Fuel Tank Pressure Build-up

I had an issue with my bike this past weekend.

While riding at highway speeds I was not having any issues but as soon as I stopped for a light or considerable slowed down to make a corner the bike would sputter and then die. I would hit the starter button and it would restart and I would go again. This was happening at every light and stop sign and then it started to take a couple and then a few hits of the starter button to get it going until it would not start. I would say I had 25 plus miles like this until it would not start ( I was going from Sedona AZ to Jerome). It finally would not start at the top of a mountain heading to Prescott at about 7200 feet in elevation. I figured it had something to do with the fuel delivery system, I had already experienced a clogged fuel injector from bad gas which was replaced in warranty. I opened the gas cap and fuel shot up, like a guesser, about 2 feet, about a half liter or so. The tank had built up a tremendous amount of pressure. I listened and either the fuel was boiling or air was causing a bubbling sound coming from the fuel tank. I waited a half hour and it started up. I rode with the gas cap slightly opened (not locked in) as I did not want to build up the pressure again. I continued to have the bike cut out on me when I stopped for the next 50 miles but at some point it cleared up and I was able to close the cap and it ran normally the rest of the way home.

To me the tank did not seem like it was venting properly. What do you guys think it could be? Clogged line, air in something, or the dreaded charcoal canister. It is a 2009 with 16,000 miles.

Andy

I had the same thing happen to me twice. Once got a healthy dose right in my eyes. Now, I make a practice to open the filler with my head turned.

However, I never had any running issues.

Hasn't happened again in the past year.

The same thing happened to me... Hot weather, constant highway speeds. Stopped, opened gas cap, extreme pressure. Relived pressure, bike stalled at stop sign, continued ride with no other problems. I do remember the service guy telling me if my bike ever died randomly and had pressure then I should bring it it in because something was up. I honestly don't remember what though, but was something with the fuel delivery system.. I didn't take it in, because I honestly haven't had time and the bike has been running well. It was about 115 outside the day it died.

It is really quite simple to trouble shoot a vacuum buildup. Take off the right plastic surrounding the tank...... Follow the line on the top of the filler neck.. It will run back under the plastic, and under the seat. Before in enters under the seat.... There is a little half inch thick aluminum cylinder about 1 inch long. If that one freezes it will not let in air in the tank as fuel is burned. The valve has a little ball inside that rolls around. It can be stuck if debris gets in there. The line then goes further down to the CC..... and even if you have a Y at the canister as the mod/recal dictates.... If the little cylinder is plugged.... the tank will not relief, and a vacuum will be generated. Cure is to either remove the little valve.....or full removal of the CC.

Thanks, I will look at that to see if that is/was the cause. I experienced it again a couple of days ago but without the fuel tank pressure. I will probably just remove the charcoal canister in the end.

remove the charcoal canister and quit over filling your bike on the side stand.

This

Or else you'll have a cracked tank like most of the US (californian spec) F800s

You could have something else going...... But is is easy to eliminate the first usual suspects. In addition...regarding overfilling.... the charcoal in the canister,can break down when gas from a overfilled tank enters it....and then when the purge valve opens to ingest the fumes......Then you have a rough ....way rich running engine.....that could ingest the charcoal paticles... which of course will just get combusted..... IMHO...the CC is crap..... Let us know how it goes....

to the carbon canister. Maybe stick an airhead fuel filter in there to keep rocks out of the tank.

Only the US gets the poorly engineered cc, everyone else gets a "vent line".

Most anything will corrupt the cc, water from it's vent line, raw wet fuel from an over full tank.

Only in places with detail emission inspections will this be a problem, and it will make it easier to export your bike to Peru.

Mine has been fine for 50k miles with one airhead filter, sometimes it has some presure in hot weather, it never draws a vacuum anymore.

bobo

Ok ... long day here so don't flame me if I hose this up eh?

If the fuel tank is suffering from high VACUUM than yeah ... likely it is a clogged canister and maybe a messed up check valve to go with that ................

If there is high PRESSURE in the tank that likely is not the canister's fault.
While only USA (maybe Canada I don't recall) bikes have a canister,
all bikes have the pressure/vacuum check valve in the vent line which could freeze or become plugged and cause too much pressure in the tank ... overfilling or topping up and adding road heat can cause this as well.

Sometimes it is hard to tell when opening the tank if pressure is venting or vacuum is being releived...
hold a piece of paper up next to the tank lid when you flip it and deflection will tell you...........

On early models the pressure/vacuum check valve is actually a ball-check as described above ... Joel mentioned that on later models it was actually a contraption with a sintered insert that was easily clogged.
The specifications for the valve (at least the older valve) as to how much pressure/vacuum they would hold before venting were pretty high....

I am personally not convinced that the CC is the reason USA bikes have cracked fuel tanks far more often than non-USA bikes... I'm not saying that the CC is NOT to blame I'm just not convinced..... that topic has been kicked around multiple places also ...

Just my $0.02...

Just remember that that line with the check valve to the cc both relieves acuum and pressure. With a good resistance away from the tank, and no ressistence into the tank...... If all is well in said line.... There is no way for either pressure or vacuum to build up. Most importantly...... The steps I mentioned above, is an easy first step to eliminate easy to check item...... Then if that does not help.... Dive in further......

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!

guys,

don't you mean UNDERPRESSURE?

it's a known failure of air valve just under the seat.

Hicks:

I was told that the valve was not supposed to "vent" until a certain threshold value was exceeded on EITHER side of "atmospheric" pressure -- both to the high side (some number of psi above zero-psi-gauge pressure) and to the low side some number of psi/inches of mercury below zero-psi-gauge pressure (i.e. a vacuum).

In other words there is a range of pressures above and below atmospheric at which the check valve does nothing and "holds" whatever pressure (positive or negative) that is sees in the tank.

did you disassembe the thing?

there is a ONE-WAY (including the arrow) valve on a rubber hose under the back edge of a seat... it can only pass air TO the tank.. ale least on my bike (5/2008).

when you hear a hissing sound when opening the tank on gas station -- it's gone.

I have personally second one.. + one repair done by myself with help of air compressor (it was simply stuck).

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? :huh

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

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 .... :huh

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....