Hi,

yesterday my K75 decided it did not want to get out of bed. 

Turned over fine but not the slightest puff from the exhaust. I realised fairly quickly that pump was not making its usual sound when I hit the button. No point in flattening the battery I had no fuel pressure.

Thanks to several good threads on the forum I fairly quickly tracked down wires and determined that although relay etc were fine and 12V was getting to the pump on button press, it was not even twitching. I also checked the other terminal did have a good earth connection and also found about 0.5V present on button press.  This implies a fair current is actually going through the motor windings.

This leaves me with a couple of questions:

1. I have a K100 in the garage, are the pumps interchangeable ? 

2. Is it credible that a fuel pump can just go tits-up overnight ??  I had not seen any intermittent issues and rode the bike the day before. It ran and stopped perfectly normally.  This seems odd to say the least.

Any answers on those two from those who have already been down this path?

TIA.

Hello!

Yes, the fuel Pump is interchangeable 
But first check the fuel tank connection at the right side!
Is there power?

Cheers

Thanks, OK that gives me some way out without waiting a week for a parcel.

Here is the reason: a bucket full of petrol-snot that someone sold me in place of fuel.I suspect there is some frog-spawn in the bottom of there too.


There is all sorts of rust and rocks swilling around in there and the pump seems to have swallowed a lump of grit. The usual trick of connecting the pump backwards does not even work, it's locked solid. It makes a kind of thump noise when I apply power in either direction.

Sadly there are no accessible parts in these sealed pumps so no way to force it to rotate backwards to free the jam.

I will try tapping and banging it and a shot wtih the air line but it seems choked up on a large lump of grit.

Not a happy beamer. Cost me two days work already !!







PS. can someone explain the logic of putting a filter AFTER a roller gear pump which is critically sensitive to being filled with crap?

Isn't this a bit like buying a device that tells you you have just gone through a radar trap and been flashed at excess speed?

Beamer wrote:Sadly there are no accessible parts in these sealed pumps so no way to force it to rotate backwards to free the jam.

can someone explain the logic of putting a filter AFTER a roller gear pump which is critically sensitive to being filled with crap?

If this is the bike you bought in March, then I think you need to overhaul the fuel system components. The debris in the tank is typical of a neglected bike. Sudden failure for the engine to run can be a sign of a fuel system clogged to the last degree.

Actually there is a post in the download section containing fully-illustrated instructions on how to disassemble and reassemble classic K-bike fuel pumps but I think you have more work to do than just that.

Regarding the pump's placement, all fuel pumps I can recollect had filters on the outlet side of the pump. These pumps also have a fuel filter sock or filter basket on the inlet side, and are durable for decades if the fuel system is reasonably clean. The filter socks or baskets can be abraded or clogged by debris left in the tank and eventually fail to one extent or another.

The problem here stems from failure to keep clean fuel in the tank, a consequence of not riding the bike enough, or storage of the bike without emptying the fuel system. You are probably aware that these tanks and this fuel system are not meant for long-term storage of fuel over months and years. If the lower seams of the tank don't leak from corrosion, count yourself lucky and work on the other components


The tank should be thoroughly cleaned using vinegar or another cleaning agent. The pump assembly should be removed to inspect its vibration damper and mount to see if they are decomposing. All the fuel lines, the fuel filter and the fuel pump pre-filter should be considered for replacement. Cleaning of the fuel rail and the injectors should be considered. At least, the injector's performance should be checked.

The fuel pump itself can be replaced—if necessary—by aftermarket substitutes.

The problem here stems from failure to keep clean fuel in the tank, a consequence of not riding the bike enough, or storage of the bike without emptying the fuel system.

Thanks for the reply.

No this bike is my everyday transport and as I said it ran the evening before it refused to start.

The crap came from the fuel station pump. A classic bottom of tank scavenge, full of crap and snot. Look at the photo! That is not just stale petrol ! 

I have managed to free up the pump having taken it out. There is a little access to the rollers at the bottom, enough to just move then with a pocket knife and free up the pump. That saves taking the K100 pump off too, so that's greatly appreciated.

The rubber shock-absorber is in pretty bad shape. Probably the result of all the polluted octane ( ethanol and '98 aromatics ) we get pushed. Several pumps round here don't even sell straight 95 any more. 

Well now the pump works again, I'll flush everything and put it back together. I need a vehicle tomorrow. ( As well as today and yesterday   )

Beamer wrote:Look at the photo! That is not just stale petrol ! 

The rubber shock-absorber is in pretty bad shape.

I did look at the photo and a fine photo it is too! You should be proud.

There'd be some bits of the vibration damper in there, I'd bet. How's the fuel sock/filter basket looking from your vantage point? Is there one?

