NF Mod/Nissan Master Tech
Joined: Jan 2006
Location: Prospect, VA
Ok...let's start with the evaporative emission issue. First, when you remove the gas cap, you are release pressure out of the tank, not letting air into the tank. A lot of people get this wrong. The gas cap has a pressure vent and anything more than 0.5 psi is released. Gasonline vaporizes and causes the buildup of pressure; the evaporative emissions canister is supposed to store the fuel vapors until they can be later introduced back into the engine and be burned through the normal combustion process. It would help if we knew what the code was, because it would be key to the diagnosis. The system will need to be diagnosed to properly determine what the exact cause is. This can involve scantool testing with a Mity-vac tool, or a smoke machine. That said, a lot of problems are often due to a bad canister vent control valve, which bolts to the evap canister. They are normally "open" to vent the system and commanded "closed" by the ECM to test the system for leaks. They often stick and the valve has been updated several times over the years. Again, without testing the system properly per the Nissan service manual procedure and using the proper equipment, it's impossible to say if this is your problem or not, but it won't cause your engine to smoke or burn oil.
The issue with the catalytic converter is that the ECM would run the engine in such a way that it would cause the upstream converter catalyst to break down. The engine used the variable valve timing for EGR as opposed to a conventional EGR valve type system. It doesn this by slightly opening the exhaust valves during the intake stroke to introduce inert exhaust gas into the combustion chamber. If the catalyst substrate breaks down, bit of it can get sucked into the combustion chamber and score the cylinder walls, creating a poor sealing surface for the rings. The recall covered a number of fixes that would vary to the individual vehicle based on testing results. In general, they reprogrammed the ECM with updated programming, added heat shields to the converter and exhaust and performed a compression test. If the compression test failed, then a new short block was installed, and later, a long block assy. was installed. Also, in these cases, the exhaust manifold/catalytic converter assy. was replaced.
For you, I would recommend a compression test and cylinder leakdown test. Not only do you have to be concerned about the ability of the rings to seal, which could be caused by the converter issue, but also of a blown head gasket between a combustion champer and oil channel. Head gaskets are not an unheard of problem with the QR25DE and a failure to an oil channel would explain smoke and why the problem occured suddenly. I'd also be suspect of the aftermarket valve cover, as the PCV baffle is part of the cover and if defective, could cause excessive oil to be sucked through the PCV valve into the engine, where it will be burned and cause smoke. I would also smell the oil on the dipstick for fuel odor, as gas contaminated oil will be thinner and cause smoking, as well.