Car Mechanics (UK)

Neg­a­tive fuel trim

- Chris Tat­ton Iceland · Austria · Belgium · Belarus

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.

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