Yes, the rock catcher is there but seems to have grit both in and outside. Maybe due to me shaking everything to wash it all out after pulling the pump.

There is a little pipe stub on the side which I don't get. Presumably the fuel is pulled in via the gauze, what is the pipe connector for?  That would bypass the filter. :?

Also a 1/8" flexible pipe going up to an ali tube just forward of the filler hole. I don't see any mention of this in the mickey mouse edition manual I have.  What is it's purpose?

Beamer wrote:Yes, the rock catcher is there but seems to have grit both in and outside. Maybe due to me shaking everything to wash it all out after pulling the pump.

I don't see any mention of this in the mickey mouse edition manual I have.  What is it's purpose?

Get a couple of adult editions of manuals even though there are fewer opportunities to color the pictures with crayons. The downloadable BMW workshop manual has a few really nice drawings for coloring though. I did a splendid rendition of the cooling system using Scarlet and Cerulean. I'm reluctant to post it because people would download it and claim creation rights.  

The design and purpose of that hose/pipe system is to prevent cavitation interfering with fuel intake through the pump. Here is a photo that likely confirms what you're seeing. That whole thread is informative.

That hose was missing from my tank for 30K miles. The engine ran well in spite of its absence. That could be because even though our politics are sometimes shabby in the USA, the gasoline supply is usually clean.

I eventually installed the hose in case I had guests.

I eventually installed the hose in case I had guests.

Well cavitation may be a corner case and it was added at some stage to production but if you have the stub to attach it you seriously need something on the end otherwise the filter becomes pretty meaningless ( whether or not you someone looking inside your tank ) .

Thanks for the explanation. It seemed the only option was to connect it to the ali pipe near the filler hole but it did not make any sense to me why it was there.

Still don't see how sucking air into the pump inlet is a good idea but I'm not here to redesign the bike, just get the damned thing running again. If that's where it is supposed to connect, I'll go along with it.

I'm may take dimensions an mould a new shock absorber out of silicone rubber. Fuel problems are only going to get worse with dumb-assed enviros trying to save the world by turning food into octane substitute and getting it legislated even when they now realise it was a bad idea.

Reversing the polarity will cause the pump to run in reverse. Alternating the polarity back and forth briefly will sometimes free up a jammed pump. Soaking in white vinegar for a week will sometimes free up a really jammed pump. As per Laitch clean your tank and always run a filter screen.
Regards Martin.

Beamer wrote:

I eventually installed the hose in case I had guests.

Well cavitation may be a corner case and it was added at some stage to production but if you have the stub to attach it you seriously need something on the end otherwise the filter becomes pretty meaningless

Having the pre-filter isn't meaningless just because the hose isn't connected. The pre-filter inhibits particles from entering the pump. You can run the system without the hose. I don't care why it worked ok that way on my bike. It just did.  

I just spotted you posted that from Cork.

Not by any chance buying fuel out near Blarney?????

Brand new fuel pump Parcel Motel ex UK £50.....correct pump and connectors. Its a Fiat/Lancia/Alfa fit too....

I don't have a spare pump on my shelf...I did until Jim Ellis rang me one day looking for one for a K75......gave it to him for what it cost me to replace it. Just haven't replaced it yet.

Beamer wrote:
Thanks for the explanation. It seemed the only option was to connect it to the ali pipe near the filler hole but it did not make any sense to me why it was there.

Still don't see how sucking air into the pump inlet is a good idea but I'm not here to redesign the bike, just get the damned thing running again. If that's where it is supposed to connect, I'll go along with it.

That hose between the aliuminum pipe near the petrol cap and the mesh filter basket on the pump is not injecting air; it is directing fuel directly around the pump inlet. This action prevents cavitation starvation with low fuel levels.

Thanks but I don't understand this explanation.

The inlet where this attaches to the rock-catcher filter is arbitrarily oriented with respect to where the pump draws fuel in its base. It also effectively bypasses the filter letting in unfiltered fuel.

There is a long path, through quite small bore tube and ali pipes, so the resistance to flow will be many times greater than simply passing through the light gauge nylon filter.


I'm not saying you are wrong but I don't see how this works.

Does this mean that when running low on fuel you are more likely to draw unfiltered  fuel via this line ?

PS, I've just had another look at the tank. There are two ali tubes which lead out through the base of the tank. One is the rain drain for the filler cap, the other goes to the block just inside and in front of the filler hole. I was thinking that this was connected to where the PVC tube from the  pump connects. I have not checked this assumption but if not it must be drawing in air from the top of the tank.  I can not see how this tube could be pulling in fuel.

Beamer wrote:. . .  I don't understand this explanation.
Does this mean that when running low on fuel you are more likely to draw unfiltered  fuel via this line ?

