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Excess fuel consumption: Lambda sensors?

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Over the last three months or so my fuel consumption has deteriorated to about 20-22 mpg on runs and 13-15 mpg in town (from approx 25 and 20 respectively). In response to an illuminated MIL, my local dealer (Gordon Lamb, Sheffield) one defective lambda was replaced and according to OBD, all four are now switching, the ECM is working and the MIL is off - but consumption is high.

The engine has done 47,000 miles but three of the lambdas are original and have done 80,000. In spite of there being no apparent problem with them, should I replace these three or might there be other explanations?

Any advice would be much appreciated. I hope to be at Donington on Sunday.

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If I got as much as 22mpg I'd be well over the moon :)

I believe Lambda sensors are pretty consumable items with a life of about 30,000 miles {imo}

As Lambda sensors fail they underread, this causes the car to richen the fuel mixture. There does come a point when they fail completely at which point the car will throw a CEL.

On the v8 Esprit the front most Lambda's are used for fuel metering, the rearward 2 {post cat} only check that the cats are working, and this itself applies only to a US car, the code associated with these does not appear in the UK ECU.

So, if the garage changed a post cat sensor then it is probably worth changing both the fronts to see what happens. However, if they changed one of the fronts then I'd defo change the other one - and i'd find it somewhat odd that they didn't do the pair - but not bother with the 2 rear units as there will be no gain.


As for your mileage, I've had mine for about 18 months and have never got more than about 200 miles on a full tank, which equates to somewhere between 16-18 mpg.

In that time she's had;

New air filters

New HT leads

New Sparks

New Coils packs

New Lambda Sensors

New Cats

and a New exhaust

She now runs soooo smooth, I love her sooo much {we have cuddles each night} yet my fuel mileage in all that time has never budged an inch :lol:

I do have a somewhat entertaining right foot :P and have never had good fuel economy in any car I've ever owned :P

So I just wonder if mayb you've been having a little more fun with her recently?

hth :)

Hey, can anyone smell fuel?????

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Thanks Colin, this is helpful.

I take you point about the consumable nature of lambdas and their approximately 30,000-mile life.

It was the left hand pre-cat lambda that was changed, shortly after having both cats replaced. I'll explore the right hand pre-cat lambda route.

What struck me was the marked reduction in fuel economy; on a trip down to Exeter from Sheffield last November, with no change in driving style, fuel consumption plummeted to 15 mpg. Shortly after, the MIL illuminated and triggered the LH pre-cat lambda replacement.

Incidentally, at 70 mph, fuel consumption used to be a tad under 29 mpg; now its down to about 24. At 100 mph it was 22ish, now about 20. What puzzles me is that there is no apparent fault registered by OBD - but I will probably buy my own with a version that can track lambda responses.

Edited by Winter

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Excessive ? Surely you jest ?

I average 17mpg and don't give it another thought.

Right then, said Fred.

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