Fuel tank pressure or vacuum?

pathfinder7377

Joined
Jun 7, 2020
Messages
47
Age
67
Location
Ohio
Bike
1999 ST1100
I am the new / second owner of a 99 ST1100 with <9K miles on it. The bike is in excellent condition and has been well cared for and stored properly. The bike runs and rides flawlessly and is living up to the reputation for these models.

I only have ~ 200 miles on it an recently fueled it for the first time. When I removed the gas cap there was a vacuum in the fuel tank. Nothing major, but definitely a slight vacuum.

This seems unusual but I also own a PC800 and know that these fuel systems are somewhat unique. Should I sort out the gas cap vent or is this normal?

Thanks in advance for any input or advice.
 
DeanR, yes, a hiss is a good way to describe it. The cap is a tight seal, but the same is true on my PC800.

New to the ST after years of BMW ownership and cannot say enough good things about this bike.
 
The ST1100 fuel cap vents both ways, pressure and vacuum. I can dig up a schematic of it if asked (probably already posted it here over the years).

John
 
The pressure equalization is normal, there shouldn't be a vacuum however (unless the filler cap vent is clogged), but a small pressure build up is normal. If there were a vacuum the bike wouldn't be running so well.
 
As the fuel is consumed and the level of gas in the tank drops, a vacuum is created that the cap relieves by letting air be drawn in. Pressure is created by ambient and engine heat causing the fuel and air to expand.

Also, as pressure is equalized the cap is known to make an audible ‘moaning’ noise, which has caused some new STriders some concern. Normal.

John
 
Slydynbye said:
Do yourself a favor, remove the rubber gasket under the fuel cap, clean under it and grease between the cap and the gasket when you put it back on. Thank me later
Click to expand...

I came to say the same. Makes a world of difference.

If you think the venting is weird wait till it gets really hot and starts moaning.
 
When I got home from a ride today, I parked my ST1100 in the garage and lifted it onto the center stand. As soon as I did that, it vented the built up pressure (i.e. the "moan"). I then unscrewed the cap and there was a good amount of pressure left. The pressure has to be quite high to overcome the spring on the cap.
 
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