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Thread: Some more diagnostic ideas

  1. #1
    Registered User naddy100's Avatar
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    Some more diagnostic ideas

    http://www.nsaddy.net/Videos/Bike/Bike.html

    Recall that a few weeks ago I had a 1988 K100 that stumbled intermittently. It never died. It just stumbled. Based on that I kinda went down the fuel path. I replaced the fuel filter and replaced the fuel pump with a IBMWR-suggested non-BMW pump, complete with the shimming required to have it fit.

    It's quite possible that it only had some water in the tank. I don't think I know exactly what the original problem was, but it ran fine for a week or so and then it got worse.

    Now it doesn't run above an idle. The link shows what it's like. It's an mp4 file.

    I'm thinking of emptying the tank, putting the BMW fuel pump back in, and replacing the fuel filter again. The tank-to-fuel-rail line is relatively new. I haven't checked the fuel-rail-to-regulator line.

    A brief peek in the tank today and the non-BMW fuel pump is still in place.

    Ideas about how to diagnose why it won't run above an idle? I never put a fuel pressure gauge on the fuel line, so I don't know what pressure the system is producing.

    Noel
    Last edited by naddy100; 05-03-2010 at 03:19 AM.

  2. #2
    Rally Rat
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    It soudns to be running extremely lean. Unplug the throtle switch to make sure the idle contact isn't the culprit-you can run the motor without it connected. If hat's not it, I would suspect the airflow meter. These began failing a few years ago.

  3. #3
    MichiganMike MICHIGANMIKE's Avatar
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    Air flow Meter

    A second here to check the Air Flow, I had a K75 with same symptoms and it was just the feed tube in the front was not secured properly.

  4. #4
    Firefighter Randy
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    Smile



    Probably a foregone conclusion, however, did you have the high idle fully on?

  5. #5
    Registered User naddy100's Avatar
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    Quote Originally Posted by RNowell View Post


    Probably a foregone conclusion, however, did you have the high idle fully on?
    By 'high idle,' do you mean the choke? As I understand the choke, it just turns the rpms up a little. And, since I'm not Campbell Tellman (congratulations to Campbell!) I will probably start messing with it tomorrow.

    Noel

  6. #6
    Registered User naddy100's Avatar
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    http://nsaddy.net/PressureGauge/PressureGauge.html

    I unhooked the throttle switch and it made no difference.

    I hooked a fuel pressure gauge into the line at the front of the fuel rail. Here are the results. The reading oscillates between 32 and 38 psi, which seems to fit Clymers. This suggests that it isn't an obvious failure on the fuel side like fuel pump or fuel filter.

    Following lostboy and MichiganMike, I guess I next started on the air side, but that's pretty simple. Either the air flow meter is sitting correctly in the top of the air filter case or not. I pulled the air box and the air flow meter is where it's suppose to be. I guess the electronics could be bad. Hmm. I don't think I've ever changed the air filter. Would that cause something as drastic as this as suddenly (relatively) as this?

    I know there's always the possibility of those tubes being broken that run from the Air Plenum Chamber to the intake manifolds(?). I guess I figured the symptom would be rough running rather than inability to run above an idle.
    Last edited by naddy100; 05-07-2010 at 10:11 PM.

  7. #7
    MichiganMike MICHIGANMIKE's Avatar
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    Air Flow

    On my K75 it wasn't the air flow meter, but just an improperly attached hose at the front of the box. It looked secure but must have been just a small leak. Just fixing that seal properly made all right with the world. I had done other maintenance and it must have been my clumsy fingers that knocked it loose in the first place.

    I would highly doubt the air filter, they are oversized and shouldn't be an instant problem.

  8. #8
    Benchwrenching PGlaves's Avatar
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    Quote Originally Posted by naddy100 View Post
    http://nsaddy.net/PressureGauge/PressureGauge.html

    I unhooked the throttle switch and it made no difference.

    I hooked a fuel pressure gauge into the line at the front of the fuel rail. Here are the results. The reading oscillates between 32 and 38 psi, which seems to fit Clymers. This suggests that it isn't an obvious failure on the fuel side like fuel pump or fuel filter.

    Following lostboy and MichiganMike, I guess I next started on the air side, but that's pretty simple. Either the air flow meter is sitting correctly in the top of the air filter case or not. I pulled the air box and the air flow meter is where it's suppose to be. I guess the electronics could be bad. Hmm. I don't think I've ever changed the air filter. Would that cause something as drastic as this as suddenly (relatively) as this?

    I know there's always the possibility of those tubes being broken that run from the Air Plenum Chamber to the intake manifolds(?). I guess I figured the symptom would be rough running rather than inability to run above an idle.
    An air leak beneath the throttle bodies would cause a high idle and lean mixture.

    Inability to run above an idle tells me one of two things: opening the throttle plates isn't providing air (obstruction),

    or

    The fuel system is failing to provide increased fuel to match the increased air.

    Check the air filter.

    Look for mouse nests.

    Then suspect either a faulty air flow meter or a bad LJetronic unit.
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
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  9. #9
    Small road corner junkie pffog's Avatar
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    My experiance says go back to the last thing you did. First, on the original problem, my experiance with the electric fuel pumps says they seldom go intermittent, but rather it just quits dead, but that is water over the dam.