I have not checked this assumption but if not it must be drawing in air from the top of the tank.  I can not see how this tube could be pulling in fuel.

I think the purpose of this system is not to draw air from the top of the tank into the system but to aid in evacuating air and vapor that would otherwise enter the pump at severely low fuel levels. Vapor entering the pump can be compressed then its sudden expansion within the unit can damage the components. I believe that is what rbm meant by "directing fuel around the pump inlet." The hose provides a means of expelling vapor that might block fuel entrance into the pump by providing a path for vapor to exit from the basket to the pipe inlet inside the tank then to its outlet beneath the tank and from there to the ground via another hose or a catch cup/hose combination.

The absence of that hose from the pre-filter basket in my bike's tank made no difference in the bike's  performance because I haven't chronically run my bike when it was extremely low on fuel. This feature might be another example of BMW over-building. I'm ok with that.

Anyway, that explanation works for me. I'm not trying to sell it. I'm giving it away.

Thanks, that makes sense to me. I can see how fuel could be sloshing around with bumps, acceleration, etc and draw air into the rock-catcher and it's good to have a quick exit route for this air when the fuel next comes sloshing back. The position and connection of the pipe is consistent with that explanation.

The fuel level sensor is broken on this bike so a couple of times I've been caught out. Never actually broken down but once I got the pump having done the last mile running on the smell of petrol left in the tank !

I've finally come up with a theory on the 'anti cavitation" hose installed between the stub on the plastic moulding of the suction strainer (not included on early models). Being a failed Mechanical Engineer, I expect to have to pay someone to accept it!

I'm thinking that BMW experienced some examples of long term pump cavitation damage to the pump internals due to a combination of hot fuel (something I still don't feel I have a good explanation for) and restricted suction mesh screens. My theory is that they must have decided that it would be better to have a situation where the fuel pressure inside the screen basket was maintained at more or less normal atmospheric conditions even if this meant that air was being introduced via the anti cavitation hose. This would mean that the pump would handle a mixture of fuel and air which would impair the running of the machine. You would know that you have a problem to be attended to but at least you would have a chance of getting home. I can see how this would be better than having the pump inlet conditions affected by the pressure drop across a semi clogged screen to the point where bubbles of fuel vapour begin to form. The fuel bubbles forming are no worse than air bubbles except that they implode in the pump mechanism when the fluid pressure excedes the vapour pressure of the fuel at the prevailing temperature. My understanding is that it is the shock waves from these minute implosions which accounts for the degradation of pump components seen in prolonged cavitation cases.

Comments from anyone with time to read this monologue ????

rawdonball wrote: Being a failed Mechanical Engineer, I expect to have to pay someone to accept it!
Comments from anyone with time to read this monologue ????

I accept it and I accept PayPal, too.

Laitch wrote:

Beamer wrote:. . .  I don't understand this explanation.
Does this mean that when running low on fuel you are more likely to draw unfiltered  fuel via this line ?

I have not checked this assumption but if not it must be drawing in air from the top of the tank.  I can not see how this tube could be pulling in fuel.

I think the purpose of this system is not to draw air from the top of the tank into the system but to aid in evacuating air and vapor that would otherwise enter the pump at severely low fuel levels. Vapor entering the pump can be compressed then its sudden expansion within the unit can damage the components. I believe that is what rbm meant by "directing fuel around the pump inlet." The hose provides a means of expelling vapor that might block fuel entrance into the pump by providing a path for vapor to exit from the basket to the pipe inlet inside the tank then to its outlet beneath the tank and from there to the ground via another hose or a catch cup/hose combination.

The absence of that hose from the pre-filter basket in my bike's tank made no difference in the bike's  performance because I haven't chronically run my bike when it was extremely low on fuel. This feature might be another example of BMW over-building. I'm ok with that.

Anyway, that explanation works for me. I'm not trying to sell it. I'm giving it away.

Great. This is the best explanation I have read here about the fuel pump intake. Thanks for your generous gift Laitch

I cannot see why this tube (and I resist calling it an Anti-cavitation tube) would be used to introduce air to the pump inlet so I therefore accept Laitch’s explanation that its purpose is to "aid in evacuating air and vapor that would otherwise enter the pump at severely low fuel levels”. I've had this happen and the sound is loud and screeching.

Cavitation is often talked about at this point but I can't see how it has anything to do with the fuel screen basket or the integrated tube. I have often wondered about cavitation in K-bike fuel pumps, as my late father, an ex British naval engineer had long ago, lovingly explained to me the effects of cavitation on propellers of Destroyer class frigates (thanks Dad. I knew that bit of information would come in handy.... one day ).

