SIMPLE FIXES

Fuel system cleaner

Modern petrol is generally formulated with a ‘detergent pack’ to keep injection systems clean. If your engine proves marginal on the MOT emissions test, pouring a bottle of proprietary cleaner into half a tank of petrol can improve injector performance.

Wiring connectors

Poor running and apparent sensor faults are often simply due to bad electrical connections. Remove suspect clipped connectors and clean with electrical contact spray. Make sure the connectors’ rubber seals are in good condition before refitting.

Air filter

A dirty or oily air filter will impede airflow. The ECU will adjust the mixture to compensate, but the engine will be down on power and emissions may increase. Change it at the intervals specified in your workshop manual, or sooner if it’s visibly dirty.

DME Relay

The DME (Digital Motor Electronics) relay controls the power supply to the ECU and sensors. Failure to start or erratic misfires can often be traced to the DME relay. It’s in the fusebox and it’s easy to replace – so if in doubt try fitting a new one.

Spark plugs

Similarly, old spark plugs will reduce engine power and increase emissions. Fit good quality replacement plugs at the specified intervals.

Take great care not to damage the coil packs or their wiring when you’re extracting the plugs.

ECU

The ECU is behind a cover on the bulkhead of this BMW, but it’s also commonly found in the passenger compartment. Moisture will damage its sophisticated circuitry, so ensure it’s dry. Testing or repairing a faulty ECU is a job best left to specialists.