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This is very close to Lambda = 1, William - which is indeed as it should be, and the basic target for engine management systems…
The more common mixture adjustments relates to CO levels - nominally set to 0,5 - 1,5%. The main reason for setting the engines fat, particularly the xk, relates to difficulties, with the rather primitive engine management system, of avoiding lean running under all circumstances.
Apart from other drawbacks; lean running means loss of power - while running fat, up to a point, has no negative effects on performance. Adding, as Robin implies, that excess fuelling contributes to engine cooling.
Be aware, that detonation is residual fuel being spontaneously ignited by combustion temps, due to too early ignition for the fuel octane used, and the compression of the engine. The xk engine was designed to use 98 octane, and the original timing specs reflects this. When ‘pinking’ is experienced the prescribed remedy is to back off the ignition timing. This basically detunes the engine from optimum power - but so be it…
Running lean may lead to inlet backfiring due to inconsistent ignition of a lean mixture - the remedial action, when experienced is to fatten the mixture. The two symptoms are different, and have different causes - and different solutions…
All that said; the mixture is adjustable; on the xk this is done by the AFM bypass - you do not mention if this has been tried? As the range of this adjustment is limited; if the desired CO level cannot be acquired by bypass adjustment - the AFM can be opened and the flap spring reset…
The ECU bases the mixture computations on AFM flap movements - the program is otherwise fixed; there are no internal adjustments.
However, the Lambda system, when fitted, provides feedback to the ECU to alter mixture based on O2 readings - basically striving to maintain Lamba = 1. The CO adjustment must be done with the O2 sensor disconnected - properly set up; the engine runs perfectly without the Lambda…
An O2 sensor has two readings; oxygen present (lean mixture) or oxygen absent (fat mixture) - it can not read actual mixture. The Lambda system simply forces the ECU to alternate between fat and lean running - to average Lambda = 1. But the system is basically disabled during pedal movement and high loads - to prevent lean running…
Frank
xj6 85 Sov Europe (UK/NZ)
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