Firstly, my understanding is that cavitation will not occur outside the pump but inside on metal surfaces where pressure differentials are far greater. During cavitation, the bubbles of vapour which form in the low pressure zones within the pump, do not explode, they implode, as Rawdon mentioned, and with some force when met with denser higher pressure liquid immediately above the surface. Their existence is very tenuous and not sustainable in such a volatile environment and so they disappear as quickly as they formed, never leaving the metal surface. This implosion is enough to cause serious erosion damage to the metal surfaces on which they formed. Cavitation has another effect, the generation of sound. A distinct problem for submarines. Props are designed for a specific rpm range at low speed to avoid cavitation.
If you’re still wondering how this cavitation works, it can be easily seen at the bottom of a saucepan of water as it comes to the point of boiling. Heat is the catalytic force here instead of vacuum. Bubbles of vapour appear but then quickly disappear never really leaving the bottom of the pan. You may also remember that science experiment we all did in High School where we boiled water at room temperature by placing a beaker of water inside a glass vacuum bell and evacuating the air. Vacuum forces pockets of vapour to form on the sides of the glass beaker. Just a heads up. Don't enter the hard vacuum of space without your pressure suit. You'll die from cavitation within about 15 seconds.
And have you ever wondered why your electric kettle increasingly makes so much noise as it comes to the boil?…. cavitation. That is the sound of vapour bubbles imploding. It soon stops as all the water reaches boiling point and the bubbles of vapour are able to sustain themselves in the evenly heated water and make it to the surface.

Your post above, Laitch, has urged me to research this a little further in relation to fuel pumps and I've found some interesting facts.

• Cavitation creates heat and the hotter the fuel becomes, more cavitation is produced. Fuel can reach temps of around 120ºc due to cavitation.  This sounds very familiar to a K-biker. Heating of fuel is of course aided by engine and ambient temps but if hot fuel facilitates cavitation then this is a good case for fitting a fuel cooler especially in hotter climates like ours here in Australia.
  
• Cavitation in the fuel pump can produce loud noise. We’ve heard a few owners reporting pump noise before too. I think that our K fuel pumps are fairly reliable but an especially noisy pump may be a good indication of cavitation and impending failure sometime down the road.
  
• Also, as a means to reduce the occurrence of low pressure zones at the pump inlet which can facilitate cavitation inside, pump inlets are wide and mostly unobstructed. It’s for this reason they are located inside the fuel tank. To have a hose connected to the inlet of the pump would create a vacuum which would promote cavitation.
  

Take from that what you want.... it's free. And take a read of this excellent article… it’s a good read. Cheers

Also interesting.... Death of the External Fuel Pump

I too am a little suspicious of the idea this is about cavitation. Simply evacuating air that will get into the rock-catcher filter when the fuel level is around that level in the tanks seems more credible.

These pumps do suck quite strongly, which surprised me. While flushing the crap out of the pump I had put some clean petrol in a plastic bowl and sat the pump in it. When I applied power, it actually stuck itself to the bottom of the bowl with the suction. That said , it was only a slight  pressure drop , not a "vacuum". I did not hear any "cavitation" noise.

Cavitation is well know naval problem but consider that you have a screw being driven with enough force to propel a large vessel and massive amounts of water being displaced very quickly. The depression arises because the sea water can not move back in to the space behind the screw quickly enough. At some point the depression is great enough to produce evaporation at ambient temperature causing the bubbles.   I do not believe this kind of situation could develop in side nylon gauze filter with a relatively large surface area with a low viscosity liquid at the moderate flow rates of the fuel pump.

I think that the idea of cavitation is a red herring and Laitch's  explanation about quickly evacuating any unwanted air ( which will get into the filter at low fuel levels ) is the correct on.

I've always assumed the hose on the filter basket was an air bleed to expel trapped air.
Regards Martin.

Allow me to throw a couple more pennies into the pot.

Looking at the plumbing diagram for the tank in the Official Factory manual the filter basket is connected to the return line from the fuel pressure regulator.  I think the rationale for the connection was to send the already filtered fuel that was coming back from the rail directly to the inlet of the fuel pump.

Since the vast majority of the pump output is returned to the tank it makes sense to plumb it past the pump strainer to reduce the amount of work the pump has to do to draw fuel in.  Because the fuel is actually pumped into the filter basket, it saves the pump the work it would have to do to pull the fuel through the screen.  Less work, means less heat generated by the pump motor, and longer pump life.  And yes, by having that very slight positive pressure in the filter basket the chance of cavitation is reduced.

I guess that in later models it was found that the inlet strainer didn't have much restriction to the pump inlet so the return line connection was eliminated.

It's too early for cabin fever.

Point-Seven-five wrote:the filter basket is connected to the return line from the fuel pressure regulator.

Have you visited Dai recently?

Dai is making good progress lately so they wont let him in.

All the best wishes for a speedy recovery.