85 K100RT Noise from Gas Tank...with Engine Off

[timbro]

Arrived home from work tonight after 28 mile commute. Bike sat outside for a few minutes while I opened the garage and let the dogs out. When I went to push the bike into the garage I could hear a noise coming from the gas tank area. The noise sounded like a small motor running in the tank.

Popped the seat, removed the "brain" and disconnected the battery. The noise continued. I lightly touched the outside of the gas tank and I could feel a very slight vibration. Pushed the bike into the garage and the noise lasted for about 5 more minutes and went away.

Bike has 160K on the clock and the only thing I have been doing differently is adding Techron at each gas fill to clean injectors and hopefully cure some hesitation the bike experiences when accelerating through each gear.

Only guess I have is fuel flowing back into the tank to equalize some pressure difference.

This is probably unrelated, but on the last 5 miles of my commute home on Friday, the bike began to surge and lurch violently. I needed to maintain 7000 RPM in 2nd gear to maintain 30 MPH. Engine never quit, but extreme surging and lurching continued until I was about a block from home. Suddenly the surging and lurching stopped and the bike ran normally.

Only thing I can figure is that some foreign substance/object lodged in one of the injectors and it took about 5 miles of extreme surging and lurching to pass the obstruction through the injector.

Any help is appreciated as always.

Tim

'85K100RT; '90K75RT; '91K75RT; '05R1200GS

[roncooper]

I think the tank is not venting. Have you had it off lately? If not, then maybe a critter got into the vent hose.

[98lee]

Also check your fuel filter.

[Pauls1150]

The combination of noise and vibration maybe points to the fuel pump not shutting off (but disconnecting the battery should kill that -?). Might be stuck or internally shorted; maybe a rubber gasket has eroded and is putting garbage into the pump, this would explain the jerkiness.

The pump pressure is higher than what the injectors actually need, so excess pressure is bled back to the tank via a return line. While it's apart, verify that line is clear.

[Bobmws]

When you have the bike running open the cap and listen closely to the tank. You may hear the same noise. The fuel pump has a pop off valve to relieve high pressure, usually caused by a clogged filter. That valve can make a sound anywhere from a mild vibration to a high pitched squeal.
The fuel pressure regulator would keep pressure in the line and cause that problem. A failed or failing regulator usually causes less than the normal 35-38 psi in the system.

[98lee]

A failed or failing regulator usually causes less than the normal 35-38 psi in the system.

Usually, yes, but I have run across one that failed in the other direction. Bike had 88psi fuel pressure and ran like crap!

I would first check fuel filter for being partially plugged and while I'm there closely inspect the two hoses and their clamps in the tank.

[98lee]

I think the tank is not venting.

Can be checked by riding for awhile (20 miles should do it, or until the bike develops poor running symptoms), then pull over and immediately open gas cap. If you hear air whoshing into the tank, as you open the cap, then there is a venting issue.



Did I mention; Check your fuel filter?

Checking fuel filter:

Remove filter
pour out ALL gas from filter
let filter dry out for about a half hour (preferably standing in a vertical position to let any gas run out)
LIGHTLY blow ( yes, you have to put your lips on it) through the filter in the direction of the arrow.
If you feel ANY resistance, you should replace the filter. If you feel mild to moderate resistance, it's JUNK!

[TinyTrains]

If you have found a way for the fuel pump to run with battery disconnected, our energy problems are over! Patent it!

If the sound continued with the battery disconnected, it sounds like pressure in the tank leaking out. Next time, pop the gas cap and see if it goes swish an stops.

[timbro]

Thanks for the great information. I will check both venting and the gas filter and will let you know what I find.

Tim