TECHNICAL Q&A: What to do (and not to do) after misfuelling your 4×4


Shortly after hitting the road on our Mozambique holiday, I pulled into a fuel station to fill up. You can imagine my horrified response when I found the petrol-attendant pouring diesel into my Patrol’s fuel tank! The vehicle has a 4.8-litre petrol engine. Fortunately, I managed to stop the guy before the entire tank was filled with diesel, but all in all, about 5-litres went in.

I filled the rest of the tank with petrol, and remarkably, there was very little difference in how the engine ran. Should I be concerned that irreparable damage has been done? A mechanic recently told me that the Patrol’s entire fuel system should be replaced, at a cost of many thousands of rands! What should I do?

The term is commonly known as misfuelling. According to the AA, the error happens almost every minute of every day, costing motorists millions of dollars worldwide. There are two possible scenarios: petrol into a diesel; and diesel into a petrol. Let’s start with the latter…

Putting diesel into petrol is generally not as bad as putting petrol into diesel. It’s also a far less common mistake, as diesel pump nozzles are wider than petrol nozzles and shouldn’t ordinarily fit into the filler hole. The attendant that made the mistake with your Patrol must’ve been a newbie on the job.


In most cases, a small amount of diesel in a petrol engine shouldn’t cause too much trouble, and if you haven’t experienced any adverse driving effects (engine spluttering and misfiring) we wouldn’t worry too much. Chances are, the diesel (which is oil based) simply dissolved into the petrol (which is solvent based) and the motor knew little of the change.


However, if too much diesel is added, you’ll obviously notice a lack of engine performance and fuel economy. If the entire tank has been filled with diesel, it goes without saying that the fuel tank should be completely drained and topped up with petrol.

Many fuel-station pumps feature an alarm or buzzer that notifies the attendant (and driver) that the diesel nozzle has been lifted from its cradle. If you drive a petrol-powered vehicle, and you hear a faint alarm sound while topping up, stop the attendant immediately.


If your filler hole doesn’t have a warning note that says which fuel type to use, it’s a useful addition, and a low-cost solution to a potentially high-cost problem.

Older generation diesel engines were designed with high compression ratios, which allowed them to achieve fuel detonation. However, these days, modern diesel engines are increasingly dependent on high-pressure fuel pumps that aid the ignition process and help one achieve better engine performance and economy. In many ways, the fuel pump can be considered the heart of a modern diesel engine, and the fuel itself is the lubricant that keeps the pump running. If it weren’t for the lubrication properties of diesel, the high-pressure, low-tolerance fuel pump would prematurely fail.


Most fuel stations use a wider nozzle on the diesel pump to prevent mis-fuelling into a petrol fuel tank, but mistakes still happen.

As mentioned before, petrol acts as a solvent when mixed with diesel, reducing its lubrication abilities. For this reason, mixing petrol into a diesel engine can be a serious problem, leading to fuel-pump wear and fine metal particles that may also damage the rest of the injection system.

That said, a small measure of petrol should have minimal effect on an old-school diesel, such as the Toyota 1HZ. Provided you fill the rest of the tank with diesel (so that the petrol percentage is low) you could probably get away with driving such a vehicle – preferably to the nearest mechanical workshop or service centre to have the tank drained. However, we would not recommend that the same be done with a modern turbo diesel.


If you suspect that your (modern) diesel-powered vehicle has been misfuelled with petrol, do not start the engine. In fact, don’t even turn the ignition half way, as this could prime the fuel line with contaminated fuel. If your tank has been contaminated, you must drain the fuel and possibly even flush the tank with fresh diesel.

Coming back to your query: with only 5-litres of diesel added to your Patrol’s fuel tank, it’s unlikely that any damage occurred, especially to the fuel delivery system. However, if it happens again, it would be wise to have the tank drained as soon as possible.

Unfortunately, there are too many variables at play to make definitive rules on this subject. No two engines are alike, and it’s impossible to say what percentage of contamination is okay, and what’s not okay; but to play it safe, it’s always best to have the fuel tank drained. As far as replacing the entire fuel system goes, that sounds a bit drastic in your case and more like a money-making (or losing) scheme, especially since the engine ran without any trouble.


For more 4×4 advice, contact the 4WD Truckin’ Company on (011) 791 3822 or (012) 803 1040, OR visit OR send your technical problems to



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