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Discussion Starter #1
I just got this bike and like it. I've noticed something at around 4K, 5th or 6th gear at highway speeds: an almost imperceptible "deceleration" or hesitation. It oscillates maybe every few seconds. No change in speed or rpm- I can barely feel it. Any ideas? Thanks in advance
 

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Had my 2003 for a little over a year and 8000 miles and I know what you mean. Feels like an occasional mis-fire. I only run premium and I’ve tried several fuel injector cleaner additives. Finally convinced myself it was normal.

While I have it all ripped apart at the moment, I’ll put in fresh plugs and see if anything changes..
 

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I just got this bike and like it. I've noticed something at around 4K, 5th or 6th gear at highway speeds: an almost imperceptible "deceleration" or hesitation. It oscillates maybe every few seconds. No change in speed or rpm- I can barely feel it. Any ideas? Thanks in advance
I have ridden and done repair on these since 2002 - assuming a well tuned "stock" engine, this is NOT normal. Can you be more specific when it feels worst: at what RPM, what gear and at what road speed ?

If you are cruising (fairly constant speed) at 4,000 to 4,500 RPM in 5th or 6th gear than this good usage of the power band for this engine.

Engine and EFI fueling behavior will fell very different if one is "forcing" this engine to run at too low RPM against wind pressure at cruising speed (like 6th gear at 45 Mph is NOT OK). One should not engage 6th gear when below 60 MPH - engine it out of normal power band and running lean at small throttle openings (simplified explanation of course...).
 

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I just got this bike and like it. I've noticed something at around 4K, 5th or 6th gear at highway speeds: an almost imperceptible "deceleration" or hesitation. It oscillates maybe every few seconds. No change in speed or rpm- I can barely feel it. Any ideas? Thanks in advance
For those symptoms I'd be looking at Lambda sensor replacement - as they get old they get lazy and switch slower than they should - this gives a hunting sensation on cruise
 

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For those symptoms I'd be looking at Lambda sensor replacement - as they get old they get lazy and switch slower than they should - this gives a hunting sensation on cruise
Given the symptoms, I agree with you there is a 50% probability the Oxygen-Sensor is out (or weak). HOWEVER for such remote troubleshooting, unless I can see / ride the bike myself, I always gather more info. I do not change parts just because "it may solve" the problem, unless I have data / evidences it will ;-)

By a process of elimination, I am trying to confirm:
1) his expectations and how he is riding it (gear, RPM, Speed)

2) if indeed the Oxygen-sensor is responding:

3) if Motronic EFI has developed bad adaptives because of one or more sensor failing. We can clear / reset memory by removing the Motronic EFI fuse for 30 minutes (ignition OFF). A change in behavior will need to be confirm over 2 or 3 rides as it may come back if Motronic sees a repeating lean or rich condition.

To confirm if O2 sensor is working properly, we can EITHER :
- use a GS911 diagnostic interface (live data log or on a graph)
- use the dealer's computer to see O2 sensor data live
- warm up the engine, shut it down, disconnect the Oxygen-sensor and go for a ride. This can be done as there is a default EFI map for all combo of RPM and Throttle openings (TPS). It will run a bit more rich, but if behavior improves we have a VERY HIGH probability the O2 sensor was not doing its job.
 

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Discussion Starter #6
Thanks for all the input. I'm riding at around 4-5K rpm, in 5-6th gear, at 65-80mph. It really is a slight- nearly imperceptible, certainly can't hear it, but can barely feel it. I guess the description of "hunting" fits. Like I said, there isn't any indication in road speed or engine speed. I can just feel it.

I'll try the TPS/Motronic reset, have some plugs on order, will look into the O2 sensor and report back. Thanks again-
 

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Thanks for all the input. I'm riding at around 4-5K rpm, in 5-6th gear, at 65-80mph. It really is a slight- nearly imperceptible, certainly can't hear it, but can barely feel it. I guess the description of "hunting" fits. Like I said, there isn't any indication in road speed or engine speed. I can just feel it.

I'll try the TPS/Motronic reset, have some plugs on order, will look into the O2 sensor and report back. Thanks again-
Because the throttle-cable(s) has a habit to become dry and stiff inside with age , it is very important to take a habit to do open 1/4 and close fully (then release) JUST BEFORE YOU TURN IGNITION ON. There are 2 throttle-cables if equipped with factory cruise-control - this makes matter even worse if throttle is not returning to idle stop freely (caused by too much cable friction with sometimes wrong cable alignment under fairings).

