View Full Version : Hissing noise from K11 gas tank

03-26-2004, 06:12 PM
When I originally posted this, I had not seen the post by a new member over on Campfire. He and I are acquainted, and we have inadvertantly posted about the same issue. (BTW, IMO, we still need a general tech forum that is not model specific).

So, several weeks back, he noticed a hissing sound emitting from the gas tank whenever he finished a ride and killed the engine.

Hearing it while visiting with him one day, I opened the tank after listening to the noise and the noise stopped.

Our first thought was that it was a suction in the tank, and that a vent hose was kinked or stopped up. We fiddled with the assorted vent hoses, but the problem has not stopped.

And now, he has noted a slight seepage of gas around the seal of the tank at the cap. I know less than nothing about the pressure valve setup of the tank cap, and would not want to hazard another guess on anything as volatile as the fuel system.

So ... what are the possibilities here? And what suggestions might lead to a fix? All feedback will be appreciated.

Thanks in advance.


03-27-2004, 01:49 AM
I have posted a similar item on the Clubhouse.
I wish I had a solution to your problem because it would also solve mine. I have taken the tank off and made sure that all the hoses (both pressure relief and fuel return) were not stopped up. I even cleaned the pressure relief valve.
I spoke with a BMW dealer in Louisiana and they told me to take the pressure relief valve off entirely. That is the grey contraption that is on the forward most hose under the fuel tank. I plan to do so this weekend.
It appears to me that the problem is not with the fill cap since it is a pressure build up and not a vacuum that is causing the hissing and fuel around the cap. That means that pressure is not being released from the tank. The part on the bike that is responsible for that is the pressure relief valve. In the shop manual it says that only US models have the pressure relief valve .... must be a polution device.
I have not received any replies from my posting - I have a hard time believing that no one else with a K1100 has not had this problem!
Please post your solution if you get it solved.

03-27-2004, 02:18 AM
With the riding season here, it sometimes take a few days for a post to get read. And then, some of us take a few days to muster our thoughts for post. And then, some of us post without mustering thoughts ...

Actually, I think you and I have a mutual acquaintance over in the Magnolia State, and my post is in response to his telling me about your hissing sound.


03-29-2004, 01:55 PM
I did not even look at who or where the post was from. I was just happy that I saw something that sounded like the same problem that I had.
This is really funny -

03-29-2004, 02:58 PM
Actually, I know a retired guy in Birmingham who has had the same issue with his gas cap. But he does not hang out on the forums, since he spends his discretionary time riding ... sometimes, around the world. As in, he once shipped his K100 to Europe, and then rode right at 13,000 miles -- all the way from Barcelona, Spain, to the Artic Circle in Finland, and a lot of places in between.

Meanwhile, I am over in Jaw-Ja with the family on a Spring Break trip to Six Flags. They are still sleeping, and I am up playing on the courtesy computer provided to customers at the Wingate Inns. This is the second one I've stayed in, and they have this neat "business room" for adults who need web access.

Anyway, when I get back to Tuscaloosa, I'll give Casey a call and get his take on the hissing sound. A retired engineer, he has been known to dissect a motorcycle and put it back together to his own specs. :brow

03-29-2004, 06:27 PM
Thanks for you help - I posted on another forum and received this reply from someone that has the identical bike: "It's not a problem - it's petrol vapour!"
I would be interested in your friends response also - our mutual acquaintance and I plan to make a longer ride and will monitor the results. My guess is that there is a small amount of leakage around the sealing gasket of the gas cap that is allowing some pressure out causing the sound. At least that is what I think - BUT I have been told that I should not rely on logic when dealing with a motorcycle???

03-30-2004, 01:42 PM
I can agree that the gas appearing around the cap is vapor. But I think the issue includes a bit more -- as yours is the only bike I've ever heard "hiss" when turned off.

Over the weekend, I asked a reputuable wrench about it, and he suggested:

1. Acquiring a manual and then throughly analyzing the tank venting system, and

2. Checking to be sure the previous owner did not try and disable the emissions control system.

Beyond that, I do have a couple of other questions:

"Is it a blowing hiss?" (pressure in the tank)

or, "Is it a sucking hiss?" (vaccum in the tank)

I :dunno

Either way, it stops when the cap is opened. And I'll be interested to hear how it shakes out.

03-30-2004, 08:05 PM
The response when the tank cap is opened is that pressure is released - the friend that helped you with the brake fluid thought it was a suction and went about taking apart the cap -and got it almost all back together.
The hissing is more like a high pitched noise - I now am not sure that sound was coming from the tank but rather from a hose that we found was not clamped properly on the fuel return line. We took the tank off and cleaned the pressure releif valve and line also - that cleaning made it easier to blow through the valve.
I rode the bike ~100miles on Saturday and did not find the gas around the tank and also when I opened the tank, the pressure was much less than in the past. I did not hear the "hissing" when I stopped either - either I think the problem is fixed or my hearing is going bad???
I am not sure when the pressure valve appeared on the K1100. Our mutual friend does not have one on his K1000 bike and therefore does not have any pressure in the fuel tank. That valve is suppose to keep (I think) 1.5 bars of pressure in the tank. I do not know what the value of this is except as a polution control device.
I have never had a performance problem with the engine which indicates to me that it is not causing a mechanical problem - the noise is aggrevating and also I have had the fuel pump make noise once. Some day when I have time, I may take it to a reputable shop and get the thing looked at and end the speculation. The guy I purchased the bike from is a regular at a motorcycle store here - he says there is nothing wrong with the bike and I should just ride it - we'll see.

