Jerking on acceleration

Q I have a 2007 Mazda6 1.8 petrol saloon. I bought it five years ago with 18,000 miles and it now has 48,000 miles. The previous owner worked for the Mazda dealer I bought it from. Most of my driving has been on motorways, where it has returned around 40-45mpg, and I am mechanically sympathetic in how I drive it. It is serviced annually by a local garage and the last service was about 3000 miles ago. Three months ago I fitted a towbar and electric socket to tow a small trailer. The car has proven reliable and economical, and other than a flat battery a year ago, it hasn't given any problems.

A few weeks ago, however, it started juddering and jerking on acceleration within a specific rev range. This continued when the engine had warmed-up. The jerking is very pronounced, but only occurs between 2500-3500rpm and only if I accelerate normally. It is most apparent in second, third and fourth gears. I filled up with fresh fuel, which made no difference, and putting Redex in the tank had no effect. My local garage tried replacing the spark plugs, to no avail.

I then bought a new coil on plug – €100 from Mazda – which the local garage fitted to each spark plug, but it made no difference. They said there were no codes showing on their computer readout and there is no dash light illuminated. On their suggestion, I took it back to the Mazda dealer, which ran a diagnostics check and again no codes showed. The Mazda dealer said they had taken out the fuel pump and cleaned it, and had tried replacing the mass air-flow sensor (€300), but this didn’t cure the problem. The Mazda dealer suggested the next stage would be to replace the fuel pump/pressure regulator assembly.

Replacing parts bit by bit is getting very expensive, so currently I am driving around the problem by either short-shifting before 2500rpm or booting it through the 2500-3500rpm range; at anything over 3500rpm, the engine pulls smoothly and strongly. Sitting on the motorway with a steady throttle at any revs doesn’t cause any jerkiness, cruise control works fine and the problem is not apparent at tickover or when the car is not under load – ie, increasing the revs while stationary.

The car isn’t worth much, but I like it and would like to fix it. Tim Byrne

A I would suspect the problem is fuel-related as opposed to spark-related, as any coil pack failure would normally result in a misfire throughout the engine rev range. There are a range of components that could be causing your problem, but I would check the 02 sensor before the cat, as this is the one that can control the mixture and may be giving incorrect information. This would not necessarily throw up a code, so might have been overlooked by the dealer. The 02 sensor can be checked using a scan tool and should have a regular pulse varying between 200-800mv in a regular and rapid pulse. Using the 500mv line as a datum, the pulse should rise above and below it in a regular pattern.

Another component that may be to blame is the knock sensor. This is located at the front centre of the engine and retards the ignition when it detects knocking (indicating pinking). A faulty knock sensor may be retarding the ignition unnecessarily. After ensuring the connecting plug is secure, a quick way to check the sensor is to tap the side of the engine block around the sensor with a metal object while the engine is running. As the sensor detects the knock you should notice a slight change in engine rpm; if no change is detected or if the engine speed changes and does not revert to its normal speed, the sensor is most likely defective.

I would also check the throttle body and throttle valve potentiometer. Again, this may not necessarily throw up a fault code, but should be easily checked with a scanner, ensuring that the response to the pressure on the pedal is even.

Cleaning the throttle body with a carb cleaner spray should also help ensure you have no residue causing the throttle butterfly to stick momentarily.