7 Trimmer Fuel Tank Leak Fixes That Old-Timers Swear By

You walk out to the shed, grab the string trimmer, and notice that familiar, sharp smell of gasoline. A quick look reveals a damp spot on the concrete floor right under the engine—a fuel leak. It’s a frustrating but common problem that can sideline your weekend chores and create a real fire hazard. For a hobby farmer, time and money are always in short supply, and sending a tool out for a simple repair often isn’t worth the cost or the wait. Learning to diagnose and fix these small engine issues yourself is a cornerstone of self-sufficiency.

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Pinpointing Your Trimmer’s Fuel Leak Source

Before you can fix a leak, you have to find it. Guessing will only lead to frustration and wasted parts. The best way to start is with a clean slate. Wipe down the entire fuel tank, the fuel lines, and the area around the carburetor with a clean, dry rag.

Once it’s dry, add a small amount of fresh fuel to the tank—just enough to cover the bottom. Now, watch closely. Sometimes a leak is obvious, a steady drip from a cracked line. Other times it’s a slow seep that’s harder to spot. You can dust the area with a little chalk powder or flour; the fuel will create a dark, easy-to-see trail as it escapes.

Focus your attention on the most likely culprits first to save time. Check these key areas in order:

• The fuel lines, especially where they bend or enter the tank.
• The rubber grommet that seals the lines going into the tank.
• The fuel cap and its gasket.
• The clear plastic primer bulb.

Replacing Old, Cracked, and Brittle Fuel Lines

Fuel lines are the number one cause of leaks on any small engine. Modern ethanol-blended gasoline is notoriously hard on the plastic and rubber compounds used in older equipment. Over time, the lines become hard, brittle, and develop tiny cracks that are often invisible until fuel starts seeping out.

Replacing them is one of the cheapest and most effective repairs you can do. You can buy bulk fuel line in various diameters from any auto parts or small engine shop. Just be sure to get the right size; a line that’s too loose won’t seal, and one that’s too tight is impossible to install.

The trick to the job is getting the new line through the grommet in the fuel tank. Use a pair of hemostats or long-nosed pliers to pull the old line out. To feed the new one in, cut the end at a sharp, 45-degree angle. This creates a pointed tip that you can easily push through the small hole. Once it’s inside the tank, just snip off the angled end to create a flat surface for the fuel filter to attach to.

Resealing the Fuel Line Entry Point Grommet

That black rubber grommet where the fuel lines pass into the tank is another common failure point. Just like the lines themselves, it can dry out, shrink, and lose its seal against the plastic tank. If you see fuel weeping from around the base of the fuel lines, the grommet is your likely problem.

The proper fix is to replace the grommet, which is an inexpensive part. However, if you’re in a pinch and need to get a job done, you can often reseal it. First, drain all the fuel from the tank. Clean the outside of the grommet and the surrounding tank surface with a degreaser or rubbing alcohol to remove any oily residue.

Once it’s perfectly clean and dry, apply a thin, even bead of a fuel-resistant sealant around the edge where the grommet meets the tank. Products like Permatex MotoSeal or a high-tack gasket sealant work well. Let it cure for a full 24 hours before you even think about adding fuel. This is a temporary fix, but it can get you through a season.

Fixing a Worn or Damaged Fuel Cap Gasket

A leaky fuel cap is a sneaky problem. You might chase leaks all over the engine, only to realize fuel is sloshing out of the top every time you tilt the trimmer. The culprit is almost always the small rubber gasket or o-ring inside the cap. Over years of tightening, it gets compressed, cracked, and loses its ability to seal.

This is an easy fix. Use a small pick or a flathead screwdriver to carefully pry the old gasket out of the cap. Inspect it for cracks or flat spots. Sometimes, if it’s just compressed, you can flip it over and reinstall it for a temporary seal.

For a permanent solution, you need a new gasket. If you can’t find an exact replacement for your model, don’t give up. You can buy a small sheet of fuel-resistant gasket material from an auto parts store. Use the old gasket as a template, trace it onto the new material, and carefully cut it out with a sharp utility knife or scissors. It’s a little extra work, but it’s a classic old-timer trick for keeping good equipment running.

