Frankly, about a thousand different things. Poor idling and performance can be down to the ignition system, fuel system or mechanical wear or problems anywhere within the engine. And within those three things can lurk potentially thousands more reasons for poor running.

A mechanic familiar with this make and model is your best bet. They will probably start with the basics such as fuel supply and quality, spark strength and timing and a compression test to make sure all is well within the engine itself. This is a case where it’s important to go back to first principles of fuel, spark, compression and then start to look at the more intricate systems that control the engine.

Honestly, though, it could be anything from a dud sensor in the intake system, the same problem with the cooling system (not allowing the engine to reach full operating temperature) or even something mechanically fundamental like a blocked catalytic converter, dirty filter or slipped camshaft timing. Or a multitude of things in between. A good workshop will be able to sort it, however, without simply swapping parts that may or may not be the problem.

The trade-in valuation you are likely to get is between $5500 and $10,500, depending on the model, condition, and odometer reading. There are no rules to say how much more you might get if you were to buy three cars.

Stick to the well-proven Japanese brands, such as the Mazda 6, Honda Accord, and perhaps the Korean Kia Cerato. As a private sale your Mazda would be worth $11,000-$12,000, as a trade-in $8000-$10,000.