Water in Your Fuel Tank: How to Drain It and Why It Happens (WA Guide)

Water in your fuel tank is a real problem. It sinks below the fuel, gets sucked into the fuel pump, and causes misfires, hard starting, or a complete shutdown on the side of the road.

Here's how to spot it, how to fix it properly, and how to stop it happening again. Written for WA drivers by Ben, qualified mechanic and owner of Rapid Fuel Rescue.

How does water get into a fuel tank?

Water in a fuel tank is more common in Perth than people think. Coastal humidity, monsoon-season flooding up north, and the odd dodgy servo all play a part. Most cases come from one of five everyday causes, not from anything dramatic.

Here are the real ways it happens.

Condensation in a half-empty tank

This is the number one cause in Perth winter. A half-full tank has a lot of air space above the fuel. As temperatures drop overnight, moisture in that air condenses on the inside of the tank and runs down into the fuel.

It's slow. It builds up over weeks. By the time you notice misfires, there can be a litre or more of water sitting at the bottom of your tank.

A leaky filler cap or seal

Your fuel cap has a rubber seal. After a few years in the WA sun, that seal hardens and cracks. Rain at the servo, water from a high-pressure car wash, or just a heavy storm in your driveway can push water past it.

If your cap doesn't click firmly when you tighten it, replace it. They're cheap.

Water in the servo's underground tank

Service stations store fuel in big underground tanks. Those tanks have seals too, and when those seals fail, groundwater leaks in. You can fill up with contaminated fuel without ever knowing.

This is more common at low-turnover rural and remote servos, particularly after heavy rain in the Wheatbelt, Mid West, or after wet season in the Kimberley and Pilbara. If your car starts misbehaving within a day or two of refuelling at a quiet servo, the fuel is a likely culprit.

Flood water or a deep river crossing

If your vehicle has been in flood water, sat in a flooded driveway, or you've done a deep crossing on a remote track, water can get into the tank through the filler neck vent or a damaged seal.

WA gets serious flooding during cyclone season up north and during big winter storms in the south. Water-in-fuel callouts spike for weeks after a flood event.

A vehicle left sitting for months

If a car, boat, or caravan has been parked for a few months with a partly full tank, condensation builds up. Diesel is worse because water in diesel grows bacteria, also known as "diesel bug", which forms a slimy black mess that clogs filters.

Vandalism does happen too, but it's rare. If your tank is suddenly full of water and the cap looks tampered with, take photos and report it.

What are the signs of water in your fuel?

Symptoms usually come on suddenly and feel like the car is "starving" for fuel. They can also come and go as water sloshes around the tank.

Watch for these signs:

• Hard to start, especially first thing in the morning
• Rough idle when warmed up
• Hesitation or stutter when you push the throttle
• Sudden loss of power going up a hill or pulling away from lights
• Engine cutting out at low speed or at a stop
• White smoke from the exhaust as small amounts of water burn off
• A "diesel" smell that's stronger or sourer than normal
• Water visible at the bottom of the fuel filter water trap (diesel vehicles only)

If symptoms started within a day or two of a fill-up, fuel contamination is the most likely cause. Compare these to the wrong fuel symptoms checklist if you're not sure which problem you've got.

Why is water in fuel so urgent?

Modern fuel systems hate water. Diesel and petrol engines both rely on fuel doing two jobs: combusting cleanly, and lubricating expensive moving parts. Water does neither.

Here's what happens if you keep driving on a water-contaminated tank.

Your fuel pump dies. Pumps are lubricated by the fuel passing through them. Water has no lubricity. A high-pressure pump can fail in hours, not weeks.

Diesel injectors get destroyed. Modern common-rail diesel injectors are expensive on their own, and most engines have four to six of them. Mechanics usually replace them as a set, so a contamination job that turns into an injector job can easily run into the thousands.

The inside of the tank rusts. Metal tanks corrode where water sits. Rust flakes then clog filters and damage the pump downstream.

Diesel bug takes hold. In diesel, water creates the perfect environment for bacterial growth. Once diesel bug is in the system, you need a chemical biocide treatment on top of the drain.

The longer you wait, the more the bill grows. Every minute counts.

How do you drain water from a fuel tank?

Honest answer: properly draining water out of a fuel tank is a workshop or mobile-specialist job. The reason is simple. You can't see what's coming out of the tank, you can't confirm the tank is clean, and modern fuel injection has zero tolerance for leftover water.

Here's a high-level look at how it's done, so you understand what you're paying for.

For diesel vehicles

Most diesels have a water trap built into the primary fuel filter. It's a clear or metal bowl at the bottom of the filter housing with a small drain screw.

Step one is to open that drain screw and let the water run into a container until clean diesel starts coming out. On a lot of older 4WDs, this can be done at the roadside.

If water keeps coming back after a few minutes of running, the water is winning. The tank itself has to be drained from the lowest point, the fuel lines flushed, and the primary filter replaced. That's a workshop job.

For petrol vehicles

Petrol cars don't have a water trap. The water just sits at the bottom of the tank, gets pumped up to the engine, and causes misfires.

The only fix is to drain the tank completely from the lowest fuel line, flush the lines through, replace the filter, and refuel with clean petrol. Same process we use for contaminated fuel removal and fuel drainage on any car.

Why DIY pumping doesn't work

People sometimes try to siphon fuel out of the filler neck. It doesn't work for water contamination, and here's why.

Water is heavier than fuel. It sits at the bottom of the tank. Fuel floats on top. If you siphon from the filler neck, you suck out clean fuel and leave all the water behind. You've wasted an hour and your tank is still contaminated.

