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KCrisp 10-23-2010 11:40 PM

15 Gallon Tank Conversion -- Safety Issue
This was my project for the day, and I am very grateful to all who made suggestions and also for the links to web pages where people write up how to do this.

All that being said.....

I took the old fuel pump out and looked at the two hoses everybody suggests should be cut down to one inch to allow the tank to fill to its full capacity. The smaller hose is just a smooth hose. The larger one has a white plastic ball in it, inside a grey plastic cage, held up against the larger filler hose by a rather weak spring with a fair amount of travel to it. It is, in other words, a check valve -- to stop tank contents going back out the filler hose.

I don't think the check valve is there for no purpose at all. It is certainly not, as one set of web instructions suggests, a float valve that rises with the level of the liquid to block something and trigger the gas pump to stop pumping. Why not? Because it is held [I]up[I] by the spring so it is always up anyway. It is tempting to think of it in flush toilet terms but that it not what it is doing. I know the author of one set of web instructions has a photo of his jeep rolled on the driver's side door, his point being that even with the valve cut out of the tank, he still didn't leak any gasoline. I don't doubt that at all, but I think this reflects a misunderstanding of what the check valve is for. In his real life event, the intact gas cap did keep the fuel from leaking out.

I think the check valve is there so that in the event of a huge rear end collision -- big enough to really compress the fuel tank -- it will be spring loaded up against the larger filler tube and in position to block the tank contents (fuel and vapor) from popping the gas cap off and spewing atomized gasoline out the side of the jeep.

You're skeptical, perhaps, and wondering about the second much smaller hose next to it. That one doesn't have a check valve. True enough, but it is much smaller in diameter. Blocking the larger one with the check valve greatly decreases the pressure on the gas cap and makes it more likely to stay on.

Having concluded that Chrysler had a safety reason for putting the cage/float/spring assembly in the gas tank, I decided that (much as I wanted more capacity in the tank) I wasn't going to cut it off and eliminate that feature. It also occurred to me that the plastic extender dipping into the tank on the small hose is much more likely to have some relationship with filling the tank to its actual capacity than the small one. I may not fully understand how a gas pump works, but I thought that when that smaller tube dips into the fuel it triggers the pump to shut off. So I cut off the smaller plastic tube only, about an inch from the top of the tank, installed the 19 gallon fuel pump/sender and reinstalled the tank. At this point I had somewhere between 2 and 3 gallons in the tank.

I started it up and the gas warning light was on, which it had not been when the 15 gallon sender was in there. Uh oh, I thought. I drove it to the gas station and put in 15.9 gallons. If I am not at a 19 gallon tank I am a few quarts away from being there. The fuel gauge (driven by the 19 gallon sender) is pegged just past the "F" mark.

So whether you agree with me or not on the safety purpose of the check valve, you can do the conversion just fine by leaving it in and trimming just the smaller hose. Anyway, thought I'd pass along these observations.

burton160w 10-24-2010 12:00 AM

That plastic ball is there for two primary reasons

1) Siphon prevention. It's designed to make it difficult to put a tube down into the tank.

2) It does prevent general sloshing around of the gas so it doesn't travel back up the fuel filler.

The second tube is vacuum release. As that part becomes blocked, air cannot travel back upwards to compensate for the gasoline filling and the gas pump shuts off. Read the HowStuffWorks article on gas pumps, they work via vacuum.

You can cut those tubes off you found as much as you want. In fact, the 19-gallon tank doesn't have any tube like that. It's simply a small nub that sticks out and that ball-assembly.

KCrisp 10-24-2010 10:22 AM

Whatever it is for, there is no reason to cut off the cage and the spring loaded check valve. The tank capacity will increase with just the plain tube being cut. It is great to have a working gas gauge again.

On siphoning, I tried to siphon gas out before I dropped the tank and couldn't get the hose in even as far as the valve. With the cage there to prevent the siphon hose from dipping much if any into the tank, it would prevent siphoning anything more than a couple quarts out of a very fuel tank.

I did put in an Airtex fuel pump since I'd already ordered it and it was non-refundable by the time I read the numerous negative posts about them. Hopefully I will have better luck with it.

Jerry Bransford 10-24-2010 10:33 AM

That info on the Airtex is good to know since I had not seen any of those negative reports and I need a fuel pump. What did the reports you found say about the Airtex? Do you have any links to them? I need to stay cheap at this point so the Airtex price looked good to me but I'll just hold out longer if it's truly junk.

jgorm 10-24-2010 10:38 AM

CAn you relocate the check valve up higher?

