mpg down by half ?

Why would you change the Lambda sensor ??

Sounds as if the timing chain might be one tooth out on one of the cogs, thus putting the valve timing out. The engine would then run inefficiently. Check that the new chain has been fitted correctly and that the valve timing is perfect. It probably won't tick over perfectly either.

ChicksFan
December 2010

You dont state which vehicle you have-this happened to my Renault and was easily fixed. I changed the plugs and used a different make-the consumption went through the floor-changed to renault recommended and it rectified itself.
Hope this helps.

Steve Walton
December 2010