Boat Gas Tank Vent Problems and System Troubleshooting

Published Categorized as Maintenance and Fixing Guides

Of all of the systems on a boat, the fuel system seems to be one of the simplest. After all, how much different can it be from a car? But, unfortunately, it is more complicated. Some of it is simply because of the presence of so much water – everywhere, all the time. And some issues are due to how little fuel some boats consume. If you only put one or two tanks of fuel in your car every year, you’d have more problems with it, too! So to sort things out and make things easier, here’s a look at some of the symptoms and signs of a boat’s gas tank vent problem.

Boat Gas Tank Vent Problems And System Troubleshooting

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How Do You Know if You’ve Got Boat Gas Tank Vent Problems?

Problems can primarily be observed during the fueling process. If the fuel tank vent is blocked or non-functional, common indicators include the fuel pump shutting off prematurely due to the lack of an air escape route, or fuel being unexpectedly expelled from the hull vent or fill hose, creating potential messes.

Additionally, if the engine stops running due to an inability to draw fuel, this could indicate a vacuum created inside the tank because air cannot enter to replace the consumed fuel, pointing to a venting issue.

So, let me explain further.

One of the most common problems is with the boat fuel tank vent. Problems nearly always occur during fueling at the gas pump. Most fuel pumps put the good stuff faster than our engines use it, so any problems with the vent show their heads first there. 

Before we dive in and look at fuel tank venting problems, let’s take a minute and reflect on why vents are so important in the first place. A vent line allows air in and out of a tank. If a tank is sealed, and you pull the liquid out for use, something has to be added back. If there is no way to allow air into the tank, the lowering fluid level will create suction. Eventually, the liquid will stop flowing out.

For this example, a fuel tank has no vent or perhaps has a blocked fuel vent. If that’s the case, fuel may flow to the engine for a short time until a suction or vacuum forms inside the tank. Once the force of that suction exceeds the pull of your fuel pump, fuel will no longer flow, and the engine will stop.

Of course, vents are also crucial during the fueling process. If you pour a liquid into the tank, the air from the tank needs an escape path. The fuel pump will likely shut off prematurely if the only escape path is through the filler neck. 

The most common problem during fueling is having gas spewed out the hull vent or fill hose. When you aren’t expecting it, it can make a real mess. It helps to fuel slowly and carefully, but sometimes you have to be ready with a fuel absorbent mat – just in case!

How to Troubleshoot Common Issues

Causes of a Blocked Vent Line

A blocked vent line can become evident in several ways. In the worst-case scenario, the engine mysteriously shuts down. More likely, however, you discover the problem during fueling. Either fuel erupts out of the vent or the filler neck, or the fuel pump begins shutting down too soon.

A fuel vent hose can get plugged up in several ways. A basic inspection is in order. Start at the most likely cause—something has gotten stuck in the outer vent on the hull design. Some designs have a bug screen that can be removed and cleaned. Yes, it might just be that simple.

If it’s not that easy, you may have to remove the vent line for inspection or replacement. Before you start messing with the line, though, take it off the inside of the outer hull vent and ensure that air moves freely through the through-hull vent fitting. 

Vent Hose Problems

It’s unlikely that something has plugged up the hose entirely. A more likely explanation is that sag or bend in the hose allows fuel to pool, preventing air from flowing freely. 

The fuel vent hose should run from the through-hull and then loop upward. It should not provide a straight path into the tank for water!

After creating a loop, it should then follow the straightest path to the fuel tank as possible. Make sure there are no sags or low spots. 

If you’re inspecting the hose and still haven’t found the cause, it’s time to look closer to the tank. Remove the hose from the tank fitting and inspect the port on the tank. You don’t want to be doing this if the tank is full!

Be careful to avoid spills, and have a few clean fuel absorber mats handy, just in case.

Related: What Should You Do if the Motor on Your Boat Catches Fire?

External Tanks

Outboard boats with external, deck-mounted fuel tanks aren’t without their problems. Recent EPA rules have mandated that a new fuel tank be sealed to avoid spills and avoid gasoline vapors escaping into the atmosphere.

