Air in fuel line, causing problems

We took a 50 mile (1 way) trip this weekend and I ran the freshly cleaned 'Aux' tank with no problems.  Then on the way home I pulled out and pushed the gas and the coach died.  I restarted and happened again.  I switched to the 'Main' tank and the problem went away and we came home.  So I switched to aux and looked under where I installed an inline fuel filter and there is a ton of air bubble in the filter . . . what is causing this and how do I fix it?

Tank is empty, or empty enough that the sloshing of fuel has let air into the fuel intake. Or an actual leak in a line.

Is there a pump to move fuel from Aux to the main fuel line or is it gravity flow? A pump will suck air if there is a leak on the intake side.

RV Roamer said:

Tank is empty, or empty enough that the sloshing of fuel has let air into the fuel intake. Or an actual leak in a line.

Is there a pump to move fuel from Aux to the main fuel line or is it gravity flow? A pump will suck air if there is a leak on the intake side.

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Not empty or close, I filled it before leaving ~40 gallons.  Could it be a leak in any of the lines?  I replaced all except the vent line going to the filler neck.  Only pump is the engine mounted mechanical fuel pump so I think it is gravity flow???  I don't understand why it worked and now it doesn't, could it be the gas cap too?

The tanks are not gravity flow.  Fuel is sucked out the top by the vacuum created by the engine's fuel pump.  This is so you have a reasonable chance of the fuel flow stopping if you get a break in the line.  If it was gravity flow a fuel line leak would empty the tank.

When you changed the lines the new lines were empty ... or full of air.   This created the air bubble you're seeing in the fuel filter.

If the engine fuel-starved after sitting for a while, you might have a slight leak that's letting air into the line.  Over-tightening a hose clamp can wrinkle the hose under it, creating a channel for a slight air leak.

Or it might be that you just had a bubble in the empty line that let gas dribble through without completely filling the line, and depending on the routing it became dislodged as you pulled out.  If that's the case it's gone now so I wouldn't worry about it unless it happens again.

Lou Schneider said:

The tanks are not gravity flow.  Fuel is sucked out the top by the vacuum created by the engine's fuel pump.  This is so you have a reasonable chance of the fuel flow stopping if you get a break in the line.  If it was gravity flow a fuel line leak would empty the tank.

When you changed the lines the new lines were empty ... or full of air.   This created the air bubble you're seeing in the fuel filter.

Depending on the routing of the lines, you may have had another air bubble elsewhere in the run to the engine.   In any case, it's gone now.   I wouldn't worry about it unless it happens again.

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After this analysis I'm convinced the aux tank is gravity flow because there are 2 tubes entering the tank at the bottom, not the top.  After I got everything put back together I wasn't getting any fuel so I took off the main feed line and no gas came out.  Then I pushed a coat hanger through the opening in the tank and fuel started running out (Must be a gravity flow?)

I am not talking about a single (or even a few) air bubbles, it is hundreds, rushing into the fuel filter every time I switch tanks.  ???

I didn't see your reply until after I finished adding to my post above.   So ignore most of what I just said. 

If large amounts of gas didn't run out of the tank as soon as you removed the hose, there's either a check valve or debris that prevents fuel flow until the pump develops a vacuum.  Or the vacuum breaker loop is inside the tank and it has a  leak.   In either case, there's a leak between the fuel filter and tank that's sucking in air when the pump pulls a vacuum.

Short but true story:

My one trip to Fl. I came across a van load of kids with fuel problems.. The fuel pump was not delivering fuel to the engine.  Turns out the owner of the borrowed van had installed a second tank with a home made crossover valve, The cross over consisted of 2 valves and a 'T" fitting

Well the main tank was slung between the rails (normal tank mount) the aux was in the fender. Above the main. if you opened both valves the fuel flowed from the aux to the main and when the aux ran dry the fuel pump sucked air (Engine mounted pump)

Closed the valve to the "AUX" tank and viola. Varroom!!!

Short story 2: Had a car acting strange, pulled the fuel line and cranked (into a cup) got just a small (way too little) fuel, but.. It looked like I was pumping a lot of air.

Fuel line has more pin holes in it than a collander, none were leaking fuel, but they sure sucked air

Thanks guys, I really appreciate your help.  I'm not sure what I'm going to do next, I guess just start trying to find an air leak, haha.

