1991 K75 RT fuel strainer hose

[xcaret]

I fueled up my bike and half a kilometer later it died . When I got it home I noticed the breather hose going under the fuel pump to the fuel strainer ,also under the fuel pump , had came off .  I asked on another site what the purpose of this breather is, and as Haynes mentions if it isn't on ,the bike could get vapor bubbles .
OK , I just discovered the  hose off last night ,and am going to see if the bike starts once it's back together today, but I'm very much interested in what exactly this breather does for keeping the engine running.
BTW I am fanatic for details, so if anyone answers be prepared to be bombarded with more relevant questions about 's function ,and routing .
I realized the hose wasn't hooked to anything as soon as I got the bike home , but no one seemed to know anything about where it went or did. That includes Clymer, and Haynes ,plus an online BMW repair manual..
 Last night I decided to remove the fuel pump and see where the hose went . Well, well, there was a nipple on the side of the fuel strainer that the hose had came off of.. Being that I had to apply a dab of lithium grease to get the hose back on the the tight fit of the nipple , I'm quite sure it wasn't on very far because The force of the fuel going into the tank as I refueled must have caused it to come off the nipple. The gas station had more force coming out of the nozzle than I had ever seen before ..
I have looked through my 3 manuals for an answer before asking here . 
Xcaret
IMGP1470.JPG (31.33 kB . 768x508 - viewed 406 times)

[Laitch]

The purpose of that hose was to prevent the intake of air into the pump when the fuel level was allowed to become excessively low and fuel at the intake became turbulent. Air bubbles within a pump can cause poor performance and pump damage. The condition is called air cavitation. Theoretically, the hose attached to the filter basket allows air to be evacuated before entering the pump. The basket-and-hose assembly was a feature on 52mm pumps. My moto never had the hose attached from the basket's port to the port on the tanks ceiling. I haven't had problems; I don't allow fuel to drop tol excessively low levels. Disconnection of that hose should not stall your engine. The basket-and-hose seems to be an example of BMW's over-cautious approach with some elements.  The basket filter-style pump was replaced with a 43mm pump that uses a permeable fuel sock attached to its inlet. The sock eliminates air bubbles and turbulence during fuel intake.

Read more here.

[Scott_]

Laitch, so why have I been thinking all these years that that hose connects to the fuel return line from the fuel rail so that returned fuel is dumped on the side of the tank with the pump.....
With the later model tank that uses the 43mm pump, they not only changed the basket to a sock, they also re-designed the fuel return and just let it dump wild in the tank.(hence a later 1994 TSB 16 018 94 to connect a hose from the return line open end and run the hose to the left side of the tank by the fuel pump.) Like you mention to prevent cavitation and pump noise by returning the fuel as close to the pump as possible.

[Laitch]

Laitch, so why have I been thinking all these years that that hose connects to the fuel return line from the fuel rail so that returned fuel is dumped on the side of the tank with the pump.

I can't answer that one, Scott_, it's too personal. 

[xcaret]

Thanks for the replies . Bike runs fine now.. I'll just ask two questions. Does the breather hose draw the air( bubbles) away from the pump by vacum, and where is the air going to once it exits the breather pipe ?
I agree with a full tank there should have been no air cavitation. hmmm strange .

[The Mighty Gryphon]

The "breather" hose does not remove air from the fuel pump intake.  It directs the return line flow to the filter basket to make sure the pump inlet has adequate fuel in the basket to prevent cavitation in the pump.

The idea is to send the fuel back to the inlet of the pump as directly as possible so that when the tank is low on fuel it won't suck air and have cavitation.  Because the tank has such a high divide to clear the frame backbone a substantial amount of fuel can be on the right side while the pump is starving on the left.

[Laitch]

The "breather" hose does not remove air from the fuel pump intake.  It directs the return line flow to the filter basket to make sure the pump inlet has adequate fuel in the basket to prevent cavitation in the pump.