For them to work without a proper vent, most fuel lines now come equipped with a special fitting that vents the tank and prevents airlocks without venting out the fumes. However, these can stick, so inspect them carefully and give them a little tap if you think the tank isn’t venting properly.

Troubleshooting Other Fuel System Issues

More than any other activity, boating teaches you the value of an ounce of prevention.

Keep a Clean Gas Tank

Many fuel problems stem from dirty fuel. Gasoline and diesel both have their demons that you must be careful to avoid. 

Gasoline problems usually stem from ethanol fuel. Entire books could be written about this problem, but suffice to say that avoiding ethanol is to every boater’s benefit. Most marinas sell ethanol-free fuel. Ethanol on its own is harmless, but left in a tank with any amount of water moisture will create a huge mess.

Diesel is prone to algae and growth in the tank if moisture is introduced and left too long. The foul muck that results will quickly clog fuel filters and stop engines. It usually happens after a bumpy ride churns all the gunk up from the bottom and sides of the tank.

The moral of the story is to keep a dry and clean fuel tank. If you haven’t used the boat in a while or you’ve wound up with some bad fuel, the best solution is to have the tank professionally polished. Also, make sure your fuel tank fill gas cap closes securely and replace its O-ring seal when necessary.

Keep Spare Fuel Filters Onboard at All Times

Stuff happens, and some water or gunk in your tank is inevitable. That’s why it’s vital that you carry spare fuel filters onboard and you know how to change them. For diesel engines, this means you may need to know how to bleed the fuel lines, too.

Install a Boat Fuel Vent Check Valve

You might be surprised at just how many different types of boat fuel vents there are on the market. Older boats are usually equipped with a simple open through-hull with a cover to keep most water out. In some cases, the opening has a screen on it.

It’s not uncommon to find this type of vent covered by a simple clamshell-shaped cover for a little extra protection.

A newer style vent opening, like the ones made by Perko, is flush to the hull. They can be disassembled for cleaning, but they are also cleverly designed to keep water out of the tank.

No matter what style you choose, make sure that it is one of the US Coast Guard-approved vent fittings.

If you have problems with fuel coming out of the vent during fueling, you might want to consider installing a fuel surge protector in the vent line. This apparatus is basically a check valve that will allow the tank to properly vent and also capture any liquid that tries to squeeze past.

Some surge protectors are integrated into new-style hull vents, while others are designed to be put in line between the tank and the vent. These are a big help on boats where it’s difficult to hear the tank getting full or where the sailboat design makes proper fueling awkward.

Common Issues and Solutions

The Vent is Not Venting

  • Turn off all ignition sources and avoid smoking near the boat.
  • Inspect the vent line for kinks, bends, or collapses that could obstruct airflow.
  • Check for blockages in the vent, such as mud, insect nests, or debris, and clear them using appropriate tools like a wire brush or compressed air.
  • Gently remove obstructions to avoid damaging the vent or hose.
  • Inspect the vent hose and the vent fitting for signs of wear, damage, or corrosion.
  • Ensure all connections are secure and there are no leaks.
  • Test the venting system after clearing blockages and ensuring everything is intact.
  • If unable to resolve the issue, consult a marine mechanic or professional.
  • Regularly inspect and clean the vent and surrounding area to prevent future blockages.
  • Consider installing screens or covers to keep out insects and debris, ensuring they do not restrict airflow and comply with marine regulations.
  • Understand the importance of a properly functioning vent for safe boating, preventing pressure build-up, and reducing the risk of explosion or fire.

