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.

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[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.

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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.

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[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.

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

 

[Sharecropper]

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

The fuel return system starts at each injector in those small flexible hoses that jump from one injector to the next. All these are routed to a hard line on each side of the engine which runs over the water pump and exits the engine just above the fuel pump on the passenger side at the front bottom of the engine. At the end of this hard line there is a 1/4" rubber line which connects the hard line on the engine with another hard line on the frame which runs back to the tank. If it were mine, I would disconnect this rubber line, remove the fuel cap, and direct compressed air into the line going back to the tank. This should clear any blockage in the return line. Hope this helps.

 

[WWRD99]

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

I guess you could do that. I just got new used ones when it did it. They'll build pressure on a low tank on a hot day, too. Could be you're getting pressure and not vacuum in the tank. No way to really know.

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[Sezzo]

Thanks to all of you. Helped a lot. I will start with gentle compressed air to clean out the vent in the filler cap and go from there. Return line is next if that doesn’t help. Buying another cap is the last option because they are crazy expensive in Germany - about $250

 

[Guyfang]

@PHOENIX-GER might be someone for you to talk to. She might be able to tell you where to get a cap cheaper.

 

[WWRD99]

Thanks to all of you. Helped a lot. I will start with gentle compressed air to clean out the vent in the filler cap and go from there. Return line is next if that doesn’t help. Buying another cap is the last option because they are crazy expensive in Germany - about $250

Leave the cap loose or swap them first.

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[PHOENIX-GER]

@PHOENIX-GER might be someone for you to talk to. She might be able to tell you where to get a cap cheaper.

To find one in Germany maybe very difficult. Where did you got one in Germany? We have installed a second original tank (sattle tank configuration). The tank, sending unit, necessary Army fillerneck complete with cap and all other needed parts did i got in the USA all new except the fillerneck, this was only in a very good used condition available.
I would check all other reasons like sharecropper mentioned before drilling a hole in the cap ....

We are looking for a cap which i can lock. Not available anywhere. We check truckcaps which could have a fitting size.

 

[Sezzo]

Leave the cap loose or swap them first.

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Will do so.

 

[Sezzo]

I cleaned the vent on the filler cap with some WD-40 and compressed air. It seemed to work. No more pressure or vacuum when opening the cap after a 20 mile trip. Thanks to all of you.

 

[Tinstar]

Thank you for the update!
Glad it works again