1995 K75 bogs occasionally when fully warmed up

[bast1472]

Hello all,

I recently picked up a 1995 K75 in seemingly fantastic condition with only 21k miles. I went through all the regular maintenance items (splines, plugs, oils, filters, valve clearances, tires, brakes, etc.), but after riding it around on a hot day I noticed some running issues:

1. It would die if idling at a red light for more than 10 or 15 seconds.
2. Sometimes, even when giving it revs to avoid problem #1, it would immediately bog when taking off from a stop. The bogging would continue for a couple seconds, then suddenly the power would come back and the bike would carry on as if nothing happened.

I tried hitting the starter button to see how it affects idle speed, which resulted in a minor increase to RPM. From what I've gathered here, that's what you want to see. Next, I tested for air leaks using starting fluid and saw the idle drop when sprayed around the throttle bodies. So I replaced the upper gaskets and lower manifolds, along with the crankcase breather hose/z pipe that came apart when I pulled it out to check its condition. I also checked the fuel pump electrical connection behind the right side panel and saw no corrosion, but I gave the pins a light cleaning and made sure the connectors feel securely attached to each other. For good measure, I replaced the fuel filter with a new genuine BMW one.

The bike ran great and idled strong. I took it on a 45 minute test ride at night and could not reproduce any issues. The next day, I took it out again in some warm, sunny weather. For the first 15 minutes it ran great. Then it started doing the low RPM bogging when taking off from a stop, but only like 10-15% of the time. It seems that the issue is strongly correlated with the operating temperature of the bike. The hotter it gets, the greater the chance it will bog coming off idle. Interestingly, when doing the starter button test, the engine speed drops by 400-500 RPM, almost dying.

I have some new fuel injectors and spark plug wires on the way, but I have low confidence that those are the source of this problem (unless spark plug wires are known to get affected by a hot engine?). To you K-bike experts out there: based on what I've described, is there an obvious culprit jumping out that I should look into next? Thanks in advance.

[The Mighty Gryphon]

Have you checked the engine temperature sensor?  It has a very important function in providing a proper air/fuel mixture as the engine warms up.

Another possibility is that the fuel pressure is not being controlled accurately at 35psi. 

Both of these parts are hidden away behind the air intake box.  You might want to go in there as it seems you have touched everything else.

[Laitch]

Welcome, bast! I've got a 9575, too. What comprises the etc. in your post? It's an important variable dependent upon budget, personality, country and spirituality and not necessarily recognized universally.

[frankenduck]

Hall Effect Sensor or coils can be affected by heat like that but at 21K those shouldn't be going bad.

Try reseating the connector to the L-Jetronic under the seat. Can't hurt and it's free and easy to try.

[bast1472]

Thanks all for the suggestions. I reconnected the Jetronic plug and will test again tomorrow. Next, I’ll look at the engine temperature sensor and the fuel pressure regulator. Laitch—the “etc.” would be a fork rebuild, a new stainless steel front brake line, and I ran a small amount of Seafoam through a full tank of gas as a first attempt at resolving the earlier weak idle. By the way, the valves were in spec with .15mm on all intake valves and .28mm on all exhaust valves.

[Laitch]

Laitch—the “etc.” would be . . . .

Frankenduck's suggestion could be the remedy. Be certain that after you reseat the plug, you pull on the right end of the plug and attempt to pivot the plug outward. If it moves outward, you'll need to bend its latch plate so that it is positively engaged with the Jetronic unit.

You should visit all the major ground and electrical connectors to clean them and verify they're tightly connected. First among them is the four-pin fuel pump connector on the foot brake side of the frame dangling from beneath the tank. They are problematic and often need replacement but clean its pins and sockets. My moto's original is still ok.

Next, check the battery ground terminal on the transmission to be sure it's bright and tight. Next, the battery terminals themselves. After those, the tank gets raised and the frame ground connections attached at one point to the frame get checked. The ignition switch plug under there gets cleaned and so does the ignition control unit's plug.

This type of condition is difficult to remedy unless an orderly process of elimination is followed. One anomaly I discovered leading to this condition was a fuel pump that had slipped down within the fuel vibration damper and bottomed out on the floor of the fuel tank. When it was reseated at the correct height according to the pressed bands around its case and secured there, the problem ended.

If you address these areas, you're likely stop this condition but if you don't, you'll have eliminated common sources of future problems and will be able to concentrate on other system tests rather than hopping from one lily pad to another.