Clogged Vent

  • Turn off the engine and all ignition sources immediately to avoid sparks near fuel vapors.
  • Locate the external vent cover on the boat’s hull and inspect it for visible blockages.
  • Use a small brush or compressed air to gently clear away any debris, insects, or dirt from the vent cover.
  • Check the vent hose for kinks, bends, or internal obstructions that might impede airflow.
  • Carefully straighten any kinks or replace the hose if it’s damaged or deteriorated.
  • If accessible, use a long, flexible cleaning tool to remove any clogs within the hose itself.
  • Reconnect and secure all parts of the vent system if any were disassembled or loosened for cleaning.
  • Test the vent by feeling for airflow or using a small amount of smoke to see if it’s drawn into the vent, indicating clear passage.
  • Regularly inspect and maintain the vent and hose to prevent future clogs, paying special attention during and after long periods of inactivity.
  • If the problem persists or if you’re unsure of the cause, consult with a marine professional to ensure the venting system is properly cleared and functioning.
  • Always prioritize safety by working in a well-ventilated area and wearing appropriate protective gear if necessary.

Motor Shut Off Without Warning

  • Check the fuel level to ensure the boat hasn’t run out of fuel.
  • Inspect the fuel lines for any signs of kinks, leaks, or blockages that could restrict fuel flow.
  • Verify that the fuel filter isn’t clogged and replace it if necessary.
  • Examine the electrical connections to the motor, including the battery terminals, for corrosion or looseness.
  • Test the ignition switch and kill switch for proper operation; replace them if they’re faulty.
  • Look for any signs of overheating, such as discoloration on the engine or an unusual smell, which could indicate a cooling system issue.
  • Check the oil level and condition. Low or dirty oil can cause the engine to shut off.
  • Inspect the propeller and the area around it for entanglement or debris that could have caused the engine to stall.
  • Review the engine’s air intake system for blockages or foreign objects that could prevent proper air flow.
  • If equipped, check the engine’s computer system for error codes that might indicate a specific issue.
  • Reset any tripped circuit breakers or replace blown fuses related to the boat’s motor system.
  • Listen for any unusual noises when attempting to restart the engine, as these could hint at internal mechanical issues.
  • If the motor still won’t start or shuts off again, consult a professional marine mechanic for a thorough diagnosis and repair.
  • Regular maintenance and timely addressing of minor issues can prevent unexpected shutdowns and prolong the life of the boat motor.

Tank Spits Gas Back Up

  • Immediately stop fueling to prevent overflow and potential fire hazards.
  • Release the pressure in the fuel tank by opening the gas cap carefully, if safe to do so, to allow any built-up vapors to escape.
  • Check the tank’s vent system for blockages or kinks that could be causing pressure buildup and preventing proper venting.
  • Inspect the fuel fill hose for any signs of collapse, wear, or damage that could restrict fuel flow into the tank.
  • Look for any obstructions in the fuel tank’s fill neck that might impede the flow of fuel into the tank.
  • Ensure the fuel cap is venting properly if it’s designed to do so; replace the cap if it’s faulty or not venting.
  • Examine the overfill prevention device (OPD), if equipped, for proper operation. These devices can malfunction and cause premature shut-off or spit-back.
  • Adjust your fueling technique by slowing down the fuel flow rate to reduce the chance of creating a vapor lock or causing spit back.
  • Use a different fuel pump or nozzle if possible, as some nozzles may not be compatible with your boat’s fuel system design.
  • If the problem persists, consult with a marine mechanic or technician who can inspect and test the boat’s fuel system for more complex issues.
  • Consider having the entire fuel system professionally cleaned if debris or sediment buildup is suspected.
  • Regular maintenance of the fuel system, including cleaning and inspections, can help prevent issues with fuel spit back.
  • Always follow safety guidelines when dealing with fuel, including working in well-ventilated areas and keeping fire extinguishers accessible.

