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Uh-Oh, I’ve Used the Wrong Fuel. Now What?

As humans, we’re prone to an error or two from time to time. In fact, we could hardly consider ourselves human if we were perfect and not making the odd mistake. And while it’s not common to mix up different fuel types when putting them into your vehicle, it can and does happen. After all, for those pump nozzles to be colour-coded, and even slightly different fits, someone must have realised there was a problem. However, even if you end up making this surprisingly not so uncommon mistake, you can rectify the issue and minimise the prospect of any long-term damage to your vehicle.

When petrol is inserted into diesel vehicles, the more common mix-up, the engine and fuel injector system are most prone to issues. Petrol acts as a solvent to reduce the lubrication within a diesel fuel pump, in turn creating weakness within the diesel fuel pump. Where metals make contact with each other, a lack of lubrication can mean that tiny fragments are created. The impact of these fragments can be notable as they make their way through the rest of the fuel system. Engines potentially may also be exposed to damage as a result of the extreme and ill-regulated compression of petrol fuel.

Although diesel used in petrol engines still has the chance to create catastrophic damage, petrol engines tend to suffer immediate performance issues that are symptomatic of a fuel mix-up. This may include a poorly running vehicle, or one that won’t start at all.

If you’ve realised you’ve made a mistake, the most fundamental rule is to refrain from starting the ignition. As soon as you switch that key, fuel is circulated right through the system, extensively expanding the areas at risk.

Your first point of call should be to ask a licensed towing agency to transport the vehicle to a workshop premises, or alternatively, you will need to put the car into neutral and push it away. The problem with the latter approach however, is that there are few places immediately near a service station where it is appropriate to syphon fuel. Doing this on the side of a road, particularly major roads where service stations are located, is not the most logical location. Furthermore, you also run the risk of polluting the environment via waterways and the ground.

If on the other hand, you don’t immediately realise your error and only start to notice performance issues a short time after fuelling up, stop immediately and arrange for your vehicle to be towed away. The damage at this point, and certainly beyond, is more likely to be meaningful if left longer without being addressed. Diesel systems are likely to require a rework of the whole fuel system or worse, while petrol engines are more likely to need the fuel drained, lines flushed and filter changed.

Using the wrong petrol grade however, is of less concern. Although a lower grade still has the potential to have wider implications for a high-grade system, this is more reserved for specific vehicles. Even some high performance vehicles will slug away on a lower grade without any lasting damage, only a temporary reduction in performance levels. And if you fill up with a high grade fuel in your 91ULP vehicle, learn from your mistake and appreciate that the only damage was a few dollars difference between the cost in fuel grades – certainly not a few thousand dollars in repairs!


  1. Ernest Lohberger says:

    At one stage many years ago while driving a 1951 XK120 Jaguar we had a faulty gauge and ran out of fuel while being 40km of road on a logging road, while there was no petrol available there was Diesel, we filled 20 liters into the car and it started on what fuel was in the fuel lines. it ran pretty rough once it started burning pure Diesel but got us home about 50km without stopping. once we stopped the engine there was no way it would start until we cleaned everything out but there was no damage to the engine. the other way petrol in a Diesel engine is looking for disaster.

    February 27th, 2017 at 6:42 pm

  2. Paul Winchester says:

    I heard the story that after the owner realised he had out petrol in a diesel tank, the garage mechanic said no problem, just put some lube oil in the tank, which he did. Don’t know the long term outcome.

    February 28th, 2017 at 11:05 am