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Crankcase Vent Repair for a K11

K1100 Crankcase Vent Repair

By Dave Soine
April 1998

“Say, what’s that oily, spooged up tube sticking out of the backside of my KLT airbox?”

The crankcase vent tube attaches to a right-angle fitting on the top rear of the engine block, near the “bell housing” – you can see it by looking behind the coils on the left side of the bike. The rubber tube ages and cracks, leading to an excess of spooge, and possibly excess hydrocarbons (gasp!) being inadvertently leaked into our atmosphere! Being the anal, eco-sensitive types that we are, of course we must replace it. But how? The other end of the tube disappears somewhere inside the airbox!

Wherzat thing go, anyway?

To sum it up, the crankcase vent tube transports crankcase gases, along with a bit of engine oil, to a small chamber inside the top section of the airbox. The chamber somehow (magically) separates the oil from the crankcase gases. The oil is returned to the engine via the “oil return” line running from the front of the airbox to the fitting on the top of the engine block(at the front, next to the cam chain cover.) The now oil-free crankcase gases are distributed to the throttle bodies(!) via the “air return” line from the front of the airbox, allowing those evil hydrocarbons to be burned away in the combustion process.

As an added bonus, since the crankcase vent lines go to the throttle bodies, if they are cracked it could lead to poor idle just like any other air leak in the intake!

Part I: Inspection

  1. Crankcase vent tube

    Locate the crankcase vent tube – you will need a light. Look for a rubber tube (~2 cm diam.) clamped to a fitting on the engine block and entering the top section of the airbox. About 6 cm is visible – use the light and look behind the coils and Throttle Position Sensor.An aged and cracked tube will be obviously oily and dirty.

  2. Air and oil return lines

    Check these when you inspect your air filter – they are a bit hard to see without the right lower fairing removed. You will find these lines attached to the front (i.e. facing the radiator) of the top section of the airbox.Again, failing lines will be obviously oily and dirty.

  3. Fuel lines, air filter

    Inspect (as best you can) and determine if you want to replace these – there will never be a more convenient time to do so.

Part II. Replacement.

Because of BMW’s use of non-reusable type hose clamps (like the clamps used on cage CV boots), you could work hard to find a bunch of hose clamps to replace the ones that will be destroyed during removal of the old lines. It will probably take less time and effort to order the OEM clamps and buy a small pair of end cutters to install them.

Parts needed:

  1. Any lines that need replacing. All of mine were bad. You will not believe how difficult it will be for your BMW dealer to figure out what the right parts are. It took *WEEKS* when I did it, so to avoid a big waste of your time, here are some of the part numbers: Important part #s for K1100s (US, anyway):
    Crankcase vent tube: ??? <= THEY can find this one
    Air return: 11-15-1-461-834 US$44!
    Oil return: 13-31-1-461-971
    Cheapo alt: 8×13 fuel hose (1.5 ft, cut to size) the OEM line has a 90 deg bend at one end
  2. Hose clamps
    • for intake tubes you need four(4) –
      Hose clamps (OEM): 11-61-1-460-900 (one time use)
    • for the vent tube and oil return you can use small hose clamps, found at any auto parts store, in place of the 15.7 mm and 17.5 mm clamps
    • for the air return, you need:
      four(4) of: 17-11-1-460-922 12.3 mm one-time
      two(2) of: ??? 15.7 mm one-time
      two(2) of: 16-12-1-180-241 17.5 mm one-time

Extra tools:

  1. Small nut drivers will be helpful, 7 mm for sure.
  2. I used a flat-blade screwdriver bit from my socket set.
  3. Some type of tool to cut the old hose clamps. Sturdy “dykes” will work.
  4. If you use the OEM hose clamps, a small end-cutter (nippers) works great, especially in the small working spaces this job requires.

Procedure (“Spartan” overview):

  1. Remove left and right fairing lowers, and
  2. Remove right fairing bracket (at bottom of airbox.)
  3. Remove the airbox.
  4. Replace all the vent system lines (air and oil).
  5. Install airbox.
  6. Install right fairing bracket and fairing lowers.

