Fuel Tank Pressure Build-up

[JRWooden]

Hicks:
That might be correct ... I'm only passing on what the dealer told me....... I've not dis-assembled mine.

Does the "tank-side" of the check-valve hose from the check valve go to the tip-over valve or is there some other fitting on the tank?
Does non-tank side of the check-valve hose go to the CC or are there two "vent" hoses from the tank?

I'm confused because if this valve can only pass air into the tank and it is also this same hose that is connected to the CC ...
Then how does the CC ever get a chance to do it's job and collect gasoline fumes?

[Hicks]

Originally Posted by JRWooden

Hicks:
That might be correct ... I'm only passing on what the dealer told me....... I've not dis-assembled mine.

Does the "tank-side" of the check-valve hose from the check valve go to the tip-over valve or is there some other fitting on the tank?
Does non-tank side of the check-valve hose go to the CC or are there two "vent" hoses from the tank?

I'm confused because if this valve can only pass air into the tank and it is also this same hose that is connected to the CC ...
Then how does the CC ever get a chance to do it's job and collect gasoline fumes?

honestly.. the way how the tank get rid of OVERPRESSURE on achthundert is a great mystery to me I think it is on the pump unit.. but who knows...

btw: what is cc?

[JRWooden]

Originally Posted by Hicks

honestly.. the way how the tank get rid of OVERPRESSURE on achthundert is a great mystery to me I think it is on the pump unit.. but who knows...

btw: what is cc?

CC - Charcoal Canister unit on USA bikes that is supposed to capture gasoline fumes and store them until they can be fed into the intake manifold of the bike and burned during operations (under computer control of course).
If I understand it correctly all vapor that leaves tank is supposed to be routed into this canister.

The bike has to somehow get rid of excess overpressure, or very bad things could happen ...
I'm confused as well

[ebrabaek]

Originally Posted by JRWooden

Erling:

I can't find any specifications on the valve in either the BMW manual or the Haynes manual, but was told by my dealer when my tank was replaced that the valve did not open until something like +2.0psi on the pressure side, and several inches of water on the vacuum side. In other words the valve, by design allows some "modest" pressure or vacuum to build up in tank... (this is on a 2009 model). I also can't find any specs on the (rumored) newer design that uses a sintered core ...
If anybody else has specifications please post!

Since this valve is a separate item from the "tip-over" valve, I'd really like to know if this valve has any function other than to reduce emissions (by reducing the amount of "breathing" that the tank does which would let more fuel vapors into the atmosphere when the bike is not running).






Thanks!

That is not the case with my valve. As I noted in a post somewhere....might be on f800riders forum.... But if memory serves me correct ...... Mine flowed free with no restrictions....INWARD to the tank....and opened up about 4-5 psi AWAY from the tank. One note.... if vacuum was applied to the away side...( against the arrow) it would flow free with no restrictions. It only too a minute suction...and it would flow free against the valve. There should be no vacuum......not even a little...... but I can see a less than 5 psi pressure build up.

[JRWooden]

Originally Posted by ebrabaek

That is not the case with my valve. As I noted in a post somewhere....might be on f800riders forum.... But if memory serves me correct ...... Mine flowed free with no restrictions....INWARD to the tank....and opened up about 4-5 psi AWAY from the tank. One note.... if vacuum was applied to the away side...( against the arrow) it would flow free with no restrictions. It only too a minute suction...and it would flow free against the valve. There should be no vacuum......not even a little...... but I can see a less than 5 psi pressure build up.

ebrabaek

Yes, that's really the way I would expect it to work ... !!!!

Hold a "slight" positive pressure in order to "force" generated gasoline fumes into the CC when the bike is turned off ...
and to "free-flow" at the slightest vacuum ... as that means the engine is running and the CC is being "purged" (and that the vent line on the bottom of the CC might be obstructed ... etc as a result of the on-board computer turning on the solinoid that applies intake manifold vacuum to the canister.

