Diesel in Petrol vs Petrol in Diesel: Which is Worse?

We see it every week at EEK Mechanical — drivers who've grabbed the wrong nozzle at Z Energy, BP, or other Kiwi fuel stations. While both types of misfuel are problematic, there's a crucial difference between putting diesel in a petrol car versus petrol in a diesel car.

Petrol in a diesel engine is far more dangerous and expensive to fix. Diesel acts as a lubricant for your engine's fuel system, particularly the high-pressure fuel pump and injectors. Petrol, however, is a solvent that strips away this lubrication. When petrol enters a diesel engine, it can cause catastrophic damage to these precision components, potentially requiring thousands of dollars in repairs or even a complete engine replacement.

Diesel in a petrol car, while still serious, typically causes less severe damage. Most petrol cars will simply refuse to start or run roughly because diesel doesn't ignite properly in a petrol engine. The main risk is contamination of the fuel system, which requires professional cleaning but rarely causes permanent damage if caught early.

Modern diesel cars have particularly sensitive fuel systems with components that can cost $3,000-$8,000 to replace. Even a small amount of petrol — as little as 5% of the tank — can cause problems. Petrol cars are generally more forgiving, though you'll still need the contaminated fuel removed and the system flushed.

What to do right now:

First and most importantly — do not start your engine if you haven't already. Don't even turn the key to the 'on' position, as this can activate the fuel pump and circulate contaminated fuel through your system.

If you're still at the petrol station, don't move your car. Put it in neutral and ask staff to help you push it to a safe location away from the pumps.

If you've already driven away, pull over safely and turn off the engine immediately.

The good news? This is exactly the kind of emergency EEK Mechanical handles every day across New Zealand. We'll dispatch a tow truck to collect your vehicle 24/7 and transport it to a certified workshop for professional fuel system cleaning. Our team understands the urgency and will have you back on the road as quickly as possible.

Call us immediately on 0800 769 000 — we're here around the clock to help get you moving again.