    That said, the fluctuation of the fuel pressure does not make sense to me, way too big of swing. If I was a betting man, I say loose clamp or hose inside the tank. My bet is a hose is "shooting" fuel into the connection, but then pushes away due to the pressure of the fuel spewing out. Jump the pump and open the gas cap, I bet there will be a lot of turbulence or if the fuel is low, you might hear is spraying about. The other quick thing would be to unhook the return line down stream of the regulator and make sure there is strong flow after the regulator. Of course it is GASOLINE and it is dangerous to ply with, use ample caution.

  10. #10
    Benchwrenching PGlaves's Avatar
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    Quote Originally Posted by pffog View Post
    My experiance says go back to the last thing you did. First, on the original problem, my experiance with the electric fuel pumps says they seldom go intermittent, but rather it just quits dead, but that is water over the dam.

    That said, the fluctuation of the fuel pressure does not make sense to me, way too big of swing. If I was a betting man, I say loose clamp or hose inside the tank. My bet is a hose is "shooting" fuel into the connection, but then pushes away due to the pressure of the fuel spewing out. Jump the pump and open the gas cap, I bet there will be a lot of turbulence or if the fuel is low, you might hear is spraying about. The other quick thing would be to unhook the return line down stream of the regulator and make sure there is strong flow after the regulator. Of course it is GASOLINE and it is dangerous to ply with, use ample caution.
    I agree. I think you may have a loose hose or split filter can.
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
    http://www.bigbend.net/users/glaves

  11. #11
    BUDDINGGEEZER
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    Quote Originally Posted by MichiganMike View Post
    On my K75 it wasn't the air flow meter, but just an improperly attached hose at the front of the box. It looked secure but must have been just a small leak. Just fixing that seal properly made all right with the world. I had done other maintenance and it must have been my clumsy fingers that knocked it loose in the first place.

    I would highly doubt the air filter, they are oversized and shouldn't be an instant problem.
    I had this same problem on a K75. the bike would start and idle @ 600rpm. ANY throttle it would die. Use the choke it would not start. An easy way to tell if it's the air flow meter is remove the snorkel tube on the air box, reach in and open the air flow flap with your hand and see if it takes throttle.

    My K75 and K100 ran at 36PSI fuel pressure with no fluctuation.

    Ralph Sims

  12. #12
    Registered User naddy100's Avatar
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    Progress?

    http://nsaddy.net/Videos/Gauge_2/Gauge_2.html

    Take a look. It's an mp4 file again.

    I returned the BMW fuel pump to the tank, pulling the Ford pump out and going back to the idea that it might well have been water in the tank at the beginning.

    I also inspected the fuel system for loose connections. As predicted, there was a pretty good candidate. When I put the hose on the Ford nozzle, the clamp was close, but not perfectly clamped around the nozzle. The clamp was sorta near the end of the nozzle. This makes it plausible that the pump was not reliably getting the fuel down the hose.

    This also fits the story where the bike seemed to be fixed earlier and after a week it got worse and was reduced to where all it would do was idle. The hose clamp more or less worked for about a week and then backed off.

    The video shows two things. One is that the bike runs reliably at any RPM to which I turn the throttle, like it's suppose to. The other is that the pressure gauge still vibrates around the 30+ to almost 40psi. At higher RPMs it holds steadier. Maybe I've still got a problem.

    I rode it around the neighborhood and down a couple of roads. It seemed to function okay. I'll go back to commuting on it tomorrow and we'll see.

    Ah -- the breather (?) hose from the crankcase (?) to the air plenum box (?) looks cracked. I've ordered another one.

    Mechanic lesson: look where you last messed with things.
    Life lesson from the gauge volatility: difficulty in establishing simple facts, in this case fuel pressure.

    Noel

  13. #13
    Rally Rat
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    Unless you pull the vacuum line off the rear throttle body the pressure gauge will pulse as the regulator reacts to each intake stroke. Some gauges are heavily damped so as to resist pulsing. Most are not.

  14. #14
    A bozo on the bus deilenberger's Avatar
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    Quote Originally Posted by naddy100 View Post
    The video shows two things. One is that the bike runs reliably at any RPM to which I turn the throttle, like it's suppose to. The other is that the pressure gauge still vibrates around the 30+ to almost 40psi. At higher RPMs it holds steadier. Maybe I've still got a problem.
    Normal. You need a damped gauge (slower time response) to get a smooth reading on the gauge at lower RPMs.

    Snippage..

    Mechanic lesson: look where you last messed with things.
    http://www.eilenberger.net/laws.htm < -- law #3..
    Don Eilenberger http://www.eilenberger.net
    Spring Lk Heights NJ NJ Shore BMW Riders New Sweden BMW Riders
    '07 R1200R (current ride) and some bimmers.. and a Porsche

  15. #15
    Registered User naddy100's Avatar
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    Quote Originally Posted by deilenberger View Post

    Snippage..

    http://www.eilenberger.net/laws.htm < -- law #3..
    Great! Right between #2; won't have the right part and #4; won't have the right tool!

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