Gas Fumes Are Making Its Way to the Cap

  • Immediately ventilate the area to disperse gas fumes and prevent potential fire hazards.
  • Check the boat’s fuel system for leaks, particularly around the fuel lines, tank, and connections.
  • Inspect the fuel tank vent and ensure it is clear of blockages that could cause pressure buildup, forcing fumes towards the cap.
  • Verify that the fuel cap is properly sealed and functioning. Replace the cap if it’s damaged or not sealing correctly.
  • Examine the O-ring or gasket on the fuel cap for wear or damage, and replace if necessary to ensure a tight seal.
  • Ensure the fuel tank and cap are compatible and correctly fitted, as mismatched components can lead to improper venting.
  • Look for any cracks or damage to the fuel tank itself, which could be allowing fumes to escape.
  • Check for improper routing of fuel lines or vent hoses that might cause fumes to accumulate in areas they shouldn’t.
  • If your boat has a charcoal canister in the vent line (common in newer models), ensure it’s not saturated or clogged, which could restrict proper venting.
  • Consider having a marine mechanic perform a pressure test on the fuel system to detect any hard-to-find leaks or issues.
  • Regular maintenance and inspections of the fuel system can prevent issues with gas fumes leaking or being improperly vented.
  • Always follow safety guidelines when dealing with fuel systems, including working in well-ventilated areas and avoiding ignition sources.

Practice Safe and Careful Fueling

While you pull over to top off your boat, it’s essential to realize that boat fuel pumps aren’t quite as reliable as those for automobiles. Most of us put the pump on automatically and walk away when we drive into a gas station. You should never do that with a boat!

Here are my tips for fueling your boat safely:

  • Don’t trust gas pump automatic shutoffs
  • Hold the nozzle yourself
  • Know how much fuel you’ll be buying and watch the pump carefully
  • Pump slowly when you know the tank is nearly full
  • Listen for the change in noise as fuel begins to back up in the fill tube – it should be readily apparent
  • Always have fresh fuel absorbent mats at hand when fueling to catch any messes.

What’s Special About Boat Fuel Tanks?

There are a few characteristics that set boat fuel tanks and systems apart from one another. 

Firstly, gasoline and diesel systems are different in design. Gasoline fumes are hazardous in boats, where they can accumulate in confined spaces and are easily ignited by wiring or heat. Therefore, boats with gasoline motors must have a vented fuel tank installed and blowers to evacuate any fumes out of the boat. 

Diesel fumes are far less likely to combust, but you should take precautions nonetheless. 

The second consideration is what type of motors are installed. For example, an outboard motor with portable tanks requires a different level of precaution and system design than an inboard boat with below-deck fuel tanks. 

FAQs (Frequently Asked Questions)

Should boat gas tanks be vented?

All fuel tanks should be vented to prevent airlocks. An improperly vented tank can restrict fuel flow to the engine and result in engine failure. On new deck-mounted outboard fuel tanks, fuel lines include an EPA-approved vent regulator. This device vents the tank but prevents fuel vapors from getting out.

How does a boat gas tank vent work?

Boats have air vents built into their fuel tanks to allow air to move in and out of the tank. That way, as fuel is used up, air can enter the tank. This prevents an airlock, which would otherwise cut off fuel flow to the engine. Vents also allow air out of the tank during fueling. Vents are typically simple—a vent tube runs from the tank to outside the boat. The hull will have a port that prevents water from getting into the fuel tank. Some newer boats are equipped with an in-line venting regulator, which prevents vapors and gas from getting into the atmosphere or water.

What happens if a fuel tank is not vented?

If a fuel tank isn’t vented, a suction or “airlock” can form as the engine burns fuel. Eventually, the suction in the tank will be stronger than the pull of the fuel pump. When this happens, the fuel will stay in the tank, and the engine will stop. Opening the fuel filler lid should restore flow to the engine.

By Matt C

Matt has been boating around Florida for over 25 years in everything from small powerboats to large cruising catamarans. He currently lives aboard a 38-foot Cabo Rico sailboat with his wife Lucy and adventure dog Chelsea. Together, they cruise between winters in The Bahamas and summers in the Chesapeake Bay.

1 comment

  1. Very helpful, as far as understanding the system. I recently purchased a 2005 Kenner 1800. Nice little boat and it runs great (with Merc 90 outboard). However it won’t take fuel. I replaced a bad fuel sender so now I know how much it won’t take, and in the process found clogged vent line (by loosening the fuel sender and hearing a rush of air into the tank). The problem is how to fix it from here. No outside vent—it’s integral in the Perko fuel filler port, and the tank is not very accessible—I can only reach the outlet to the engine on top. What would you do? Pull the filler port? Seems better than removing the entire center console, the only other option I see short of sawing up the deck!

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