Why DIY doesn't work for water contamination

Even if you have the right pump, draining a water-contaminated tank yourself is a bad idea for safety and legal reasons.

You can't see what's coming out of the tank as you pump. You can't verify the system is clean. Modern fuel injection systems have no tolerance for water, so "good enough" leaves you stranded again next week.

EPA fines for spilled fuel in WA are real, and a domestic driveway is not a legal place to dump petrol or diesel. Petrol vapours from an open tank also catch fire easily. One spark from a phone, a static jolt off your jumper, or a nearby pilot light is all it takes.

It's not worth it. Call someone with the gear and the insurance.

What we do when we come to you

Here's what a water-in-fuel job looks like when Rapid Fuel Rescue rolls up to you. Same process whether you're at home, at work, or pulled over on the side of a back road.

Verify the contamination. We draw a sample into a clear container and check for water separation. If your fuel smells off, looks cloudy, or has a layer at the bottom, you've got water.

Drain from the lowest point. For diesels, we start at the primary filter water trap, then move to the tank itself if needed. For petrol, we go straight to the bottom of the tank or pull a low-line.

Drop the tank if we have to. Some vehicles, especially modern utes and SUVs, don't have a low drain point. In that case we may need to drop the tank to get every last bit of water out.

Flush the fuel lines. We push clean fuel through the lines all the way to the engine to clear any remaining water.

Replace the primary filter. A water-contaminated filter goes in the bin. Diesel filters also act as a water trap, so they're saturated and need to be swapped.

Refuel and run. We add fresh, clean fuel and start the engine. You should hear it idle smoothly within a minute or two.

Document it for insurance. We give you a written report, photos of the contamination, and an itemised invoice. Some comprehensive policies cover fuel contamination, particularly if it came from a servo's faulty supply.

A standard car job takes 60 to 90 minutes. Trucks and 4WDs with bigger tanks take longer. We come to you, anywhere in Perth metro, 24/7.

How do you prevent water in your fuel tank?

Most water-in-fuel jobs are preventable with a few small habits. None of them cost much.

• Keep your tank above a quarter full in winter. Less air space means less condensation. This is the single biggest one in Perth.
• Check your fuel cap seal every service. If the rubber is cracked, hard, or the cap doesn't click firmly, replace it. They're cheap.
• Avoid suspect rural servos after heavy rain. If you're driving through the Wheatbelt, Mid West, Pilbara, or Kimberley and there's been a recent downpour, fill up at higher-turnover servos in the bigger towns where you can.
• Drain your diesel water trap monthly. It takes 30 seconds. Open the screw on the bottom of the primary fuel filter, let any water run out, close it.
• Store vehicles with a full tank. If a car, boat, caravan, or work ute is going to sit for more than a few weeks, fill the tank right up before you park it.
• Use a stabiliser if storing long-term. For vehicles parked for months, a diesel biocide or petrol stabiliser is cheap insurance.

A good habit beats a callout every time.

Frequently Asked Questions

How much water in fuel is too much?

Any visible water is too much. Modern fuel pumps and injectors are not designed to tolerate water at all. Even a small slug of water hitting a common-rail injector at high pressure can damage the tip. If you see any water at the bottom of a fuel sample or coming out of your diesel water trap, get it drained.

Can a small amount of water in petrol hurt a car?

Yes. Petrol engines have no water trap, so anything in the tank ends up at the injectors. A small amount might just cause rough idle and hesitation. A larger slug will stall the engine and can damage the fuel pump. Petrol cars are generally less tolerant of water than diesels are.

How do I know if there's water in my diesel?

Open the drain screw on your primary fuel filter and catch the first bit of liquid in a clear container. If you see a clear layer at the bottom with diesel above it, that's water. Most modern diesels also have a "water in fuel" warning light on the dash. If that light is on, drain the trap and call us if it comes back on.

Will an octane booster fix water in fuel?

No. Octane boosters change the burn rate of petrol. They don't bind to water and they don't remove it. The only real fix is physical drainage of the water out of the tank. If someone at a parts shop tells you a booster will fix water contamination, they're wrong. Save your money.

Can I add a water remover to my fuel tank?

Methanol-based fuel dryers can disperse very small amounts of water in petrol, but they're a maintenance trick for tiny condensation issues, not a fix for actual contamination. They do nothing for diesel water problems, and they won't save your fuel pump if there's a real slug of water in the tank. Drainage is the only proper fix.

How much does it cost to drain a water-contaminated fuel tank?

Pricing depends on the vehicle, how much water is in the tank, and whether diesel bug has set in. Ben confirms the price on the call before any work starts. No Fix, No Fee. Whatever the figure, it's always far cheaper than one new injector, and a lot cheaper than a new pump. Call 0416 692 022 for a quote on your situation.

What's the difference between water contamination and wrong fuel?

Water contamination is when water has entered your fuel tank through condensation, a leaky cap, a dodgy servo, or flood water. Wrong fuel is when petrol gets put in a diesel or diesel in a petrol car at the pump. Symptoms overlap, but the fixes are different. If you're not sure which one you've got, call us and we'll work it out on the phone.

Call us if your car's running rough after rain

If your car's running rough after rain, after a fill-up at a quiet servo, or after sitting for a few months, you might have water in the tank. The fix is straightforward when you catch it early. The bill gets ugly fast if you don't.

We come to you, anywhere in Perth metro, 24/7. We drain, flush, refuel, and have you back on the road in about an hour. Fully insured, qualified mechanic on the truck, no towing required.

Call Ben on 0416 692 022. Wrong fuel happens to anyone. So does water in fuel. We'll sort it.