1985 K100 RT rides well for 20km then goes to idle with throttle unresponsive

[Chris85K100]

Hello,
        after 33 years of riding and tinkering I'm in a bit of a jam with the old girl.  I'll be driving around with occasional micro second power drop hiccups not affecting bike road performance and then after 20km the throttle becomes completely unresponsive, the bike usually goes to idle but on occasion has stalled.  Upon turning the key off and on it usually starts again.  I've noticed the speedometer moves up in neutral frequently.  To me it feels like the computer is forcing the bike to idle and intentionally ignoring throttle position.  Bike pulls hard like new when problem not present. I'm looking for someone who has had a similar problem who could give me some advice. 

Work done so far.   Battery and spark plugs replaced, battery and under tank grounds and 3 engine to frame contact lugs cleaned.  TPS replaced. I've cleaned as many connectors including the FCU.  Cooling fan works as expected however I noticed the problem is worse in hot weather with the bike often not starting for 5-10 minutes.  I have let the bike run hot and have run a hair dryer on exposed Hall Effect Sensors getting them very hot and not the issue.   I've noticed the frame to engine mounting bolts have about 0.2 ohm resistance when bike off, however once on and at idle varies from 0.2-3 ohms.  I'm thinking of running a grounding wire from the under tank ground lug directly to the -ve battery post, however I doubt this will solve all my problems.

Anyone been here?  I've got a new coolant temperature sensor on order however the fan works as expected.

Thanks Chris  Toronto Ontario Canada

[daveson]

I'm thinking of running a grounding wire from the under tank ground lug directly to the -ve battery post, however I doubt this will solve all my problems.

Good idea, and the power side of the circuit could also be weak.

I haven't had this problem but my first thought was a problem with the air flow sensor. I've seen a problem like this on a car and when the air flow sensor was unplugged, it could rev again.

I would check the voltage across the battery at rest, at idle, at 2,000 rpm, and again when it gets stuck in idle mode.

One, quick, easy, check, would be to remove the regulator to see if one or both of the alternator brushes are too short.

[Laitch]

Test the resistance value between terminals of the coils. Early model coils tended to have a shorter life than later coils. Orange bands around the secondary terminals indicate later coils. Resistance values in the diagram below come from BMW's LE Jetronic diagnosis guide, and the troubleshooting guide found on this site.

[Chris85K100]

Good idea, and the power side of the circuit could also be weak.

I haven't had this problem but my first thought was a problem with the air flow sensor. I've seen a problem like this on a car and when the air flow sensor was unplugged, it could rev again.

I would check the voltage across the battery at rest, at idle, at 2,000 rpm, and again when it gets stuck in idle mode.

One, quick, easy, check, would be to remove the regulator to see if one or both of the alternator brushes are too short.

Hello Daveson, thank-you for reaching out ,yes I agree checking the power side is probably a good idea.  I did orginally test across the battery at rest 12.55V and at 2000 rpm with high beam on 13.75V.  Alternator is charging battery however maybe something with vibration is causing bushes open circuit or shorting?  Thinking one possibility is not enough power to the plugs causes bike to hiccup and if severe enough voltage drop resets the FCU.  Condition would have to be intermittent since bike starts right up after turning key off/on.  Will put checking the brushes near top of the list. 

Hello Laitch, will look at coils once I figure out what's happening with the power ground or supply issue.  I think I've got two issues here and best to get clean power sorted out first.  I will look into this LE Jetronic diagnisis guide too. Thanks for that!

[Laitch]

. . . will look at coils once I figure out what's happening with the power ground or supply issue.  I think I've got

I suggest going to this page and downloading from the link LE Jetronic Training Manual to get a complete picture of how this engine management system works. The main ground connection can be found forward on the frame backbone.

[daveson]

I often ask myself, is it possible that there's nothing wrong with the bike. Yep, the battery is sad, the Voltage is sad but it might be much worse in the Amps department. You want to get it tested, or if you can get hold of a good one, just as a test, give it a try.

It would probably be good to video the instrument cluster so you have time to look at the many clues which might show up when it goes into idle mode. Like check if the choke light, clock, and backlights work, and if they also work in stuck idle mode. No choke light might point to a power problem, only two backlights working might point to an earth problem. No clock, probably a power problem closer to the battery, or the battery. They're just a couple of quick checks.

I like the suggestion by Laitch about the possibility of bad coils, especially if yours is the earlier version where the grab handles are incorporated with the seat. I have one of those models, both coils failed the tests suggested by Laitch, they both also had cracks on their cases which were easy to see.

[Chris85K100]

Hello,
       I've been out of town working for last 6 weeks.  Got into the garage today and checked HT Coils. No visible cracks. Coil 1/4 primary 2.3 ohms, secondary 10.1 Kohms.  Coil 2/3 primary 2.3 ohms, secondary 9.64 Kohms.  No problems here I believe.  I also put a rebuilt alternator in prior to leaving.  Bike seems to have some hesitation continuously now and goes to idle/stall mode often going down hill in idle then throttling.  I'm seeing the speedo move in neutral parked.  Going to remove tank and clean the main electrical ground and run a wire direct to the battery next. Ideas welcome!  Chris 

[Laitch]

Do you know how to check the intake manfolds and the crankcase breather hose for air infiltration? That's a step I'd be taking.

[Chris85K100]

The crankcase breather hose, the S shaped one, I replaced a few years ago.  It looks like new.  To check the manfolds I would have to take off much of the RTs fairings I believe.  I'm still thinking this is electrical because it's sudden and somewhat random, however it does seem to occur more often going down hill and reducing throttle then bike stuck at idle or stalls with throttle unresponsive.  Seems like the electronics are holding bike at idle.  Perhaps something physically wrong on the air intake side setting off a sensor?   

[Laitch]

I, too, have often used the approach that if fixing something seems too burdensome, what's actually wrong is something easier to fix. Occasionally, it's worked! However, I have learned that testing a rubber intake fitting by spraying it with a thin stream of volatile carb cleaner or Mapp gas when the engine is running fully warmed up is liable to be more accurate than looking at it.
There is an LE Jetronic training manual and diagnostic/testing manuals downloadable from the Service Bulletin section of the Technical Directory on the site for backup when needed.

[Chris85K100]

Laitch will give the carb fluid test a try on the weekend.  It's possible i'm running a bit lean.

[Chris85K100]

Hi Laitch, unfortunately tried carb fluid spray on throttle body and air box, no change in idle speed.  I've got the tank off now and going to run a new ground from under tank instrumentation collector direct to battery -ve and clean the ICU connector, replace the fuel pump relay for good measure.  Anything else anyone would recommend with the tank off? I've looked at the Bosch Jetronics manual and not much there can be done without the breakout box and BMW multimeter.  I'm going to look at that meter and see if a generic auto meter could be used instead.