Split-second surging

I have a 2009 Ford Mondeo 2.0 petrol with 70,000 miles. Following a major service two years ago, the engine occasionally seems to surge for a split-second – this is most noticeable when cruising. Acceleration and cruising is otherwise fine. This surging occurs randomly, sometimes a couple of times over 20 miles and then not again for the next 50 miles. It is more noticeable in cold weather; in warm weather it seems to disappear.

At first I wondered if it was a sticking throttle, but I understand this car has an electronic throttle. The car is regularly serviced, but as the fault is so random and infrequent it never occurs when it’s at the garage and the car runs fine otherwise. Any thoughts about what the problem could be? G Marriott

The problem may be the throttle body, which incorporates the butterfly and control motor. Sometimes the butterfly will stick slightly on the spindle, meaning the throttle will not react as it should.

By removing the induction pipe from the air filter to the inlet manifold, you should be able to see the electronicallycontrolled butterfly. I would spray this with a carb cleaner – such as this one

http://bit.ly/2jbiiyy – to dislodge any carbon deposits and free up the operation of the spindle.

The other possibility is that the surging is actually a slight misfire. As this happened not long after the last service, I would check that you do not have a spark plug or coil pack that is failing or poorly connected.

Even though you have no EML showing, if you can gain access to a scanner then you may find that a fault has been logged in the ECU, which will guide you to the source of the problem.