That depends upon which tank your moto has. Mine has fuel return dribbling over the vapor collector into the fuel pool (as described by Scott_), and a port attached to that collector designed to connect to the pump filter basket. You and Scott_ have different layouts from mine and likely xcaret's. Maybe he'll post a photo.

In this photo, the collector is at the top right arrow. The port that connects to the filter basket is the lower right arrow. The pipe that moves vapor to atmosphere via a hose or accumulator cup beneath the tank floor is at the upper left arrow. The pipe that transfers water from around the perimeter of the fuel tank cap to the a hose or accumulator cup beneath the tank is at the lower left arrow.

Does the breather hose draw the air( bubbles) away from the pump by vacum, and where is the air going to once it exits the breather pipe

Vapor probably travels by the old standby route of going from a place of pressure to a place of lesser pressure; to wit, through a hose in a filter basket to a pipe in the tank or a vent in the tank's cap, out a port in the tank's floor, through a hose connected to the pipe in the tank's floor or directly from the pipe in the tank's floor into an accumulator cup, and then into somebody's lungs and/or directly into the atmosphere.

Bricks—especially negelcted, elderly Bricks—intermittently stall from any of several reasons–clogged fuel filters, faulty ground connections, fuel mixture faults, faulty electrical plug connections, faulty coils, faulty relays, fuel separation, oxidized ignition switch components, et. al..  It's part of their charm.

[xcaret]

My tank and tubes look just like Laitch shows in his photo. except what's on the right in his photo is on the left in my tank.. probably the photo was taken using a mirror, or the negative got reversed .
I'll go with the theory that the " breather hose " is really a fuel return line . In my fumbling around inside the tank , wondering where the loose hose went, I hit the starter . right away fuel was shooting around inside the tank, probably from the loose line . I ,being new to efi, thought it was probably a fuel return ,and never wondered if it was coming from the loose hose . I didn't hit the starter after that. That was before I removed the fuel pump and noticed the port on the fuel strainer .
In my Haynes repair manual that covers K100,s and K75's it mentions the fuel strainer ,then says something about different systems .. Clymer says nothing..   
So far after reading you guy's comments ,I see the logic that the line is a fuel return line.. Especially when fuel shot around inside the tank . but why would the bike quit 2 blocks after being fuel up ?
One more thing , instead of killing the battery trying to start it , I waited 20 minutes and tried again. It started ,and I let it run at around 2300 rpm's for 30 seconds or so, then headed for home on the highway.  One kilometer later it quit again . for good that time ,and I had it trucked home .  The first thing I thought was water in the fuel, and took the filter out and blew through it . Gas shot out.
once I hooked the hose back up and put the bike back together it started just fine. I let it run for about 15 minutes with the choke on, and no problem. It continued running when I took the choke off.
Is there any logic why the bike would die with a full tank of gas ? remember I think the force of the gas could have caused the hose to come off the strainer . The first thing I did was put my hand in the tank to see if the filter or something was loose. i noticed the hose coming from under the fuel pump and with a very gentle pull ,it came out from underneath. leading me to believe it may not have been on all the way because it was hard putting it back on ,and now a good yank on it wouldn't get it to come off easily .
I always hate when something like this occurs ! Before taking the fuel pump out I tried starting it many times , but not even a cough ,but now with the line hooked back up it appears to be the problem , but so far I don't see the logic with the fuel tank being full. Somewhere in one of my manuals I think it mentions gauze or something in the fuel strainer. If that's the case , maybe the bike depends on the fast return of fuel.. But why wouldn't it start after sitting over night beside the highway ? 
It snowed here in Calgary today, but melted , tomorrow I'll keep the bike tow guys cell number in my pocket and nervously go for a ride around here ..       
 

[daveson]

Just between you and me and the gate post, here's some random guesses, I'm talking random as.

