Sale ends todayGet 30% off any course (excluding packages)

Ends in --- --- ---

Can someone please explain false lean?

General Tuning Discussion

Forum Posts



Tech Articles

Discuss all things tuning in this section. News, products, problems and results. 

= Resolved threads


Hi just wondering if some could explain false lean. I have a car that as I increase the load it slowly creeps round to rich then at around. 0.5 bar of boost the afr jumps to 21afr. I have increased the fuel and it doesn't seem to make much difference. For obvious reasons I don't want to lean the mixture out till I'm happy that it's rich.


False Lean can happen when there is a misfire and uncombusted oxygen passes through the cylinder and is detected by the O2 sensor. Is your engine misfiring? Are you in a boost-cut situation that would cut spark?

Could you be running out of injector? What is the Injector Duty Cycle?

Finally, since it "jumps" to 21 AFR (which is close to the lean limit on many Wideband controllers), could this be a wideband fault that is expressed as a specific voltage output? Do you have any wideband diagnostics you can look at when this occurs?

I'm pretty sure I haven't got a misfire the vehicle runs really well and is giving good results on the dyno. I don't have any wideband diagnostics but I have changed the afr sensor for a brand new bosch sensor and getting the same reading.

So a false lean is more so if you had say a faulty injector that say just wasn't working and the air entering the engine goes straight out the exhaust diluting the fuel mixture of the other cylinders. Giving a leaner mixture in the exhaust not necessarily in the cylinder.

As David said - the lambda reads the amount of free oxygen in the exhaust gasses, if the engine misfires that free oxygen hasn't reacted with the fuel and is till present in the exhaust, along with the unburned fuel. It is possible to get into a rich mis-fire condition but it is very uncommon as it needs a REALLY rich mixture. It isn't a 'lean' mixture in the normal sense - this is one of the reasons a 4/5 gas analyser is important for dyno' operators to use, and understand, as a misfire would also show elevated HC levels.

From your description it sounds more like a mapping problem where the lambda is going out of range on the mapping software, and defaulting to that value AFR. If you were running out of fuelling capacity, I would expect it to get progressively leaner, and the fuel pressure to drop, as the demand increased as increasing the injector open duration would have negligible affect.