In response to my last post about why you should’t Top-Off your gas tank, and the damage to your car that can result from it, I found out that apparenty not everyone knew what that term means. For those that don’t know, it means that you’re literally overflowing your fuel tank on purpose. The reason people do it usually has to do with the strange, sinister obsesion with getting even-numbers on the sale. Maybe it’s OCD? Maybe you suck at balancing a checkbook? I dunno. And then Trunk Impaired 318 also brought up a great comment about how his MR2 always stops before he gets that last 1½ gallons in.

So let’s talk about just what in the hell is going on with the supply side of your gas tank, what to check if you’re having an issue, and why you’re probably doing it all wrong. Admittedly, it’s probably not your fault though. We never really get taught how to properly fill a gas tank anyways.

For this example, I will be using the fuel tank of the Jeep TJ (aka Wrangler) as an example of what the internals look like of a tank looks like. And you’ll just love why at the end.


Now in this picture, this is the backside of what your typical fuel inlet looks like. The narrower hose that sits on top is the vent hose, and the larger one that sits below it is the filler hose. Doesn’t matter if it’s a Jeep, a Civic, an F150, or what. This is what lurking behind all of that sheet metal. Or fiberglass. I have a Flareside F150 with fiberglass fenders, so there’s that. Anyway, all vehicles have this setup.

Whenever you fill your fuel tank, there is vapor inside, and the liquid you pump in will displace that vapor. It’s just the nature of physics. Now normally as the pressure builds up from the large volume of liquid fuel entering the tank. the vapor will escape by being sucked back up through the vapor recovery system on the fuel pump nozzle. The problem though is that it can’t go up the same hose as the where the fuel is pouring in since the gasoline is already taking up that space. Enter the Vent Hose on your fuel tank.

The idea here is that the displaced vapors are instead pushed up through the vent hose where they are then dumped into the Fuel Inlet Restrictor area. There is a separate hole on the nozzle for the vapor to removed by.


Now the idea of how a fuel pump automatically shuts off is this: As long as vapor is freely flowing through the inlet hole on the pump nozzle, the valve on the nozzle continues to remain open. However, there is a butterfly valve connected to a venturi inside of the fuel pump. Once there is a significant change in pressure, it cuts off by snapping the main valve shut and ceasing fuel flow. How? Good question.


What you’re looking at here is the inside of a Jeep fuel tank, though again, it’s pretty much going to be any gas tank on a vehicle. The large pipe to the right is the inlet where the gasoline flows in, and the shorter pipe to the left is the vent pipe where the displaced vapors flow out. Which, BTW, in case you’re wondering, the ball and spring in the inlet pipe is an anti-siphon restrictor to keep nefarious mofos from stealing your gasoline. The TJ had this integrated internally with the tank, but other vehicles may instead have this device further up the inlet hose, as well as no protrusion into the fuel tank at all.

Anyhow, both of these pipes connect to the corresponding pipes on the filler neck that we previously looked at via hoses. As gas gasoline pours down through the pipe on the right, the vapors that it displaces get forced up through the pipe on the left to the inlet restrictor on the filler neck, and into the vapor recovery system on the pump handle that we talked about. So what happens when the fuel level reaches the vent hose and submerges it? Easy: The remaining vapor cannot be displaced from within the tank via that vent tube, so it then has no choice but to go up the inlet hose where the fuel in pouring down. As a result we start to get bubbles that force their way up the inlet hose. As the bubbles rise up, they will also begin to reduce the flow of liquid. What happens is that this causes a temporary flooding inside of the lower section of the filler neck/inlet restrictor as the pump is dispensing more fuel than the tank can accept. Gasoline then covers the vapor recovery hole on the nozzle, blocking the pathway of returning vapors, and trips the venturi to snap the main valve shut. Viola! Your tank is full! You’re done! What you normally do at this point is just wait about 5 seconds or so. This lets the filler neck burp, and the rest of the fuel to drain back down into the gas tank. Why is that important? Because the nozzle is still submerged in liquid when it clicks off. If you pull it out right away, you’re gonna drip fuel everywhere. So just wait 5 seconds to save your paint before you run inside for that beef jerky and stale coffee.


Now as you notice in the last cut away picture, the vapor hose is of course higher than the inlet. That is as I said to stop filling the tank at a pre determined point. The Vapor Recover Inlet that goes to the Carbon Canister and EVAP system, that’s even higher up. In some cars it even runs another line up to the inlet restrictor on the filler neck itself. All to keep liquid fuel out of the EVAP system.

