Q&A

 Master

First off check your coolant level "a strange sweet burn type smell hard to describe" could be burning antifreeze, antifreeze being burned in the exhaust which could be a sign of a mildly blown head gasket this would be accompanied by a drop in the coolant level in the radiator and or coolant expansion tank. A leaking heater core behind the dash could also allow you to smell that strange sweet kind of smell. Pressurizing the cooling system and doing a block check to check for the presence of carbon monoxide in the cooling system will rule out head gasket or cooling system heater core problems.
Low coolant level can effect the idle speed as well.
A vacuum leak can effect the idle and make the truck want to stall as well. Is the Service Engine Soon/Check Engine Light on? If so valuable information is stored in the on board computer in your car which needs to be retrieved using a scan tool.
I am anxious to find out what turns out to be the problem with your Pathfinder, it is hard to guess without being able to drive your truck first hand and perform some diagnostic tests.