But ... if it really DID work that way then the "TEE modification" would not have been required to the canister drain line becuase having the "low down" drain line plugged by water or debris would not have resulted in a stalling problem ....

I've said this before, but I'll say it again anyway...

I'd pay some serious money to share an evening's worth of beer with the German design team and hash some of this stuff out. Those guys are plenty smart or they wouldn't work for BMW ... we're kind of like the blind men and the elephant if you know that old joke....

[ebrabaek]

Originally Posted by JRWooden

ebrabaek

Yes, that's really the way I would expect it to work ... !!!!

Hold a "slight" positive pressure in order to "force" generated gasoline fumes into the CC when the bike is turned off ...
and to "free-flow" at the slightest vacuum ... as that means the engine is running and the CC is being "purged" (and that the vent line on the bottom of the CC might be obstructed ... etc as a result of the on-board computer turning on the solinoid that applies intake manifold vacuum to the canister.

But ... if it really DID work that way then the "TEE modification" would not have been required to the canister drain line becuase having the "low down" drain line plugged by water or debris would not have resulted in a stalling problem ....

I've said this before, but I'll say it again anyway...

I'd pay some serious money to share an evening's worth of beer with the German design team and hash some of this stuff out. Those guys are plenty smart or they wouldn't work for BMW ... we're kind of like the blind men and the elephant if you know that old joke....

Ha ha.... Beer with a German... I see an iceberg ahead....... Yes there is a need for the T... well...at least a vent...not from the tank. You see if the only vent into the CC was through the tank....when the purge valve would open and suck in the fumes, it would cause the engine to run too lean..... so they put a vent line down in front of the rear tire.... wonderful.....now the motor is breathing in a mix of tank fumes.....and fresh air...when the purge valve opens. Awesome.... But when the Germans tested this.... they did so i Denmark, where the weather is always sunny 360 days of the year...... Ha ha Joke.... but seriously....no rain....because when it was driven in the rain, and the purge valve opened.....fresh air from the vent in front of the tire was sucked in rather than the over pressurized tank ( remember the tank has to produce about 4-5 psi to vent.... In rain.......perhaps...cold weather....that is not likely to happen....therefore.....the little tube in front of the tire now is a vacuum cleaner.... whater is now sucked up....and into the intake manifold...and combusted.... well...trying to combust.... result....Stalling. Solution...either take out the one way check valve, and risk offending the Bavarian engineer who created the system...... or simply put a T so it now will suck from under the seat.....well ....we know the results. Reason for the hose going down..... You have all these people who want to maximize the endurance....and tip top off the tank....on the side stand...... Awesome.....but the fuel has to go somewhere....and gravity rules.....fuel with expansion now enters the CC...( Charcoal Canister)....and drips down in front of the tire.....Nice. The short and great fix would simply have been to remove the Check valve..... along with installing the T.....so fuel can still drain down.....and when driven in water.....the under the seat hose vents, when water enters the down pipe. OR.....Simply leave it like the Bavarian's did for them self... ( smart people)... simply don't install the CC.... Ha ha.... they are still laughing at us Yanks..... or in this case Danes......ehhh...uhmmmm... living in the US.

[soph9]

My bike stalled on two recent trips...symptoms are easy....riding in over 28 C for hours, gas is about 1/2 full and bike dies mid stream. Down shift bikes comes a live but then dies...can't get into high gears without stalling so i cruise at lower speeds and lower gears. bike finally gives up..pull over. Turn key off and wait. At first I did not pop the tank lid and the bike would start and go for a few hours. After awhile I did pop the lid when the stalling began..after riding for hours in extreme heat...seemed to work.

Bike into the shop...they can not find a fault code and replicate the problem...I told them they would not be able to replicate the problem unless they were riding in the same conditions with the tank not full. Said they checked all the hoses etc...everything in working order. Nothing was done. So, I will pick up my bike tomorrow and because the temps are cooling down here I am certain there will be no issues until next time we ride in crazy 90-100 degree heat.

[JRWooden]

So ... when you popped the tank lid did it release PRESSURE or was there a VACUUM?
Sometimes it's hard to tell....