Sealing Hairline Cracks with Fuel-Safe Epoxy

Sometimes, the fuel tank itself is the problem. A drop onto a concrete floor or a knock from another tool can create a tiny hairline crack in the plastic. These are tough to spot and often only leak when the tank is full and under pressure. This repair should be considered a last resort; a new tank is always the better option.

If you’re going to attempt a repair, preparation is everything. Drain the tank completely and leave the cap off for at least a day to let all the fuel fumes evaporate. Do not skip this step. Next, use some fine-grit sandpaper to rough up the area around the crack. This gives the epoxy a surface to bite into. Clean the sanded area thoroughly with rubbing alcohol and let it dry.

Use a two-part epoxy that is specifically rated for gasoline and plastic, like J-B Weld TankWeld. Mix it according to the directions and apply a thin, smooth layer over the crack, extending it about a half-inch on all sides. Let it cure for longer than the package recommends—at least a full 24 hours. A rushed epoxy job will fail every time.

Swapping Out a Leaky or Cracked Primer Bulb

The primer bulb is designed to be pressed, so it’s made of a soft, flexible plastic. Unfortunately, that same material is vulnerable to UV light and fuel, which makes it brittle and cloudy over time. Eventually, it will crack and start leaking fuel or sucking in air instead of priming the engine.

Replacing a primer bulb is one of the quickest repairs you can make. Most are held in place by a retaining ring or simple plastic tabs. A small flathead screwdriver is usually all you need to gently pry the old bulb assembly out of its housing.

The key is to pay attention to how the two fuel lines are connected. One line brings fuel from the tank, and the other sends it on to the carburetor. If you mix them up, the trimmer won’t start. The easiest way to avoid this is to take a quick picture with your phone before you disconnect anything. Snap the new bulb in, reconnect the lines correctly, and you’re back in business.

Clearing a Clogged or Malfunctioning Fuel Vent

This is a problem that fools a lot of people. Your tank needs a vent to let air in as fuel is consumed. Without it, a vacuum forms in the tank, which can stop fuel flow or, if pressure builds up from heat, it can force fuel out through the weakest point—like the fuel cap or a line connection. You might think you have a leak, but the real issue is a clogged vent.

The vent is often a small, separate part connected to a short hose, or sometimes it’s integrated into the fuel cap itself. A tell-tale sign of a clogged vent is a loud "whoosh" of air when you open the fuel cap on a warm engine. That’s built-up pressure escaping.

To fix it, locate the vent. If it’s a separate valve on a hose, you can sometimes clean it with a shot of compressed air or by carefully clearing the opening with a thin piece of wire. Often, it’s easier and more reliable to just replace the part; they’re very cheap. A functioning vent not only prevents leaks but also ensures your engine runs smoothly.

Securing the Fuel Line to Carburetor Nipple

Sometimes the leak isn’t coming from the tank at all. A common spot for a slow seep is where the fuel line pushes onto the small brass or plastic nipple on the carburetor. Over time, the end of the fuel line can stretch out or harden, losing its tight grip and allowing fuel to weep out.

For a quick and temporary fix, you can add a small clamp. A tiny spring clamp or a small zip tie can provide enough pressure to reseal the connection. Be very careful if using a zip tie on a plastic nipple—overtightening it can crack the fitting, creating a much bigger problem.

The better, more permanent solution is to restore a tight fit. If you have enough slack in the fuel line, use a sharp blade to snip off the last half-inch of the stretched-out end. The fresh-cut section of the line will be tighter and should seal perfectly when you push it back onto the nipple. If there’s no slack to spare, it’s a clear sign that it’s time to replace the whole fuel line.

Fixing a leaky trimmer isn’t just about saving a few dollars; it’s about understanding the tools you rely on. These simple, time-tested repairs build confidence and self-reliance, which are the true currency on any hobby farm. By learning to diagnose and mend these small issues, you keep your equipment running reliably and spend less time in the workshop and more time on your land.