Question of the week: Should I worry if I misfuel by a small amount?
Fri, 05 Sept 2025
Dear Honest John,
"I have a 2011 Mercedes C200 CDI. Yesterday, I accidentally put 0.57 litres of BP Ultimate Petrol in, thinking it was Ultimate Diesel. Pity they don't use a different name. I had to get home for family reasons so filled up with Ultimate diesel and drove home - 3 miles. Now uncertain what to do.
Three draining specialists have said the amount is too small to worry about, another says it must be drained or I'll regret it. The draining cost plus the cost of lost fuel is approaching £400, but by all accounts if damage is caused, the car - in excellent condition - would be a write-off.
0.57 litres is less than 1% of the tank capacity and doesn't seem that much and an unnecessary £400 would be unwise. What is your most up-to-date opinion?
- LH
Dear LH,
Any amount of incorrect fuel in an engine is bad news, but how bad the potential damage is depends on the type of diesel engine.
Older diesel engines are less sophisticated which presents means misfuelling is less of an issue, but common rail diesels which became the standard approach for majority of diesel engines from the late 1990s onwards are more sensitive to misfuelling.
That's because they use a high pressure fuel pump lubricated by diesel and solenoid injectors, both of which are designed specifically to work with the properties of diesel. Introduce petrol to these parts and they can seize or be damaged beyond repair.
The fact that you stopped the misfuel quickly and have diluted the petrol to less than 1% does reduce the potential for damage. A general rule of thumb is that contamination of petrol in diesel of 3% or less is an acceptable risk, but it is still a risk - ultimately you have to balance the cost of playing safe and having the fuel drained against the chances of engine damage which would be considerably more expensive.