If the tank is full and keeps stalling after a short time, but starts after a longer time, the vent might be blocked. If it restarts straight away after lifting the fuel cap, that would be a pointer to it.

I'm thinking it a better description as siphoning air bubbles away rather than vacuum in your earlier post. The left and right is reversed in Laitch's post cause the tank is upside down for the photo. Mine's like that too.
 
I'm thinking it is a vent hose so I'll guess that the fuel strainer port and the one above it are 8mm and they are connected together with an 8mm hose. The fuel hoses are 10mm.

On mine the fuel returns to the right side even though the pump is on the left,  first I thought that doesn't make sense. What I now thinks is the fuel return dribbles down on the right side so it cools down a bit until it hits the fuel level. That would result in bubbles in the fuel,  so better to be on the right side away from the pump.

The fuel filter might be swollen, maybe you want to replace it, especially if you don't know when it was last done.

I'm thinking like Laitch I thinks, when the fuel level is low,  instead of the pump siphoning up air, the air escapes up the vent.

[The Mighty Gryphon]

Fuel from the return line doesn't shoot around inside the tank.  It mostly just dribbles down from the return line.

Check the hoses from the fuel pump to the filter and from the filter to the hard line going to the outside.  One of them could be loose or split allowing the high pressure fuel from the pump to leak back into the tank rather than going out to the rail.

[Laitch]

Is there any logic why the bike would die with a full tank of gas ?

Gryph's correct and yours wouldn't be the first to experience that.

In addition, the bike could quit with a full tank for any of a number of other reasons, some of which were described in post #6. Three causes occurred with my moto. 1)The Jetronic fuel injection control's plug was loose, sometimes making connection, sometimes not. The control is under the seat. The plug should be cleaned and securely coupled to the unit.2) The fuel pump slipped from the vibration damper and bottomed out on the tank floor, intermittently restricting fuel intake. When it was raised and secured the problem didn't occur until two years later when it unpredictably started stalling then restarted and ran fine for days or weeks at a time until the intervals of malfunction happened increasingly more often. That was caused by 3) oxidized contacts within the ignition switch. After they were cleaned then lightly lubed that condition ended.

[Laitch]

If anybody is still interested in Brick tank lore after this scrum, here is a summation of tank construction observations by Brian Curry, a regular contributor to IBMWR's body of knowledge, and Anton Largiader of Virginia Motorrad.

[frankenduck]

If anybody is still interested in Brick tank lore after this scrum, here is a summation of tank construction observations by Brian Curry, a regular contributor to IBMWR's body of knowledge, and Anton Largiader of Virginia Motorrad.

I noticed an error on that page. It says for the 85 and earlier tanks "No provision for a screen on the fuel pump."  That's not true. Those bikes did have a fuel pump screen.  But it differed from the later screens in that it did not have a connection for the anti-cav hose.

85 and earlier screen: Part 16121450714


86 and later 52mm fuel pump screen: Part 16121455126

[xcaret]

Hey Laitch , Your mention of the fuel pump bottoming out on the tank floor sounds very logical because after removing and replacing it back in the tank the bike runs.. But could it have bottomed out with the strainer attached ?
I have ridden bikes since late 60's ,but only one other BMW, and it never quit . My Harleys were all old style with no efi ,and I never had one in a shop ,but fixed them myself. I still ride one .  So I'm lost with all this EFI and ECM stuff..
I first suspected a vacum lock as I had just fueled up, and I opened the fuel cap .
I'm going to pull the " breather hose "off the upper port and see what happens .
I'm sure everyone has had a vehicle that has an intermittent problem, and when it's at the dealer it runs just fine until your 5 miles away.. That's what worries me ! I'll be back after after checking a few things you guys mentioned ..   

[Laitch]

It says for the 85 and earlier tanks "No provision for a screen on the fuel pump." That's not true. Those bikes did have a fuel pump screen. 

