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A bad O2 sensor can certainly lead to poor fuel economy. It's located on the drivers side exhaust manifold of non-turbo cars, and on the turbo downpipe of turbo cars. Pick up a Haynes service manual for your car - it will become the best buy you ever made for your car. It will also aid in troubleshooting problems you may have.

If the timing belt has not been changed in the last 60,000 miles then change it. Also change the following (since they're easy to get to at the same time): cam and crank oil seals, water pump, thermostat, the 2 coolant hoses (one elbow shaped one between the distributor and fuel pressure regulator, and the short one that goes from the thermostat housing to the metal tube on the front of the engine), timing belt tensioner and the stud that holds it to the engine block, and the chts (cylinder head temp sensor).

If you have documentation that this (or at least the timing belt) was done recently, then you're ok. It's recommended to change the t-belt every 60,000 miles. I would recommend changing it also if it's over 7 years old (rubber doesn't last forever).

Call 1-800 NISSAN1 with your VIN number and verify that the fuel injector campaign has been done. This is VERY important, the injectors leak and your car will BURN! No joke, a good friend of mine lost a good Maxima due to this recall - burned to the ground in his driveway. (Maxima's of that year have the same VG30 engine as a Z).

Cold start problems can also be related to the air regulator. It's on the pass side of the upper intake - kinda horn shaped with a 2-wire connector plugged into it. The kind of connector that has the metal clip holding it on (similar to the connectors on the fuel injectors). Check for corrosion on the connector and regulator. There are other things that could cause your problem but those are the ones I would check first. A simple tune up might also solve your problem. Start with a good set of NGK spark plugs, new spark plug wires, oil and filter change, and an air filter change. Use a timing light and check the timing.

Start with the simplest things first - like cleaning electrical contacts, and making sure things are actually plugged in that are supposed to be. That kind of stuff is free! Then move to
stuff that requires more.

And get the Haynes manual (I like it far better than Chiltons, and I had been a long time Chiltons fan....). If you REALLY wanna spend some money, order a Factory Service Manual from http://zcarcreations.com or watch for one on Ebay - they show up fairly often.

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autozone carries haynes, o'reilly's doesnt tho, not sure bout napa or carquest or pep-boys.

they run around 15.00 each. u can find them on ebay too, but not much cheaper.