One of the easiest ways to check the timing chain "stretch" is to remove the valve cover & line up the marks on the cam and read the pointer on the crank pulley. The # of degrees it reads would be the "stretch". This has been covered repeatedly in previous threads so a search should bring up pictures & other goodies. Larry Bible will probably say it needs to be done with a dial indicator. However, many of us are getting useful #'s without using the dial indicator. Have a new valve cover gasket handy when you do this if the gasket hasn't been changed in a while. There is NO reason to change the chain just because your car has 230Kmi - unless you like spending money. Same with the rails. Diagnose the problem before throwing money at it. With fuel economy decreases, there are a lot of EXPENSIVE things that can be changed without fixing your problem. Its not like changing a $2 part to see if it helps. Also worthwhile to check pump timing. A restricted catalytic converter, dirty injectors, air intake issues, dragging brakes are also possibilities. Operating in the winter will reduce fuel economy. The fuel is lower BTU so it won't gell. Cars take longer to warm up (and they don't run as efficiency when cold). Fuel QUALITY also makes a big difference. In my MB's fuel economy can vary 20% based on station to station differences even in the same season. Even if your head gasket is blown you may not have any fuel economy issues with that.

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Brian Toscano