It doesn't say the tanks had no screens; it indicates there was no provision for screens which could possibly mean ridges, tabs, stops or something else besides the tank floor to position the screen. Regardless, if there were no screens at the beginning of production, it's likely BMW started supplying screens as soon reports of pump cavitation started hitting the decks. 

You could possibly have a valuable oem sample there, duck, worth even more with its original box and extra special if it were autographed by Michel, Gevert or Probst, if what Wikipedia tells us it accurate..    Get admission to Antiques Roadshow when it's in your neighborhood and have it evaluated.

Hey Laitch , Your mention of the fuel pump bottoming out on the tank floor sounds very logical because after removing and replacing it back in the tank the bike runs. But could it have bottomed out with the strainer attached ?

The pump intake was squatting right down on the bottom of the filter. Odd but true. I repositioned it and that was that, as my story goes.

[frankenduck]

It doesn't say the tanks had no screens; it indicates there was no provision for screens which could possibly mean ridges, tabs, stops or something else besides the tank floor to position the screen. Regardless, if there were no screens at the beginning of production, it's likely BMW started supplying screens as soon reports of pump cavitation started hitting the decks. 

You could possibly have a valuable oem sample there, duck, worth even more with its original box and extra special if it were autographed by Michel, Gevert or Probst, if what Wikipedia tells us it accurate..    Get admission to Antiques Roadshow when it's in your neighborhood and have it evaluated.
The pump intake was squatting right down on the bottom of the filter. Odd but true. I repositioned it and that was that, as my story goes.

Every 85 I've ever dealt with had a fuel pump screen.  And it's there for more than just anti-cavitation.  It's also there to keep crap out of the fuel pump since the fuel filter isn't in the system until after the fuel pump.

The used to be NA but now Max BMW shows them as available aqain.

[xcaret]

I took another look today , after removing the breather hose from the top inside of the tank , The bike still ran fine. Oh, yeah the return fuel was coming out from around the top of the right front of the tank..
 I also lifted the pump up in side its flexible housing incase it was sitting to  low.( as someone mentioned) .
Until I removed the pump and put the breather hose back on , the bike never even coughed , now it starts like it should.. Everyday for a week or more I had hit the starter just incase it had cleared a fuel line or something..   The only thing I did was check things inside the tank ,and now it runs great..
 Thanks all you guys for your advise and knowledge.
I'm satisfied that whatever the problem was ,it has been corrected..
As soon as it gets to  +12 C here in Calgary I'm heading out for a nice ride ..I'll worry about it if it happens again..   

[Laitch]

I'll worry about it if it happens again..   

[xcaret]

Since I was asked to post the problem , I'll tell you my conclusion.. I don't think it was the breather hose after all .
As I mentioned I lifted the fuel pump out of the tank and put the breather hose back on the fuel strainer, and the bike started right up.. ..
 Since I had just fueled up 2 blocks before the bike sputtered and died ,I emptied the tank into a large gas container I had ,and began looking for the problem.. Well since the bike started right up once I put the hose back on I just thought what ever I did removing the fuel pump and connecting the hose "must have cured the problem" I still had my doubts about the bike being cured .. So I rode my Harley instead .. The Beemer just sits in my garage ..  Since the tow trucks charge so much I bought a $200 atv tilt trailer incase the Beemer died again. Here in Alberta there's miles of road with no cel phone service ,and even owning the trailer I was to nervous to take the bike out.
I had been using the gas I took out of the Beemer in my lawn mower tractor . Today it sputtered and died , I replaced the fuel filter and the mower ran just fine .. Since you guys advised me that the breather hose being off should not have mattered , I now am confident I got water in the gas..  I had used the gas only in my lawn tractor and had used up about 3 gallons of it from mowing 3 times ,and I'm thinking the water accumulated in the filter or ,maybe pouring the gas out of the gas container ,only the top is what had been pouring out and now getting down I'm getting water coming out ..   Anyways That's my conclusion.. Tomorrow I'll be riding the Beemer again..