KCrisp 10-24-2010 10:49 AM

Why would you want to relocate the check valve higher when it does not prevent you from filling the tank to 19 gallons so long as you cut the smaller tube next to it? To answer the question, I don't think you could relocate it higher; once you cut it, it's off. With one hand reaching a long way to get to it you are never going to be able to glue it back on after shortening the tube. Again, why would you want to?

On the Airtex pump, if you use the search function on this forum you will find numerous criticisms of them. Most of those are from one person, repeated several times, but some others who worked at auto retailers confirm many returns. You can find some satisfied customers if you search the web, but also people reporting 30,000 mile and much earlier failures on something that ought to be good for 100,000. As an ISO certified company, Airtex should be looking at failed returns and fixing the problem, so maybe (boy do I hope so) the current production ones are good.

The unit looked every bit as well made as the Mopar one, it is nice and quiet in operation. The jeep starts and runs normally. (The gas gauge actually seems to work for the first time in my ownership of the jeep.)

On the other hand, I've driven it only 19 miles since I put it in. If it dies on me, I will absolutely report that.

Jerry Bransford 10-24-2010 10:52 AM

Great, thanks for the feedback. I may take a chance on it as my Jeep funds are, for now, very limited.

KCrisp 10-24-2010 10:59 AM

I paid $222 at Autozone. I think you can cut $60+ off that if you order it from Amazon or wherever. Make sure you get one for the size tank you have, since the sender is calibrated for one or the other.

burton160w 10-24-2010 11:51 AM


Whatever it is for, there is no reason to cut off the cage and the spring loaded check valve.
Oh no, I hope no one suggested to do this. Definitely you don't want to cut off that blocker! I didn't realize that we weren't on that same page. Yea, that prevents fuel from coming back up and from your tank being siphoned. It also gets to a pain in the ass when it jams and you can't fill your tank up. Sorry about that, you're correct you don't want to touch that pong ball assembly. The other tube for the vacuum line is okay to cut.

You're correct, there really no way to locate that assembly up higher, and there's no point to. That's not the part that's blocking 4 gallons from filling up your tank. It's that tubing the put on the air-vacuum line that becomes blocked up and shuts the gas pump off.

KCrisp 10-24-2010 12:03 PM

The reason I posted this was that web sites do suggest cutting off the check valve in the main fill line. From the "Got Trail" website, for example:

"Now for the fun part. If you reach inside the tank you will feel two filler tubes, one will be larger than the other. Take a razor blade on a scraper handle (pictured right), and rock it back and forth with firm pressure against each tube about one inch from the top of the tank. This will allow you to cut the filler tubes without having to worry about plastic pieces in the bottom of the tank. Take your time, as reaching into the tank may be awkward. "

burton160w 10-24-2010 12:08 PM

Woah, I bet you I posted that link to, sorry about that. So for the 15 gallon, there is just one rubber tube that connects to the smaller line (vac line)? Can you post of a pic of what piece you cut out? He is incorrect, you DEFINITELY don't need to cut that pong ball assembly. The only reason he had no spills is that it just went up to the cap.

KCrisp 10-24-2010 12:31 PM

On the left upper side of the tank, there is an oval ABS looking plate glued on. A big hose and a smaller hose from the filler neck connect to it on the outside with hose clamps. The big hose is where the gas goes into the tank when you fill it up. Looking from inside the tank, the big plastic fitting lining up with the large fill hose has a grey plastic cage on the end. Inside the cage is a white plastic ball held in the up position by a rather weak spring.

A smaller hose off the filler neck also connects to the same oval plastic plate glued on the tank. On the inside it has a black extension that dips down into the tank. It looks exactly like the end of a tapered funnel. That is what I cut off. Cut off the end of a black tapered funnel and you know exactly what it looks like. Leave the cage check valve alone.

On the top of the tank there are two elbows that connect to check valves on either side of the top of the tank. These are vent lines, you can easily recognize them by the 1/4" fuel line going to each one. They don't dip down into the tank, the check valves are more or less flush with the top of the tank if you feel them from the inside so there is nothing you could cut off the tank if you wanted to. The two 1/4" fuel lines on those elbows run to a fragile plastic Y connector (well I broke it....) and off the Y is a nipple that connects to the vacuum line that is a short distance of 3/16th rubber fuel line then pushes onto a hard metal line right next to the fuel line. The fuel line has disconnects on it, and is labeled "fuel," and runs to the regulator on the top of the fuel pump.

You don't do anything with the 1/4" vent lines or their check valves except maybe replace lines if they are looking old.

KCrisp 10-24-2010 12:33 PM

I tried emailing him through the Got Trail website to tell him this but the email bounced back.

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