This BMW 325i (E90 N53 B30A) came to the workshop with a rather unusual emissions failure. It had failed the NCT emissions test, with a very high level on the Lambda reading.

The first step taken was to scan the engine management system for fault codes, but no codes were stored. The data for the emissions system was analysed to check for lambda sensors and the plausibility of engine sensor information. All the data for engine temperature and mass air flow, tested as normal.

As the emissions are monitored by signal output from both upstream and downstream lambda sensors, these where the focus in the next steps. This vehicle is fitted with 4 sensors, 2 upstream wideband sensors and 2 downstream narrowband sensors. The best practise is to monitor the output of these sensors, and to also utilise EOBD data, as it is focused on the emissions system.

The upstream sensor data is monitored in microamps. Either a positive or negative reading, on either side of zero. Zero micro amps is equivalent to a Lambda of 1. On the EOBD data, there is a PID for equivalency ratio. This reading is equal to Lambda. Monitoring this EQ reading will indicate the mixture level before the catalyst. We made certain the system at idle, was in closed loop, and controlling the fuelling correctly. The measured EQ was running slightly lean, with a figure of 1.03, and there were no running issues.

The rear sensors were analogue, with an output DC voltage between 0.1 volts to 0.9 volts. At idle, the rear sensors were putting out the expected voltage of 0.75 volts, indicating there were no issues.

But the moment you raised the engine above idle, the system went into open loop. At the same time, the front sensor EQ ratio went off the scale (1.99), and the rear sensors dropped below 0.2 volts, indicating a lean mixture.

The 4-gas analyser also indicated a failure of the lambda reading and the O2 level failed high, and CO2 dropped very low, 7%.

Next, we chased the lean condition problem by checking the fuel system. Fuel pressure and fuel flow measurements came back perfect. MAF signals were checked against known good values, a MAF Calculator was used to confirm air consumption figures.

As a last check, we checked the injector opening duration on acceleration. The values increased from 3.7 ms @ idle to above 12 ms on acceleration. This indicated that the mixture enrichment was normal.

Everything that was checked was found to be in proper working order and within normal ranges.

The workshop commented on the condition of the vehicle. They said it looked like it had stood for a long time. The customer had failed to mention it had been a recent import, and the car had been unused for at least 6 months. With this new information, we suspected the fuel might be causing the problem, even though it started and drove normal.

The fuel system was drained completely, and a fuel additive was added along with 20 litres of E5 petrol. The results came back with perfect mixture control, and the emissions passed with Lambda 1 low Oxygen level 0.2 % and a high CO2 figure of 15% on the analyser.

Conclusion
Poor quality or degraded fuel is not always going to log faults within the system, and may not create any drivability issues.