M1009 fuel tank builds vacuum when driving

[Sezzo]

Good morning everyone.

I used the search function very well but could not find what I am looking for.

My M1009 builds a vacuum in the fuel tank when driving. It does not vent properly obviously. When I open the fuel filler cap after driving I can clearly hear a „pfffff“.

What is wrong here?

Thank you very much in advance.

 

[WWRD99]

Fuel cap is bad.

Sent from my SM-S906U using Tapatalk

 

[Sezzo]

Is there a vent in the cap? Where and can I fix it?

 

[Sharecropper]

Your fuel return line from the engine is either stopped up or kinked, thereby not allowing the excess fuel to be returned to the tank. Your fuel pump is sucking fuel from the tank which is causing the suction. Clear the obstruction in the return line and the problem should be solved. Just my 2 cents.......

 

[Sezzo]

Fuel cap is bad.

Sent from my SM-S906U using Tapatalk

Fuel cap looks the same as on my other CUCVs. I can see there is a small white-ish vent in the center but there is no difference to the other fuel caps and vents.

 

[WWRD99]

Fuel cap looks the same as on my other CUCVs. I can see there is a small white-ish vent in the center but there is no difference to the other fuel caps and vents.

Swap caps from another.

Sent from my SM-S906U using Tapatalk

 

[Sezzo]

Your fuel return line from the engine is either stopped up or kinked, thereby not allowing the excess fuel to be returned to the tank. Your fuel pump is sucking fuel from the tank which is causing the suction. Clear the obstruction in the return line and the problem should be solved. Just my 2 cents.......

Not kinked anywhere. Maybe some gentle compressed air will help to blow it out? And where should I loosen it to blow it out?

 

[Sezzo]

Swap caps from another.

Sent from my SM-S906U using Tapatalk

I read to drill a small hole in the fuel cap fixes it? What do you think?