If a vacuum, then I would say your charcoal canister is plugged or a related hose has a kink in it or your check valve is bad...
and ... (AFAIK) the bike is not equipped with instrumentation to detect that fault and record it to the computer logs...

If pressure ... well then I've got no clue .........

I carry a little "straw" from a can of WD-40 in my tool bag so that if I ever get the problem I can close the tank lid with the straw inserted and keep the tank from moving away from 1-atmosphere of pressure ... have not had to use it yet...

[soph9]

there was a pop once...but really not that mush sound. They said they checked out everything...I will find out more tomorrow.

[JRWooden]

Thanks!
Keep us posted.

[itsatdm]

Based partly on this thread http://www.advrider.com/forums/showthread.php?t=500968 I think the valve on the fuel tank is a 2 way valve. It allows air in to deal with vacuum in the tank via the canister and allows pressure out also via the canister (tube that goes to the throttle body).

If some one wants to test that theory, take the hose off at the tank that goes to the canister and see if you can suck air in and blow it out. My guess is it only works in one direction, or at least easier to suck than blow.
I suspect the breather valve in Yeti's diagram is an on demand switch that controls when the fumes in the canister are drawn into the throttle body.

I know it takes pressure to over come the tank valve to allow fumes out. I have heard someplace it is 4.5lb.

Even if you remove the charcoal canister you will still get the build up of pressure in the tank until you trip the pressure release valve at the tank. That is how mine works. BTW if you do remove the canister do not cap the hose to the tank.

[JRWooden]

Yeah Gary ... it is my understanding as well that the valve on the throttle-body side is an electric solenoid valve controlled by the computer.

[PatrickM]

Plenty of things like this: http://www.ascycles.com/detail.aspx?ID=71245 to help deal with pressure on the negative side, but are there two way valves which allow the valve to be tripped at a set (albeit low) pressure level, bi-directionally?

Prefer to not vent to atmo, and I do find myself going from sea level to 7200' on a weekly basis, including some 30+ degree temperature changes.

[HighTechCoonass]

one can blow it the tank with the vent line removed but you get a very very small flow with a vacuum. (a check valve). From what i understand there is a "stone type" vent on the pressure relief. I used to have high pressure also when I opened my tank... not anymore....

see my drawing.... I have the flow arrow drawn in reverse)

Originally Posted by HighTechCoonass

[COLOR=DarkOrange]


Before:

Originally Posted by itsatdm

Based partly on this thread http://www.advrider.com/forums/showthread.php?t=500968 I think the valve on the fuel tank is a 2 way valve. It allows air in to deal with vacuum in the tank via the canister and allows pressure out also via the canister (tube that goes to the throttle body).

If some one wants to test that theory, take the hose off at the tank that goes to the canister and see if you can suck air in and blow it out. My guess is it only works in one direction, or at least easier to suck than blow.
I suspect the breather valve in Yeti's diagram is an on demand switch that controls when the fumes in the canister are drawn into the throttle body.

I know it takes pressure to over come the tank valve to allow fumes out. I have heard someplace it is 4.5lb.

Even if you remove the charcoal canister you will still get the build up of pressure in the tank until you trip the pressure release valve at the tank. That is how mine works. BTW if you do remove the canister do not cap the hose to the tank.

[itsatdm]

I thought it was a ball and spring, not a stone.I thought that was what I said, did not notice which way the arrow was. It is the only line so it must do both, obviously not with the same amount of pressure required.

It has to go through the canister both ways. When I said verify, I meant the ability to draw vacuum in through the canister and difficulty in blowing it out through the canister. When this valve on the line from canister to throttle body opens, engine vacuum could not only evacuate the canister but assist in opening the valve in the tank. How hard did you suck on the tank line?

My tank line is disconnected with a filter on line end. It still builds up pressure occasionally on hot days.

I guess I could pull the valve out and check exactly how it works, but it has been that way for 3 years with no harm. It would be different on a carb'd bike.