What is happening behind the scene: when you turn ignition ON, Motronic EFI looks at idlestop positon of throttle-bodies (using TPS). If this is out of normal because the throttle stay opened a few degrees above expected idle stop, the EFI will try to compensate at start and later. This may have an impact to any issues related to idling, cold start or surging (surging is often the term used for symptoms you have described).

When you clear the Motronic memory (with fuse or disconnect main battery for 20 minutes), it is even more important for the EFI system to see the "real" idle-stop during both phase:
- ignition ON
- while starter button is pressed
When / IF all is functioning properly, you should not have to opened the throttle manually during any start (cold or hot start) as there is an idle actuator for fast idle if needed (controlled by EFI system).

P.S.: if you are changing the spark-plugs, check for any white / gray damaged spot on the high tension wires to plug caps. With age, the original plug wires have a tendency to develop cracks that will leak current to ground (to cylinder-head surface or cover where the contact spot will show).
 

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Given the symptoms, I agree with you there is a 50% probability the Oxygen-Sensor is out (or weak). HOWEVER for such remote troubleshooting, unless I can see / ride the bike myself, I always gather more info. I do not change parts just because "it may solve" the problem, unless I have data / evidences it will ;-)
Nature of forums is that people are spread out and not local - my money is on Lambda - that's all it was hat in the ring so to speak

By a process of elimination, I am trying to confirm:
1) his expectations and how he is riding it (gear, RPM, Speed)
None of which will make much difference - symptom was hunting on cruise - cruise to me means top gear and m-way speeds

2) if indeed the Oxygen-sensor is responding:
I think it's responding - but just too slowly

3) if Motronic EFI has developed bad adaptives because of one or more sensor failing. We can clear / reset memory by removing the Motronic EFI fuse for 30 minutes (ignition OFF). A change in behavior will need to be confirm over 2 or 3 rides as it may come back if Motronic sees a repeating lean or rich condition.
Hard reset always a good starting point sometimes it even unmasks the problem that the ECU has worked around

To confirm if O2 sensor is working properly, we can EITHER :
- use a GS911 diagnostic interface (live data log or on a graph)
- use the dealer's computer to see O2 sensor data live
- warm up the engine, shut it down, disconnect the Oxygen-sensor and go for a ride. This can be done as there is a default EFI map for all combo of RPM and Throttle openings (TPS). It will run a bit more rich, but if behavior improves we have a VERY HIGH probability the O2 sensor was not doing its job.
Or a really easy forth way

Warm the bike up and then in N open the throttle to get it to the same rpm that the hunting is being felt - hold the throttle still and listen to the engine

it should be waaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

if is is wooooooooooowaaaaaaaaaaaawoooooooooooowaaaaaaaaawoooooooooooowaaaaaaaaa

Then it's a failing Lambda

the length of the wooooo and waaaaa times tells you how fucked it is

short pulses and it's just started going down hill

longer pulses and it's seriously degraded

a completely knackered lambda will appear like a perfectly good one because it won't be switching at all but you will have to have driven with it failing for quite a while and failed to notice it before that happens

So fundamently you don't need a scope to test a failing lambda
 

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Discussion Starter #9
Update: I replaced a missing exhaust manifold bolt (eliminating a nasty rattle), installed new iridium plugs and disconnected the Lambda sensor. Runs like a new bike. My question now is- will running with the sensor unplugged create any issues? I plan to install an exhaust (sans catalytic converter) further down the line.

Thx
 

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Discussion Starter #10
Update again: surge or hunting is still there, with the O2 sensor unplugged. I guess my next move is to look for vacuum leaks?
 

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I haven't had this problem with my K1200RS, however I did have a similar issue with my car (Subaru Outback) a coupe of years ago, although my issue was at idle and only after warmup. Finally fixed it by replacing the two Oxygen sensors (one either side of the cat). All the advice was around vacuum leaks and intake air flow sensors.

Because no-one touches my vehicles but me, I was pretty sure there would no be vacuum problems. I went through the testing of these two suspects and cleaned the air flow sensor. No joy. Genuine Oxygen sensors are quite expensive, but I was able to buy after market units. Once fitted, all fixed.

The problem with these sensors is that they don't necessarily fail completely all of a sudden, but the oscillations become noticeable when you can actually feel them or see the tacho oscillate.

There are a lot of very good suggestions in this forum. Always start with the simple tests and least expensive replacement items like spark plugs, leads etc. You have tried some good, simple tests already.

Don't know how many your bike is fitted with but, although I don't know and can't see, hear, feel or ride your bike, my money is on the Oxygen sensor(s) as well.

Be interested to read the final solution. https://www.k-bikes.com/forums/images/k-bikes_toucan/smilies/tango_face_wink.png
 
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