Fuel Tank Vacuum

[bruceatlam]

Whenever I loosen my fuel tank cap, the tank always seems to be under vacuum ---- is that normal?? I don't have any fuel supply issues --- it runs fine. With the tank under vacuum all the time, the plastic tank contracts/caves in a little. I already had one tank replaced because it developed a leak; I would rather not do that again.

 

[Creamer]

For emissions purposes fuel tanks should run a slight vacuum - this keeps the hydrocarbon vapors in the tank. It should not be enough that you would notice though and certainly should not be sucking the sides of the tank in - that is abnormal and the air introduction valve is not working properly.

I am not sure what this is on as you did not mention that car, truck, tractor, gas, diesel???

 

[DT86]

While its not a LS, my RTV does the same thing when I run it from a full tank of fuel until the gauge is in the red. I had never given it much thought, if any.

For emissions purposes fuel tanks should run a slight vacuum - this keeps the hydrocarbon vapors in the tank.

Won't the vapor just escape when you open the cap??

 

[Jhiphop72]

While its not a LS, my RTV does the same thing when I run it from a full tank of fuel until the gauge is in the red. I had never given it much thought, if any.

Won't the vapor just escape when you open the cap??

Gov't regulations and common sense don't belong in the same sentence, or in this case, fuel tank.

 

[bruceatlam]

I guess it probably is a slight vacuum. When I loosen the cap I can hear the hiss of the air entering the tank. Obviously the pressure equalizes quickly.

To answer the "what is this question" --- it's my 2012 diesel LS R3039 tractor.

 

[Creamer]

Diesels do not have the hydrocarbon vapor issues that gas tractors do but they still pull a slight vacuum to keep the vapors in. If the tank is a closed volume and you are running the engine there is going to be fuel used and a vacuum created.

In the old days the fuel caps had a hole in them with a baffle to keep it from splashing and dirt out. This kept the pressure virtually equalized. The issue is that as the fuel heated, either by temperature changes or engine heat, it expanded and created a pressure in the tank - again with a vented gas cap no issue. Modern fuel systems attempt to capture those vapors and run them through the engines, i.e. the valve is more of a check valve with a pressure relief for safety.

Yes - when you open the cap the hydrocarbon vapors can come out but most do not for a lot of reasons, the biggest one being that as the pressure increases due to the seal of the cap being broken air goes in not the other direction - which is why they like to keep a slight vacuum.

Tractor fuel systems are not nearly as sophisticated as an automobile's, but they are doing some things to keep the hydrocarbon vapors out of the environment.

 

[Short Game]

Sometimes my 4010's cap is stuck so hard that I can barely get it loose with my hands. Once I even had to go get a strap wrench to get it open.

 

[Creamer]

Short Game - You need to check to make sure the vent is not plugged. That could be a very dangerous situation.

 

[bruceatlam]

Short Game - You need to check to make sure the vent is not plugged. That could be a very dangerous situation.

I'll speak for him, if I may: Where's this vent???

 

[Short Game]

There is no sign of extreme vacuum in the tank when the cap does open. It just sticks shut. The vent on my Cub got plugged up once and that tank caved in. Compressed air brought it back though.

Well, I'm headed out to test it with a full day of mowing the disc golf course.

 

[ozley]

I'll speak for him, if I may: Where's this vent???

My vent is in the tank's cap.

 

[Johnp33]

I do believe you will find that there is preasure in the tank not a vacuum. The carbon canister on gas units vents the tank into the air inlet. Part of the emission tests for a gas unit is the checking of the cap to be sure it holds the proper preasure.
Just a thought!

 

[bruceatlam]

My vent is in the tank's cap.

Thanks --- I see it. I'll check my cap to see if it's the same ---- on my cap, that disk that has the vent hole in it will freely fall out of the cap. Is yours the same?

 

[ozley]

Thanks --- I see it. I'll check my cap to see if it's the same ---- on my cap, that disk that has the vent hole in it will freely fall out of the cap. Is yours the same?

It's in there tight. I don't see coming out unless the cap was broken in 1/2

 

[bruceatlam]

OK --- I took that disk with the vent out of my cap. It looks like yours definitely has a pin hole in the center of that soft, flexible membrane that's dead center of the disk. Mine has no such pin hole --- it's a solid, soft flexible membrane. Maybe that's my problem ---- picture below without the cap.

 

[bruceatlam]

Lousy pic --- hopefully, this one is better.

 

[ozley]

Lousy pic --- hopefully, this one is better.

<img src="http://www.tractorbynet.com/forums/attachment.php?attachmentid=317206"/>

When you look into the center of the disk on mine, that light colored gray is a spring.

 

[bruceatlam]

So, there's no pin hole in that membrane?? I could swear that I see a pin hole in that first pic that you posted --- And, I agree --- the light colored gray around the outside edge looks like a spring to me too.

 

[Creamer]

Typically the vent is in the cap. If not there is another opening somewhere on the tank but since the cap is on top and there is a already a hole in the tank there, the cap is the easiest place to put it.

 

[Dan Hunter]

Running a vacuum reduces foaming for diesels with return lines from the injector pump to the tank.