Procedure (detailed instructions):

  1. Remove the both left and right side lower fairing.

Starting with the left side:

  1. Disconnect the electrical accessory socket (connector on the back of the socket) and remove the plate holding the socket (2 phillips screws, attaching to the coil bracket.) This will allow easier access to the crankcase breather tube.
  2. Now for the destructive part. Carefully cut the hose clamps which attach the intake tubes (from the airbox) to the top of the throttle bodies. Be sure to recover all the metal bits that will fall into the throttle linkage, etc.

Now to the right side:

  1. Remove the fairing bracket that attaches to the block near the bottom of the airbox. (10 mm bolt)
  2. Remove the front intake tube (2 – 8 mm bolts).
  3. Loosen the hose clamps on the oil and air return lines on the front side of the airbox and detach the lines. A section of the air return line may come off as you do this (OK). Have an oil rag handy, as all these lines could drip a bit.
  4. Disconnect the wires for the air temperature sensor (connector on the rear of the airbox). Pop the spring clips (3!) and remove the air filter.
  5. Reach through where the air filter used to be and loosen the hose clamp(7 mm – ratchet) which attaches the crankcase vent line to the engine block – there is not much space! If your nut driver, screwdriver, etc. won’t fit you can get a bit more space by propping up the rear of the fuel tank (remove the clips first if they are still there) and pushing up the electrical box a little. Watch out for the fuses!
  6. Pull the vent line from the fitting. Have a rag handy for when it drips oil.
  7. Pull the intake tubes off at the throttle bodies. Try not to get too much dirt in the throttle bodies. 🙂
  8. Remove the top section of the airbox, then the bottom, from the right side of the bike. Cover the throttle bodies with a rag so no dirt falls into them.
  9. Flip over the top section of the airbox – you will see where the crankcase vent line attaches. Note the orientation of the old vent line (the “engine” end), then cut off the old hose clamp and pull off the line. Pull it out of the airbox and replace it with the new one – make sure the new line seats properly where it enters the airbox. Use one of your new hose clamps, of course.
  10. Cut the hose clamp on the oil return line and detach it from the fitting on the block. Removing this line is a real PITA, you might just want to cut it off. This fitting may come off when you do this – no worries, just put it back in upon reassembly, with some gasket sealant. Attach the new oil line to this fitting, again with a new hose clamp.
  11. Carefully cut the old clamps attaching the air return line(s) to the throttle bodies. Remove the old air return. Note where the piece of foam-rubber heat barrier is located so you can put it back later.
  12. If you want to replace the fuel lines, do it now. One line is specially formed and can be obtained from a dealer, the other two are just 8×13 fuel line, cut to length. You will need some new hose clamps, as usual. 🙂


  1. Assemble/add clamps on the new air return (this will be obvious upon looking at the old one, using 15.7 mm & 17.5 mm clamps) Take care to properly align the two parts of the air return.
  2. Install the new air return. Make sure to push the lines all the way on to the nipples on the throttle bodies, and use the 12.3 mm hose clamps.
  3. Install the little piece of foam-rubber heat barrier.
  4. Put the clamps in place on the intake tubes attached to the top of the air box.
  5. Put the bottom section of the airbox back into place (watch that oil return line!) then the top section. Slide the intake tubes over the throttle bodies or at least get them close. Make sure the clamps on the intakes are in place! Also watch and insure that the foam-rubber heat barrier that lays on top of the airbox is in place.
  6. Finally, slide the crankcase vent line (with the clamp) over the fitting on top of the block and, again reaching between the sections of the airbox, tighten the clamp on the crankcase vent line. Your cursing will really become creative at this point.
  7. Push the intake tubes all the way onto the throttle bodies and tighten the clamps.
  8. Install the air filter(be careful to orient it properly!), air and oil return lines (at the airbox) and temperature sensor connector.
  9. Install the fairing bracket and intake tube (9 Nm).
  10. Double check to see that everything is in place. Install the fairing lowers, etc. and anything else you had to remove.

That’s it! I noticed better idle after replacing the lines, probably due to the leaks in the old vent lines.

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