Negative fuel trim

My 2008 Vectra C with the Z18XER 1.8 VVT engine has an issue with persistent negative fuel trims, both long-term and short-term. When driving around town, cruising up to around 50mph and idling, the long-term trim drops to around –9 with the short-term fluctuating between 0 and –10. It rarely goes positive, but if you increase the cruise speed to 70mph, the LTFT gradually rises up to –2, with the ST flicking between +3 and –3 in time with the front O2 sensor.

The front O2 sensor swings high and low, however the voltage only goes between 0.3-0.7; it doesn’t do the full 0.1-0.9, but I’m not sure if this is correct. The post-cat 02 sensor reads a steady 0.8, which does suggest a rich mixture. It’s possibly also worth noting that both O2 sensors react instantly to a fuel cut on the overrun, then rise again once fuelling is restored. The MAF is reading around 2-3g/sec at idle and rises relative to the engine speed and throttle position.

I’ve read that this engine can have issues with the PCV valve splitting, resulting in excessive crankcase vacuum and oil carrying over, but there is only a slight suction if you remove the oil cap, so it seems fine.

With the exception of the manifold and catalytic converter, the full exhaust system was replaced a couple of months ago, so a blocked exhaust shouldn’t be an issue. As for the VVT system, there was a ‘P0011’ code thrown up a few weeks ago and I replaced the inlet timing solenoid, which cleared the fault; the system is now quiet and rattle-free. The car runs well enough apart from an occasional hiccup, almost like a slight misfire on light acceleration; full throttle acceleration is smooth.

A short-term fuel trim figure will fluctuate and this is normal. The long-term fuel trim should normally stay around the 0 mark, indicating that fuel adjustment is not needed to maintain the optimum 14.7:1 fuel air mix. A negative fuel trim indicates that the mixture sensed by the 02 sensor is slightly rich, so the ECU is reducing fuelling to maintain the optimum air fuel ratio. If the issue were the PCV valve, the mixture would weaken and the fuel trim would become a positive number as the ECU attempted overcome the excess air intake.

Normally, fuel trim levels that remain in single figures, either negative or positive, are acceptable, but you are on the edge with the –9 reading. This does indicate a slight overfuelling and I suspect that the injectors may be becoming worn and allowing slightly more fuel into the cylinder than would be desired. This could account for the slight misfire/hiccupping you mention and may indicate that just one of the injectors needs attention. It may be worth having the injectors flow-tested and serviced. This can be done by mrinjectoruk.co.uk for £17.50 per injector.

Another possibility is that the fuel rail pressure is too high, possibly due to a regulator problem. The fuel pressure figure I have for your Vectra is 3.8 Bar.