MPG and rattling 97Q 113k

[SardarQ]

2 questions:

1. I have a rattling sound coming from underneath the car at exactly between 650-800 RPM. I'm guessing its the under body cover?? Its a metal rattling sound. Is there any metallic sheild that is under the car? Only happens in that RPM range. Very annoying. Someone lese must have had this problem - please share!

2. MPG in the winter is very low - not becuase of fuel (oxyginate, etc.); I think this has to do with the EGR temp sensor. Does the EGR temp sensor tell the EGR value to bypass EGR function at low temperatures - this would explain the low MPG. The big question - if this is so - WHY? Why cut off EGR at low temps?

CJ

[Q45tech]

EGR is only operative after coolant temp warm up [165-174F] at a steady cruise and very very mild acceleration.

At best EGR can improve efficiency [reduce almost closed throttle pumping loss] by 5-7% ............mpg improvement.

The temperature sensor is there to feed ecu data to set MIL light if clogged [no egr flow].......thus cool sensor even if valve is open.

Common for Nissan thermostats to need replacing in 4-5 years as the coolant leaks around thermostat seal and results in a cooler coolant temp.

ODB2 scanner will read coolant temp.

[bullittandy]

The rattle is likely the exhaust system, the pipes are covered with heat?/noise? shields underneath the passenger compartment.