But what about that vent tube, and why did I insist upon the Jeep gas tank to reference it? Easy. As usual, I have a story for you. Chrysler had this awesome money making idea on how to screw over Jeep Wrangler buyers. The Jeep came standard with a 15 gallon fuel tank, but you had the option of getting the bigger 19 gallon tank instead for only $65 more. But if you looked in the accessory catalog, skid plates never cared about size, and the dimensions were the exact same, unlike the options on most full sized trucks. So what gives? Well, remember that vent tube in the picture above? Yeah, the truth is that there is NO difference between the 15 & 19 gallon tanks. All Chrysler did was insert a longer vent tube to purposefully reduce the amount of gasoline the tank could hold by triggering the gasoline pump shut-off earlier, and inserted a recalibrate float to compensate the difference on the gas gauge. They apparently did this up until about 2000. They literally made you pay additional money to use an existing feature you already had on your Jeep. That’s like putting a lock on the cupholders, and making you pay to use them.


When you top off, what you do is flood the entire inlet hose with fuel. There is no more vapor inside of the tank to displace, and you’re trying to take up what’s left inside of both the Filler hose, as well as the Vapor hose on the inlet. And thanks again to physics, just like how a water tower builds hydraulic pressure in municipal water mains by keeping it’s supply source higher than it’s destinations, the same thing happens as fuel gets forced through the vapor recovery lines. Vapor is one thing, but raw liquid fuel can, and WILL destroy the carbon canister. Loose charcoal, mushed membranes, melted rubber lines (applied to older cars never designed for ethanol blends), it all destroys the vapor system. Aside from pouring out wasted fuel all over the ground, you also end up with vapors literally pouring out of the fuel tank whenever the fuel gets hot. And don’t think that just because you stopped the engine that the leakage has stopped. Oh no, that fuel is still hot and won’t stop venting out of your gas cap until it either cools or the pressure equalizes. And it doesn’t just come out of the gas cap either. Nope. I’ve seen and smelled it leaking all around the fuel pump and fuel gauge seals as well. Not to mention there is a good chance of rupturing your gas tank since it was never meant for any of this (see your appropriate TSB for Mazda 3 & Ford Focus spider problems).


Now yes, these are things that I’ve covered before. So what does it have to do with filling up? Plenty. Aside from damaging your EVAP system, you also run the risk of damaging your vapor hoses, including the vent hose that goes back to the filler neck. These hoses can close off, and you can still get the vapor issues. This leads to the secondary symptoms of Topping-Off: Problematic Gas Station Pumps & Burned-Out Fuel Pumps.

The first, again, is that damaged vent hose. Now, sure, you can have hoses just get clogged with dirt and debris over time. And you should check for that. But like the EVAP system, remember, that hose wasn’t meant for liquid; only vapor. So if you’re constantly having to fight a fuel pump nozzle that is permuting clicking off before you get done filling, check that vent hose to make sure it’s not damaged internally. Probably even blow out the pipes and even replace the hose. Again, if it’s not letting vapors vent, it’s forcing bubbles up through the inlet, and everything is going to flood and click off constantly.


The secondary problem to Topping-Off is all that vapor sits in your tank, and really has no where to go until you open remove the gas cap and try to fill up. In the mean time, depending upon your in-tank pump setup, you may very well have rubber pick-up hoses inside. When you do, excessive vapor pressure in the tank can cause a severe pressure differential on inlet hoses, and can actually squeeze them resulting in a restricted flow. This is a common problem with many DeLoreans, and I’ve seen the exact same issue on a 1990 Jaguar XJ40 we used to own. Loud, buzzing pump and crappy engine performance. But if you pop the gas cap and open up the restrictor to let all the vapor blow out, the fuel pump almost magically goes silent again and the engine gets peppy. The reason it’s buzzing of course is that fuel pumps are meant to be fully submerged in liquid fuel for both cooling properties as well as lubrication. The buzzing you hear are the brushes inside grinding as they heat up and expand since they’re being as starved as your engine now. And who doesn’t love a seized fuel pump leaving them stranded on the side of the road?! I hope you do, because if you keep topping-off that gas tank, it’s exactly what you’re gonna get.

Share This Story

Get our newsletter