First off, I suggest making a fuel log. Next I would get a multimeter and with the help of the helms manual, check the ohms value of each ground to the negative battery terminal. If the ground exceeds more than 1 ohm, then you should clean/repair the ground until it gets down to about 0 ohms. One other ground you'll quickly overlook is the o2 sensor ground which is the exhaust manifold. To check this ground, use one probe to body of o2 sensor and another probe to the battery negative post, if it exceeds 0 ohms, then you must clean either the hole the o2 sensor screws into (rust) and or clean other electrical grounds around the engine bay such as the alternator ground, etc.
Next thing I suggest is replacing the PCV valve. There may be some guides online that say the VX does not have a PCV valve but it in fact does. The PCV valve is either located in a black box attached to the back of the engine called a 'breather chamber' where a vacuum hose connects to the intake manifold or... You start in that same place and if you find instead a hollow tube, follow the vacuum hose from that black breather chamber all the way to the intake manifold and you should find the PCV valve. The PCV valve is a one way check valve that may be encrusted in dirt/oil.
Third idea: Replace all vacuum hoses. My guess is on a car of its age, they're starting to crack and go bad so you should replace them. They're suppose to be dirt cheap to replace and are good insurance in case one does finally give up the ghost. Replace the hoses one at a time so you don't get confused.
Fourth idea: Clean EGR chamber... (I'm going to stop with the details unless you ask for them)
Fifth idea: Clean IACV and Throttle body (if you doesn't need a cleaning now, it will soon)
Sixth idea: Adjust valve lash back to factory specifications: This isn't an option but kind of mandatory if you're looking to keep your car for a while and not risk burning a valve. Valve lash being out of adjustment will negatively impact fuel economy.
Seventh idea: Give the car a solid tuneup if it hasn't had one recently. What ever one does in a tuneup (that includes PCV valve!) do that. I don't want to say what needs to be done just yet because I want you to consult your maintenance schedule in the owners manual to find out for yourself which you won't do if I just tell you!
If you replace any parts, stick with OEM or OEM equivalent. If given the option of Bosch vs Denso, always choose Denso! If you can't choose Denso/NGK/OEM equivalent, go somewhere else!!! Cars running like crap is commonly attributed to users using non OEM or equivalent parts! Bosch is fine on European cars and Denso on American and Japanese cars but mixing the two only leads to disaster on BOTH vehicles.
Finally: Read this entire thread:
http://www.gassavers.org/showthread.php?t=12045
Guy had similar complaints to yours about poor fuel economy in his VX but after working with him for a while, now his fuel economy is at 67mpg!
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