R1100RT Gas Tank pulling vacuum when I open gas cap?

[DIYBeemerMan]

I need help. I have a 96 R1100RT that sucks in a lot of air when I open the gas cap after riding for a while. In addition, the fuel gauge now only operates correctly above 4 bars. The bottom four LCD bars stay constantly on. The bike starts and runs fine. The bike has 70,000 miles on it and the gas tank has been removed numerous times for service. Any suggestions on what causes this vacuum?

[dirtrider]

Morning DIYBeemerMan

 

A somewhat common complaint.

 

Your evap canister (that black thing on the rear of the bike) is plugged allowing engine vacuum to enter the fuel tank.

 

On your gas gauge problem that excess vacuum has probably crushed the fuel gauge tube inside your gas tank (another common problem).

 

You can remove then by-pass the evap can then vent your gas tank directly to behind the R/H foot peg. (just find the hose that runs from the tank to the black can then connect that to the hose running from the black can to behind the R/H foot peg. Then plug off all the others.

 

To repair your fuel gauge you will have to remove the sensor tube from the gas tank then CAREFULLY drill a small hole in it’s side across from the crushed area then insert something with a blunt end in that hole and pound the tube round again. Be very/very careful of the wires and internal float.

 

[DIYBeemerMan]

Such great service!! Only 2 minutes of waiting for a response. Thank You very much! I will check your suggestions out on my bike and see if it's the fix. Thank You.

[dirtrider]

Morning DIYBeemerMan

 

 

If you check around on the internet (maybe even in the archives here) there is a repair procedure including pictures on how to repair that fuel gauge tube.

 

Hopefully someone that reads this thread has a link to the procedure for you.

 

 

[dirtrider]

[Hotdog]

I am perplexed as to how an internal metal float level sensor tube (at atm pressure?) in the fuel tank can be crushed by a vacuum. Seems like the tank must have been pressurized at some point to do this damage.

[dirtrider]

Afternoon Hotdog

 

I haven’t ever stood there and watched one crush but I can assure you that it happens.

 

My guess would be the plastic tank sucks in on the sides and displaces the same real-estate that the sensor tube occupies at that same time.

 

That plastic tank couldn’t take the pressure it would take to crush a metal tube especially when that tube is open to internal tank volume to allow gasoline into the float area. (same pressure inside the tube as outside the tube)

 

If you have ever seen a GS tank suck in you would see the massive tank deformation. (no Tupperware covering the GS tank)

 

[AndyS]

So what you are saying is that it is not vacuum that crushes the metal tube, but the vacuum deforms the plastic tank which in turn bends / crushes the metal tube. When the tank filler cap is opened, the air rushes back in and the tank returns to pretty much its original shape, leaving a 'mysteriously' damaged tube.

 

Andy

 

[dirtrider]

Afternoon Andy

 

That would be my best guess -- engine vacuum evacuates the inside of the tank to negative pressure so external atmospheric pressure then forces the sides of the plastic tank in on the float tube. I just can’t imagine it happening any other way.

 

That gauge sensor tube is open to allow the float to read the gasoline level so whatever negative pressure is on the outside of that tube must also be present on the inside of the tube so neither positive or negative pressure should be able to (directly) deform it.

 

[Mark K]

I will check your suggestions out on my bike and see if it's the fix. Thank You.

 

 

Oh, that is the fix. These folks know their stuff. Do a search for 'cannisterectomy' and you will find plenty of backup.

[Mark K]

I forget. Is the fuel gauge tube accessible without removing the plastic surrounding the filler cap? I believe it is, but my concern for him would be the same -- the o-ring that won't go back in without completely drying out first.

[dirtrider]

I forget. Is the fuel gauge tube accessible without removing the plastic surrounding the filler cap? I believe it is, but my concern for him would be the same -- the o-ring that won't go back in without completely drying out first.

 

Evening Mark

 

No not really as the tank has to come off and get drained as the fuel pump pass through has to come loose to unplug that fuel sensor tube wire (it’s plugged into the back of the fuel pump pass through inside the tank).

 

I always tie a wire or string on that fuel tube sensor wire plug before removing the tube as that makes it’s a bunch easier to pull the connector back through the tank when the tube is reinstalled.