'I'm a car expert - this is one thing drivers must check before buying a used car'

A leading motoring expert has urged second-hand petrol and diesel car buyers to always follow one simple check before agreeing on a deal on a vehicle. YouTuber ‘ScottishCarClan’ said one sign underneath the cabinet can tell all road users if a vehicle is on the verge of developing “serious engine problems”.

It’s possible that a creamy “mayonnaise-like” substance may form under the car oil cap of second-hand models. This should be a major red flag for those considering buying a second-hand vehicle and could force some to reconsider going ahead with the purchase. ‘ScottishCarClan’ stressed road users should “always” look for the issue before handing over their cash.

He said: "Here's one thing you should always check when buying a used car. You should always remove the oil cap and have a look inside it for any gunk or what looks like mayonnaise. If you see that in there, it could suggest some serious engine problems like failure.”

The car mayonnaise refers to a creamy, slightly off-white substance that can form underneath the engine oil cap or in the oil system. This is a mixture of water and oil and is often caused by the emergence of condensation in the engine bay.

It’s also one of the tell-tale signs of head gasket failure which could be a serious blow for those taking on a second-hand vehicle.

The RAC explained: “As well as checking the oil level, it’s worth looking under the oil cap for signs of a white mayonnaise-like substance.

“This could be caused by condensation but is usually created by coolant mixing with oil - a sign that the head gasket could have failed. Also check the coolant expansion tank looks clean.”

A head gasket is a seal located in the car’s engine bay designed to prevent oil and coolant leaks under the bonnet.

The tool ensures the engine runs smoothly and stops the component from getting too hot and potentially overheating.

The RAC has previously warned that a blown head gasket is a “very serious problem” that could set road users back hundreds of pounds.

The average fee for replacing the part is around £662, although specialists have admitted that fees will vary depending on the make and model affected.

The RAC added: “If you suspect your head gasket is damaged or faulty, have it inspected by a trusted local garage before it results in engine damage.”