Fuel air / ratio dropping off sharpish??!!

[CaptiV8ed]

Took the Chim along to a TVR rolling road shoot out today in Cornwall (brilliant! )

I've had a hesitation / misfire at higher revs for a wee while, but today showed what's actually happening. At 4600 revs, the lambda value goes from a near constant and correct 0.83, then suddenly leaps to 0.9! It's not gradual either, it's like a switch being flicked. This shows that for some reason, the fuel supply is being reduced and weakening the mixture. You can really feel it on the road too - the acceleration falls away immediately.

Anyone got any ideas? For info, it's a '99 500, all std except for removal of precats.

Any help most gratefully received.

Nick

[GreenV8S]

If it maintains the same lambda across a range of throttle/revs then I would suspect this is caused by the map. Fuel starvation would be more likely to get worse and worse as the load and revs increase. Perhaps the modifications you've done have increased the air flow at the higher revs and it hasn't been remapped to suit? Lambda 0.9 doesn't sound catastrophically lean to me though.

[CaptiV8ed]

I'm guessing that the lambda change isn't the cause of the drop off, but an indication. It is almost like a soft cut rev limiter. This isn't some sort of limp home mode? Oh for my own super duper diagnostic machine... Oh yeah, and some knowledge to go with it!

[GreenV8S]

Is it possible you are picking up a slight misfire? This would show a lean reading but that would be the effect not the cause.