, restricting fuel flow, which raises the fuel pressure, and then it runs like crap. The original hose had a formed ninety degree bend at the tank barb to prevent kinking. Over fuelling wouldn't be as bad on the freeway, but worse at lower speeds. Next morning when it's cold and less likely to kink, it seems ok.

You want to check that the previous owner used the correct fuel hose in the tank, suitable for submersion in fuel.

A kink could result in the fuel pressure going higher and even up to 60 psi, it should be about 36. I think the higher pressure could explain why the pump was sounding much louder, and for the smell of fuel, and for the stalling at idle.
 
You have a 4 lt fuel tank light, so you have the earlier '85 model brick. Don't worry about the light coming on early, that seems to be normal for the earlier sensors.

Because you have the earlier model, you want to look at the coils for swelling or cracks, and test them. They weren't very good. If replacing them you want the newer coils, which have red bottoms. But I think the hose is more likely to be the problem.

Could you post a photo of the gaiters, or describe them in more detail?

[Kaos]

Also it is worth to check everything inside the gastank.
Insufficient fuel supply could cause the bike to stutter. As the bike and all components warm up, bad hoses wil expand under the pressure causing fuel to escape from the hose. Same for bad or untightened hoseclamps.
Also works and counts for bushings or other rubber components.
Furthermore check the air filter/supply to the engine. Not enough air and those sort of things can cause the bike to run rich and sputter because of it.
And topping it off, havent seen cooling fan being mentioned once. Have you checked the fan coming on? Engine or oil temp? Sufficient coolant? The bike could simply be overheating due to a lack of any of the above.
For what i am aware of, older models could or can have the charge light coming on under 1500? Rpm even when they rode it for it a while.
Hell, thinking of it, it might even be a busted spring of the throttle body.
Man you have a lot to check. Luckily most are easy to check.

[PeterA]

Thanks Daveson, Kaos,

Radiator temp is good - gauge sits just below 1/2, thermo fan is good. Fuel is hot and fuel condensation on fuel cap when you open tank, fuel rail hot to touch so pretty sure fuel is vapourising and causing the motor to break down i.e. no fuel sporadically to injectors. Will fix this first and see what temp the fuel in the tank gets to. I cant image 45 degrees Celsius after 1 hour of sitting is good. I've read BMW actually insulated the fuel lines on later K1100's so there is an issue with fuel temp and fuel line touching radiator is not good so will re-route and insulate - pretty sure this will fix the issue. If fuel is not hot bike runs fine.

Regards
Peter

[daveson]

If the temperature gauge usually sits just below half, I'm thinking it might be running a bit hot.

If the fuel is vaporising, that would be a lean fuel problem. If it's running lean, I'd expect problems at high revs on the highway, but I think you're describing a rough running after getting off the highway at lower revs, and when idling for example. One test might be to reproduce the problem, then alter the hose position to eliminate the kink. If it then runs better and the pump noise goes back to normal, that might be the problem. Since it's ok on the highway, but stalls at idle, that made me think high fuel pressure.

The smell of fuel also points to a rich mixture, rather than a lean one.

If fuel is prevented from returning to the tank by a kink in a hose, the fuel circulation would stop, then the fuel temperature would rise. Also, at sixty psi, that too would increase the temperature.

Another way to verify the problem would be to connect up a fuel pressure gauge and see if it's high or low when the rough running happens.

[PeterA]

Thanks Daveson,

Yes we will re-route the hose and insulate as a first step and then do some testing - there still could be issues around Coils or HES but fuel starvation seems most logical at the moment - tank and fuel rail was abnormally hot!

Regards
Peter

[Laitch]

Upon inspecting fuel line from front of tank to fuel rail it seems 20-30mm too long so it kinks against top radiator hose and touches actual radiator tank on left side under the top hose which will be radiating heat straight into hose and then heating fuel as its circulating.

A kink could result in the fuel pressure going higher and even up to 60 psi, it should be about 36. I think the higher pressure could explain why the pump was sounding much louder, and for the smell of fuel, and for the stalling at idle.

A return hose shouldn't be kinked for the reason daveson indicates. Although there could be some heat picked up from  the radiator, diminished fuel circulation would increase fuel temperature, too, and is the likeliest reason for what you're describing. The OEM return fuel line has molded curves to accommodate the circuitous route back to the tank. Some owners don't want the expense of using the OEM solution and sometimes kinks will follow. Here is a product used by some to avoid hose kinking.

Martin, a member here, has posted his fuel insulating methods here and in a link in that post's text. You seem to be doing some of those measures. He lives in a tropical paradise with seasonal hell like you're describing, in fact, it could be the same paradise.
 
 
Early model Bricks had fuel tank caps that could vapor lock causing poor engine performance, but yours has the improved cap.

[PeterA]

Thanks Laitch,

There is no kink in the pipe, its just a little too long so it bends such that its touching top radiator hose and running along left radiator tank for about 50mm. I should be able to turn the pipe so bend is facing to rear of bike and then not touching anything. I read a guy on a Flying Brick who seems to have had exactly the same issue: https://www.flyingbrick.de/2021/forum/index.php?thread/6137-vaporising-fuel-lines/

Regards
Peter