Engine pops on overrun

We have recently bought a 2009 MINI Cooper S 1598cc. It’s a little firm on the suspension, but it goes well. The only problem is that the engine ‘pops’ on the overrun, especially when you are accelerating up through the gears. When you lift off the accelerator to change up, the ‘popping’ is quite pronounced.

I was told by a colleague that it could be due to a manifold leak, but it has been back to the main dealer where we bought it and they couldn’t find an induction leak. Any suggestions as to what I should look for? I am quite handy with the spanners, but I’m not familiar with MINIS. James Harrower The popping is a common feature of the Cooper S and some owners seem to find it quite agreeable. It is caused by over-fuelling, which allows unburned fuel into the exhaust, which then ignites to give the popping sound.

This is really a performance feature. Once the cylinder has fired and the exhaust stroke is completed, the cylinder is then ready to receive the fresh air/ fuel mixture for the next stroke. The inlet valve opens, but the exhaust valve also remains open momentarily (valve overlap) to allow the fresh mixture to flow through. The velocity of the exiting exhaust gases will draw through the fresh mixture, which allows slight wastage but increases the possibility of a fully-charged combustion chamber, increasing performance. Although the injector won’t begin to inject the fuel until the exhaust valve is closed, the residue in the manifold from the previous injection allows the combustible mixture through the system.

There was a software update in 2010 to address the issue and it may be worth checking with your dealer if your MINI is on the latest version. Provided all other systems are operating correctly, this is a feature, not a problem!