There are two ways to mount an aux tank. In one method, the aux feeds the main tank via gravity (or a separate pump) and the engine fuel pump always pulls fuel from the main tank. This method is often used when the primary fuel pump is mounted in the main tank itself, or if the fuel system uses two pumps (an injector pump at the engine and a lift pump in the tank).  If you have a separate engine mounted fuel pump, you can have a Y valve in the main fuel line and pull fuel from either of the main or aux tanks.  You may still need a tank mounted pump, though. Some systems may rely on the tank pump to move fuel to the engine, where a separate injector pump handles the rest of the job.  You didn't mention the type of vehicle or the brand of aux tank, so we can't guess which system you have.

You appear to have used the Y method and rely on the engine pump to do all the work. It really sucks hard, so you have to have a steady fuel supply to the valve. If you do not, the pump will suck air through even the tiniest pin hole or "wrinkle" under a clamp. You mentioned seeing bubbles when you turn the Y valve, which suggests the valve itself may have an air leak.  Can you switch to Aux and then purge the air (disconnect at engine end, perhaps?) and then see if it then continues to run ok?

It appears you ran ok once for 50 miles but the engine pump lost prime after you stopped and was unable to get fuel moving steadily again. Hard to say whether there was no leak initially or you were just lucky enough to have a strong fuel flow and the leak did not matter. After using a few gallons of fuel, there is  more air in the tank and the fuel sloshes around more, introducing bubbles.

I'm also a bit concerned about poking the coat hanger up into the aux tank. You probably penetrated a sediment screen in the tank outlet or maybe even a diaphragm if the tank has a pump inside.

You mentioned cleaning the tank.

What kind of stuff did you get out of it?
Is it a steel tank?

Also you may want to remove the filter you installed and look to see if you have a bunch of sediment or rust. If during the cleaning you loosened a lot of it, it may have settled when you stopped and plugged the outlet. Sticking the coat hanger opened it back up.  Also use the small screw clamps on the lines and don't over tighten them, just snugged up with lines that are not too big, they will wrinkle like mentioned above. Steel tanks that are starting to rust inside are pretty hard to do anything with that wiil last. You can clean the rust with a bunch of small pebbles and rinse them. They make a liquid rubber coating to seal the inside, but it usually doesn't last very long. I've tried it several times on ATV & snowmobile tanks and it doesn't stay very long.

I should have stated this is a 1986 Chevy 350 motor, 26TB Mallard mini motorhome.  Here is a link to all the pictures I took after removing the aux tank (so I could get all the hoses hooked up correct when I got it back) to get it cleaned.

Pics of aux tank and tank selector

Today I replaced the 'T' in the main fuel line closest to the tank and found that it had a blockage in it.  I also replaced the fuel filter that I had added.  I took it out and drove about 20 miles with no problems.  It still has the air bubbles in the filter but isn't effecting the way it runs now.  I think the blockage was the biggest problem.

The filter I put on a filter that looks like
This One
Mine will accept 2 sizes of hose (3/8 and 1/4) . . . should I have cut off the smaller end before putting on the hose?

sugarshane said:

The filter I put on a filter that looks like
This One

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Sugarshane,

Be sure that you put that filter where there is no possibility of excess heat. Those plastic-bodied filters have been know to melt in high heat areas. It is better to use a metal-bodied filter.

When those filters are mounted horizontally it's common to have air in the upper part.

getting this tank clean is going to be really hard!  I just went out and took a short video and there is already tons of rust pieces in the filter. . . I'll post the video here shortly.

Nevermind, the video was virtually impossible to see.

What do you guys think about adding something inline (before the filter) that I could drain to get the particles out and not have to change the filter everyday?

It's not going to be long before it starts leaking. A Plastic Tank is a better solution. The rust is the inside of the tank deterioting. Watch for damp looking areas.

There is a solution to keeping a metal tank from leaking (or stopping it)  Some radiator shops offer this service.. It is called a tank liner.

Basically it's a plastic/synthetic rubber type chemical they pour it into the tank, tape off all the openings, then "roll" the tank (If I were doing it I'd put the tank in a roller cage and rotate the cage but I don't think they do this, I think they just turn the tank every your) to distribute the chemical, as it dries it forms a tough rubber like coat on the inside of the tank.  Resistant to fuel, oil, alcohol and the like.

When done they remove tape, cut holes where the holes belong, replace the outlet/sender and other lines and.. Give you back a leakproof tank

POR-15 can be used to seal the inside of a rusting tank. I have used it many times and have never had it fail. You get what you pay for so be careful with regular auto house sealers.

Wayne