Nissan Technical Advisor 10/14/2010Probably you have a bad fuel pressure regulator and it's best to have it checked, and if replacement is needed. Or, your oxygen sensor is giving a lean mixture input to the ECU resulting to pump an increased fuel mixture to the combustion. It is also possible that your air filter is blocked; restraining sufficient air flow to the combustion, and thus it causes a rich mixture. Check also your catalytic converter it may be choked resulting for increased backpressure that can strangle engine performance, particularly at highway speeds. Fuel economy may also be down.