The Dreaded Diesel Dilemma: What Happens When You Put Diesel in a Petrol Car?
It’s a common enough mistake to put petrol in a diesel vehicle, but the opposite scenario – accidentally filling a petrol car with diesel – while perhaps less frequent, can still lead to a world of automotive woe. The thicker nozzle of a diesel pump often acts as a natural deterrent, preventing it from fitting into the narrower filler neck of a petrol car. However, this isn’t a foolproof safeguard. Some petrol stations might employ similar-sized nozzles for both fuels, and the colour-coding of pump handles can vary, leading to confusion, especially during a moment of absent-mindedness. A long day, a borrowed car, or simply being on autopilot can be enough to cause a driver to grab the wrong nozzle.
While specific Australian statistics are scarce, a UK comparison site, GoCompare, has reported that as many as one in five people have admitted to putting the wrong fuel in their car. This highlights the prevalence of such errors, underscoring the importance of understanding the consequences and, crucially, what to do if it happens to you.
Understanding the Engine’s Heartbeat
To grasp the impact of misfuelling, a basic understanding of how an internal combustion engine operates is essential. At its core, an engine relies on a precise interplay of fuel, air, and ignition. Fuel is injected into cylinders, where it mixes with air. This combustible mixture is then ignited by a spark plug, creating an explosion that forces a piston downwards. This downward motion is converted into rotational energy by a crankshaft, which, through the gearbox, ultimately powers your wheels.
The Diesel Intrusion: Petrol Engine Under Siege
When diesel finds its way into a petrol engine, the fundamental differences in fuel properties and engine operation lead to immediate problems. Diesel fuel possesses a significantly higher flash point than petrol. Crucially, petrol engines rely on spark plugs to ignite the fuel-air mixture. Diesel, on the other hand, combusts through compression ignition – it requires much higher temperatures and pressures to ignite than a typical petrol engine can generate.
- Ignition Woes: In most cases, a petrol engine simply won’t have the compression necessary to ignite diesel effectively. While a rare chance of ignition might occur if there’s a significant mixture of petrol and diesel already in the tank, the engine will likely run extremely poorly, if at all. Expect significant smoke billowing from your exhaust.
Spark Plug Sabotage: The oily nature of diesel can quickly coat and clog your spark plugs. This is akin to trying to smother a flame with your hand; the spark plug is the flame, and the diesel is the smothering agent. This fouling will lead to misfires and, eventually, complete failure of the spark plugs, rendering the engine inoperable.
Fuel Pump Frustration: Diesel’s higher viscosity poses another threat. It can clog the fuel pump, impeding its ability to deliver fuel effectively in the required volumes. This can starve the engine, leading to a condition known as “running lean.”
The Perils of Running Lean: Fuel plays a role in cooling the engine. When insufficient fuel is supplied, cylinder temperatures can skyrocket, potentially leading to catastrophic engine failure.
Hydro-locking Horror: If your car doesn’t start, the temptation to repeatedly crank the engine can be overwhelming. However, this can exacerbate the problem by flooding the cylinders with diesel. This can result in “hydro-locking,” a dangerous situation where liquid fills the cylinders, preventing the pistons from completing their stroke. The consequences can be severe, including bent connecting rods, lifted cylinder heads, and even a bent crankshaft.
Injector Irritation: Even if the engine doesn’t immediately suffer catastrophic failure, clogged fuel injectors are a common and costly consequence. The oily diesel can obstruct the fine spray of fuel needed for efficient combustion, again leading to fuel starvation and the aforementioned lean conditions.
The Immediate Action Plan: What to Do If You’ve Misfuelled
The absolute best course of action if you realise you’ve put diesel in a petrol car is to not start the engine. If you’ve already started it and notice it running roughly, stop the engine immediately.
Resist the “Dilution” Temptation: You might hear advice about diluting the diesel by filling the tank with petrol. While this might seem logical, it’s not a viable solution. It will still result in a compromised fuel mixture that can cause significant issues, especially in modern cars with sensitive fuel systems that require premium-grade fuel.
Call for Professional Help: The most sensible step is to contact roadside assistance if you have a membership. Alternatively, seek out a specialist “wrong fuel” service. These professionals are equipped to handle such situations. They will either arrange for your vehicle to be towed to a mechanic or, in some cases, can perform the necessary draining and cleaning on-site, particularly if the car hasn’t been run on the incorrect fuel.
The Mechanic’s Role: A mechanic will typically drain the entire fuel tank, flush the fuel lines, and replace the fuel filter. If the car hasn’t been run for an extended period on diesel, these steps are usually sufficient to get you back on the road.
The Financial Fallout: Cost of Misfuelling
The cost associated with rectifying a misfuelling incident can vary significantly. Factors influencing the price include the service provider, your geographic location, and, most importantly, whether you attempted to start the car with the wrong fuel in the tank.
Typical Service Costs: Expect to pay anywhere from $200 to $500 for a professional wrong fuel service. This usually covers draining the contaminated fuel, its disposal, fitting a new fuel filter, and ensuring your vehicle is ready to be refilled with the correct fuel.
Beyond the Service Fee: Remember that this cost is in addition to the price of the incorrect fuel you purchased and the cost of refilling your tank with the correct petrol.
Insurance and Warranties: A Risky Proposition
Unfortunately, putting the wrong fuel in your car is unlikely to be covered by your standard insurance policy. Most insurance providers explicitly state that damage caused by misfuelling is not covered. Furthermore, attempting to rectify the situation yourself or, worse, continuing to drive the vehicle on the wrong fuel will almost certainly void your car’s warranty. In Australia, finding an insurance policy that offers coverage for misfuelling errors is exceptionally rare, if not impossible. It’s a costly mistake that generally falls outside the purview of insurance protection.
