LS400 feel sluggish when cold

[NormLS400]

Hi, I have a 1999 LS400 Mk4... had it for about 5 years. Recently I have noticed that it feels rather sluggish when cold, but seems to improve when up to operating temp. Been like that since last summer actually. I also notice the fuel consumption has gone up. I wonder if anyone has have ideas what might be causing this? Spark plugs are fairly new still... not really had much done to the car of course... being a Lexus!! 

[Titch LS]

If your mpg figures have become worse for no apparent reason, the favourite culprit is the coolant temperature sensor. A faulty one fools the ECU into thinking that the engine is cold when it's hot so it keeps it running too rich and the mpg figures go down.  Fitting another one yourself is fairly straight forward and they don't cost much either. LS owners on the American forum also claim that changing this sensor restores performance somewhat. Makes me think there could be something it.

[NormLS400]

Many thanks, I will give that a try.