[bast1472]

Thanks, Laitch. I had a couple more hours to work on the bike tonight, and was able to locate the ground connections you mentioned. They were all clean and tight, but I took them off and gave them a good cleaning before mounting them back. I once again checked the 4-pin connector for the fuel pump, and verified that the pins are clean and the plug is fully seated. I also double-checked that the Jetronic plug was fully seated inside the metal clip, as you mentioned. I forgot to take a look at the ignition switch plug and the ignition control unit's plug, so those will have to wait till tomorrow. While inspecting the wiring around the bike, I found two weird things:

1. There was a single pin connector for a brown wire (presumably a ground) under the tank. On one side, near the plug, the wire sheath had been worn away, or perhaps chewed on by a mouse. I replaced the bad section with new wire of the same gauge.
2. The left-most ignition coil had a significant amount of corrosion around its top mounting point. I cleaned the top and bottom contact points for each ignition coil before remounting them.

I took the bike on a test ride and had to work to get it running hot since I wasn't able to take it out until the evening. I practiced some figure eights and low speed maneuvers in a parking lot until that fan started to kick on. After another 5 or 10 minutes of low speed, stop-and-go riding, I was able to start reproducing the issue. On the way home, for the first couple blocks after leaving the parking lot, I could reproduce the bog while cruising in third gear. I noticed the bike was making a ticking noise while bogging, and I also noticed an off-and-on whirring sound. I'm not sure if this was the fan or something else. Is there something else that could make rhythmic a "WHRRRrrrrrrWHRRRrrrrrWHRRRrrrr" noise? The fan is typically a constant noise. After a minute of idling at home I got it to bog again:

(Volume warning!)
With throttle to try to save it from dying:
No throttle:

Note that you can hear the fan kick on toward the end of the first video, which makes me think the temperature sensor might be working fine. Tomorrow I'll take a look at those ignition plugs and the fuel pump, but hopefully the videos help.

[Laitch]

There was a single pin connector for a brown wire (presumably a ground) under the tank. On one side, near the plug, the wire sheath had been worn away, or perhaps chewed on by a mouse. I replaced the bad section with new wire of the same gauge.

This item was not connected to anything? A photo might be helpful.

As you seem to have learned, the whir in the first video is the fan. It's the only component that sounds like a fan. After you clean the ICU plug and the ignition switch plug, tell us the condition of the spark plugs when the engine bogs down and quits—which is wet and which is dry. The fuel pump mislocation that I discussed previously caused the engine to stall straight off, not bog down and stall.

See if you can reproduce a stall by wiggling and slightly twisting the four-pin connector. There is plenty more to check. Be patient.

[frankenduck]

Try putting lots of ice in the gas tank. 

On a more serious note, since you can recreate the dying by stationary idling try pouring some water over that Hall Effect sensor cover plate to see if that makes a difference. As I pointed out earlier, Ks dying when they heat up can be a symptom of a HES going bad.

[bast1472]

Thanks for the water tip, frankenduck. I'll give that a try later today, along with Laitch's wire wiggling suggestion, and checking the plugs after the bike dies. To Laitch: Yes, the plug was connected to something. I just traced it and it goes down toward the Hall effect sensor alongside a wider cable. See the attached image of the wire and my not-so-pretty repair job wrapped in red electrical tape.

I checked the plug on the ignition control unit and that looks good. The Hall effect sensor plug also looks good, aside from the aforementioned single ground wire that had some damage. I'm going to keep digging to find the plug for the ignition switch, as well as the throttle position sensor. I'm also thinking of hooking up some longer fuel hoses so I can run the bike with the tank out of the way and do some heat testing with a blow dryer. Would 2ft fuel hoses mess with the fuel pressure?

[frankenduck]

The only plug for the TPS is the one that plugs into the TPS.  It's part of the engine wiring harness that plugs into the L-Jetronic.



Foilow the wires from the ignition switch to find its connector. It has four wires: Red,  green, gray & gray/blue.

[bast1472]

I checked the following today: throttle position sensor connection, ignition control module plug, ignition switch plug, Hall effect sensor plug, spark plug wire plugs (both ends), and saw nothing of note. I also inspected the brass air adjustment screws on the throttle bodies, which were clean, and put them back the same number of turns as I'd found them. Frankenduck: I tried pouring water around the HES cover but it had no effect on the issue. Laitch: I tried wiggling the 4-pin fuel pump connecter, along with the spark plug wires and fuel injector connectors, with no effect on the bog. See the attached image of the plugs immediately after the bike died (1-3 from left to right).

Does anything look amiss? Those plugs only have a couple hours on them.

By the way, I noticed my tail light is not functioning. I verified that it's neither bulb nor socket related. I doubt it's related to the running issue but I thought I'd mention it just in case it might be.

[Laitch]

I checked the following today: throttle position sensor connection, ignition control module plug, ignition switch plug, Hall effect sensor plug, spark plug wire plugs (both ends), and saw nothing of note. I
I noticed my tail light is not functioning. I verified that it's neither bulb nor socket related. I doubt it's related to the running issue but I thought I'd mention it just in case it might be.

