Upon receiving a customer complaint of a rough running vehicle once it gets to operating temperature, we meticulously inspected the car for any potential issues.

We checked the vehicle's fault codes, of which none were present, and carried out a relative compression test, all of which returned normal results.

Further investigation led us to examine the camshaft and crankshaft signals, as well as injection and spark signals from the ECU. Upon detecting an intermittent secondary fuel injection occurring at approximately 83 degrees, we analysed the spark high tension output and current draw on injectors. When the fault was present, the secondary injection pulse was half current draw.

We checked throttle position signal and found dirty signal out of Throttle position sensor when fault present, leading to additional fuel injections and erratic misfire. This issue coincided with erratic engine cooling fan operation and air conditioning operation.

We determined that the original earth cable from the battery to the engine was losing partial connectivity at 83 degrees engine temperature, which caused the throttle position sensor to not receive correct earth, thus sending incorrect information to the ECU, over-fuelling the engine, and causing erratic misfire.

We replaced the engine earth cable and checked the operation, finding that all issues had been resolved.