Non-sealing gas cap

[radan2]

My '89 R100GS has always had a very slight smell of gas around it. After a lot of checking, I have determined that the gas cap is not sealing well. The locking mechanism does not work well. Is there a gasket or something that needs to be replaced?

 

[fracture]

Someone else may have some thoughts on this but this is what I would look for:

Your bike is probably of the vintage that contains the fuel vapor recovery system. This system traps fuel vapors in the tank and also dumps some vapor into the crankcase. I think the gas cap on these contains a pressure relief valve to relieve fuel vapor pressure in the tank if the pressure rises too high. The cap probably also contains a valve to allow air in as fuel is consumed to prevent a vacuum from forming in the tank.

If the pressure relief valve is working then on a warm day you should hear a hissing sound as the cap is removed. This is the pressure being relieved as the cap is removed. If not, then this part of the cap may be bad and is not holding pressure, thus allowing vapor to escape all the time. The other valve may also be bad as it may be stuck open all the time, thus allowing vapor to escape.

There may be other connections in this system that are also leaking vapor.

If you can find a Haynes manual it may contain a sketch and an explanation of this entire system.

 

[bikerfish1100]

21 51251453148 GASKET 0.01 1 $6.93

The old gas caps used to come with a red bottom gasket, which sealed gasoline. It was never available as a replacement part until about 1985 when BMW made a black sealing ring available. This is the gasket for the actual cap. It replaces the old red gasket that breaks down. This ring goes right over the threads to where the red

(above is direct from fiche page; http://www.maxbmwmotorcycles.com/fiche/fiche.aspx )

 

[radan2]

Someone else may have some thoughts on this but this is what I would look for:

Your bike is probably of the vintage that contains the fuel vapor recovery system. This system traps fuel vapors in the tank and also dumps some vapor into the crankcase. I think the gas cap on these contains a pressure relief valve to relieve fuel vapor pressure in the tank if the pressure rises too high. The cap probably also contains a valve to allow air in as fuel is consumed to prevent a vacuum from forming in the tank.

If the pressure relief valve is working then on a warm day you should hear a hissing sound as the cap is removed. This is the pressure being relieved as the cap is removed. If not, then this part of the cap may be bad and is not holding pressure, thus allowing vapor to escape all the time. The other valve may also be bad as it may be stuck open all the time, thus allowing vapor to escape.

There may be other connections in this system that are also leaking vapor.

If you can find a Haynes manual it may contain a sketch and an explanation of this entire system.

Thank you for this information. There is NO hiss as the gas cap is removed. [DUSREGARD: Is it possible to repair the current cap, which also does not lock well, or do I need to replace it? If so, where can replacement caps be found?] UPDATE: I have found the answers to these questions. I thank both fracture and bikerfish1100 for their prompt and helpful replies.

 

[radan2]

21 51251453148 GASKET 0.01 1 $6.93

The old gas caps used to come with a red bottom gasket, which sealed gasoline. It was never available as a replacement part until about 1985 when BMW made a black sealing ring available. This is the gasket for the actual cap. It replaces the old red gasket that breaks down. This ring goes right over the threads to where the red

(above is direct from fiche page; http://www.maxbmwmotorcycles.com/fiche/fiche.aspx )

Thank you for this information. I followed the link and found that my bike has a SHED cap, which is not recommended. I will replace it with the Euro cap they recommend.

 

[fracture]

Did you buy this bike used? If so, a previous owner may have switched gas caps.

With the stock vapor recovery system components, the engine is hard to start on a warm day due to the fuel vapors being dumped into the crankcase. Also, vapors are sucked out of the tank and into the intake system upon starting. This rich mixture will not burn, the starter will crank for quite some time until these vapors are purged.

A different gas cap may not allow vapors to be stored, thus eliminating the hard start on warm days. If you install the proper gas cap you may now experience hard starts at times if all of the other vapor recovery components are still installed and working as designed.

 

[radan2]

Did you buy this bike used? If so, a previous owner may have switched gas caps.

With the stock vapor recovery system components, the engine is hard to start on a warm day due to the fuel vapors being dumped into the crankcase. Also, vapors are sucked out of the tank and into the intake system upon starting. This rich mixture will not burn, the starter will crank for quite some time until these vapors are purged.

A different gas cap may not allow vapors to be stored, thus eliminating the hard start on warm days. If you install the proper gas cap you may now experience hard starts at times if all of the other vapor recovery components are still installed and working as designed.

I did buy the bike used, but it has the original gas cap, that locked (when it worked) with the ignition key.

I had to have the lock disabled because one of the internal springs had broken (according to the locksmith). There was a lot of debris in the lock, which might have happened when the bike went down--the previous owner did reveal that it had been down once.

Max BMW recommends a different gas cap from the original one for the same reasons you mention.

 

[radan2]

Problem now solved

I received the new gas cap today and it seals great. No more gasoline smell! I appreciate all the help and advice I got from this forum!

 

[radan2]

The new gas cap has had a very good effect on the gas mileage of my r100gs. Whereas before I would need to turn on the reserve at about 130-135 miles, I am now able to get into the 140s. I assume that it is the result of better sealing of the tank, so that the vapors cannot escape.

What kind of total mileage do others who own the '89 100gs get from a tankful of gas?

 

[guenner]

200+ miles until reserve with 2-up, above 7,000ft

R100GS 9/88, regular 5.5 gal tank

In the summer with the tank not 100% full (so fumes can build pressure) I get that "pffffff" sound when turning on the ignition. That's when the solenoid opens the vent connection from the tank to the air filter housing.

And there is the little one-way pressure relief valve which sits next to the starter and releases higher pressure into the crank case - no solenoid here.

Guenther