03-30-2004, 09:21 PM

The early K-bikes had this problem, by the cure is likely different, given the configuration.

Assumption: This is your K1100LT and the vent pipe and hose is plugged!

I recommend that you remove the fuel tank. To do this do the following:

Drain as much fuel from the tank as possible.

Diconnect the fuel pump/guage connector near the rear of the tank (It may be fastened to the frame with a wire tie). Be careful when you separate the connector, so as not to break the ears or stress the wires. At this point, I think the RT/LT configuration required that your remove the inner side panels or kneepads.

Lift the tank up enough to remove the fuel hoses and have a rag handy to soak up the gas when you pull them off.

Lift the tank off, remove the cap assembly and remove. Locate the vent hose inside the tank and check to make sure it is clear. My guess is that it IS plugged.

Any help?


03-30-2004, 11:39 PM
The friend that helped you with the brake fluid thought it was a suction and went about taking apart the cap -and got it almost all back together.

Yikes! Everybody should have an engineer for a friend. Assuming it was a sucking hiss and not a blowing hiss, you are in good shape. But until time proves that assumption to be correct, I would caution against lighting a celebratory cigar while sitting astride that mount! :brow

Black Fly, thanks for the feedback. I think Lee has already done that exact routine with our engineer buddy. Maybe a lengthy ride before long will verify a fix.

Rick (yes -- I'm curious about which parts were left off) in AL

04-03-2004, 07:21 PM
Rick and fl,
The problem may be twofold. The seal on your fuel filler cap 'may' need to be replaced since these tanks did not leak when new. OR You may also have a 'kinked' fuel overflow hose somewhere.
Here is what I did to fix my '93 K11LT when it exhibited the same problem a few years back.
Go to the dealer and get the little "Air Accumulator" cup (16 13 2 307 467). It mounts under the two stub pipes at the right rear of your tank. One of those pipes runs up to the rain-drain around the filler cap and the other one runs to the 'biscuit' inside the tank.
Find the HOSE that vents to the ground (this was for the rain-drain) and attach it to the new cup you installed. This hose will now carry both fuel and water to the ground.
The other hose is routed to the crankcase vent and has an inline one-way valve that mantains some level of pressure before it will vent the fuel vapors into the crankcase (not a good idea in my opinion). Disconnect this hose and plug it. Either stick a screw into the end or remove it at the crankcase vent pipe (just in front of the airbox and almost centered left-to-right at the top of the crankcase) and plug the fitting with a rubber cap like the ones used for your throttle body vacuum gauge attachment points.
You wil never have a hissing tank again. :clap



04-06-2004, 03:17 PM
As clarification, I read the fix you made to your bike for the hissing sound. When I took the tank off (a couple of weeks ago) to see if that one way valve you described was causing the problem, I only noticed that one single line and the fuel return line.
The line attached to the valve is for what you describe as the "rain-drain" I do not recall seeing another line to the "biscuit" inside the tank. I will take the tank off again and refresh my memory.
If that line is not present on my '97 K1100, in your opinion, would taking the valve off of the line to the rain drain and putting a splice tube in the line, be acceptable? I think you indicated in your response that line should be disabled entirely? :dunno
I ordered the replacement seal for the filler cap and also the accumulator just in case I did not see the other line. I will put the seal on but wonder if taking that valve off entirely would take the pressure off the tank and relieve the hissing sound coming from it??

04-06-2004, 03:48 PM
I might suggest that you take the bike to the dealer, rather than spend any more time on this and potentially creating more problems for yourself than you know. In my previous post, I figured that an insect may have nested in one of these pipes.

Frankly, I had the same problem on an earlier K75 and a K100 and it seems you are making a mountain out of a mole hill (if you know what i mean).

The later bikes already have an accumulator cup on the frame rail. BMW made that change in 1990-1991 and since this bike is newer - there you have it.

What's dealer going to charge you? an hour's labor? Go figure!


04-06-2004, 04:48 PM
Originally posted by fltemple
The line attached to the valve is for what you describe as the "rain-drain" I do not recall seeing another line to the "biscuit" inside the tank. I will take the tank off again and refresh my memory.

Once you remove the tank it should all be clear. Do NOT simply remove the one-way valve. You need to verify where the hose with the one-way valve goes before you use it for a rain drain.
Somewhere in the late 80's or early 90's BMW stopped shipping bikes with the 'air accumulator'. This was to meet EPA Evaporative regulations. All of the K1100's had this one-way valve that routed fuel tank pressure to the crankcase. A lot of dealers removed it and re-installed the 'air accumulator' at riders request. I'm not sure a dealer would even do this modification for you any more with all the new EPA laws.

I sent you an E-Mail too.

04-07-2004, 03:06 AM
Lee ... I've heard your hissing noise.

And I think you and our mechanicking buddy with the garage full of scooters and tools can guide you through it without paying a dealer to do the simple chore of routing hoses.

Personally, I am always reluctant to tamper with the emissions system. IMHO, it would be better to get it back to "stock," i.e., with no un-necessary noises, than to (possibly) aggravate the issue by chunking stuff smart engineers put together right.

FWIW, it is even further for FL to get to dealer than it is for me.

And finally, don't get bummed out over this. That is one fine looking, low mileage, rock solid, can carry you for an half million miles K11 High Line. My take is that the former owner simply did not ride it enough, and that he is right in his assessment that you simply need ride it often.

But if you do get bummed out, can I have first shot at buying it? :cat