What would something of note look like? Contact cleaner should be applied to both plugs of each connector that you've inspected.

Although the tail light could be a separate issue, how did you verify the condition is neither a bulb nor socket issue?

[bast1472]

I was looking for corrosion, bent or damaged pins, loose connections, or damaged wires. I used contact cleaner on each of those connections. As for the tail light, I swapped out new bulbs of the same type and rating, and swapped the socket with the brake light socket. The brake light works fine, but something must be going on with the tail light wiring. I visually checked the bulb monitoring unit and didn't see any corrosion, bent or damaged pins, loose connections, or damaged wires inside the box. I guess the next step there would be to check the tail light pins for voltage.

[bast1472]

I believe I've finally found the source of the issue. Today I emptied the tank and removed the fuel sending unit to inspect its condition. While pulling it out, I noticed one of its two wires was not attached to the fuel pump. After pulling the fuel sender out of the tank, I noticed some signs of damage on one terminal (see attached image). I put everything back together and the bike started up fine. I did another 30 minutes of testing and was not able reproduce the bogging issue. I noticed that the bike was idling much lower than before, so I had to turn it up a bit to get it around 1000 RPM. Any idea why the idle could have changed? I also noticed that the rubber damper that the fuel pump sits in is starting to come apart, so I intend to replace the rubber if I can find just that part. Thanks all for your suggestions.

[Laitch]

Congratulations!

bast's tale shows the importance of honing observation when doing maintenance tasks. Early on during investigation, bast checked the height of the pump in the vibration damper but didn't notice the damper itself was disintegrating nor the weak electrical connection—or partial disconnection—at the fuel pump.  On the other hand, readers need to ask more questions about exactly what's being asserted and avoid assumptions like bast's maintenance of "filters" included the fuel filter.

bast, I recommend your performing another throttle body balance and CO setting to square away the idle. The cause of the idle change is likely rooted in the electronic system's ability to compensate performance from varying signal input. Vibration dampers are available from MaxBMW, Euro Motoelectrics and several other sources. The fuel pump is either 52mm or 43mm O.D.; measure the pump and order it according to that size.

[The Mighty Gryphon]

Those terminals are for an aftermarket pump and were installed by a previous owner.  They may not be crimped properly for a good connection.  I would give them a good tug to see if they will stay put on the end of the wire.  Also, they are for 16/14AWG wire, the ones that should be used are the red ones for 18/22AWG. 

The burn on one of them is kinda scary.  Imagine the spark that could make a burn like that in a fuel tank full of gasoline vapor.   Seeing that terminal is a good reason for using insulated terminals in the tank.

As for the idle speed, make sure you didn't disturb the throttle cable when you moved the tank.  Set the throttle position sensor and balance the throttle bodies.  That should get the idle back where it should be.

[frankenduck]

Those terminals are for an aftermarket pump and were installed by a previous owner.  They may not be crimped properly for a good connection.  I would give them a good tug to see if they will stay put on the end of the wire.  Also, they are for 16/14AWG wire, the ones that should be used are the red ones for 18/22AWG.

Look at where the spark traces are. It's not a wire crimping issue.  It's the parts of the female blade terminal that grip the male terminal being loose. Can be resolved by pinching those down a tad with some pliers.

[The Mighty Gryphon]

The crimping wasn't the issue.  That connector possibly had 12v on it and touched some bare metal.  Hopefully, it was outside of the tank when it did.

Still, those connectors are the wrong size for the wire and should be replaced with the red ones that are the right size.  The blue ones are too large where the wire goes in to get a good crimp.

[bast1472]

Upon further inspection, I suspect the wire came loose after the fuel pump slid down to the bottom of the tank. The vibration damper wasn’t coming apart, someone fitted an aftermarket fuel pump that was too small for the opening and shoved some random pieces of fuel hose to fill the gaps. I went ahead and ordered a proper replacement pump, along with a new rubber damper, plastic ring, and the correct rubber sleeve for the 43mm pump. On a side note, I fixed the tail light (and evidently a front parking light that I didn’t know existed) by disassembling and cleaning up the ignition switch components. Unfortunately I had to drill out the set screw because my extraction attempt failed. I’m not sure how to keep the switch from coming apart over time, but for now I’ve filled the void with silicone and will test how that holds up.

[Laitch]

On a side note, I fixed the tail light (and evidently a front parking light that I didn’t know existed)

There's a 1994 K75 Rider's Handbook in the Service Manual section of the site. There was no significant change in features from '94 to '95. In it, you will find information and diagrams of other things about your moto that you likely don't know.