Postby smj999smj » Mon Dec 02, 2013 10:42 pm
That usually a bad sign. Assuming the engine hasn't been neglected as far as oil changes, the first thing to check would be the PCV system. The PCV is on the right bank valve cover. Make sure the valve is not stuck and the hose is connected and not collapsed. A PCV issue would be the best case scenario, as it is cheap and easy to fix. The other two issues that sometimes arise on these engines usually require replacement of the engine.
The VQ40DE does not use a conventional EGR valve-type system. Rather, it uses the variable camshaft timing to crack open the exhaust valves slightly during an intake stroke to accomplish exhaust gas recirculation. As the exhaust valve is opened during the intake stroke, exhaust gas is pulled directly from the exhaust system, past the exhaust valve and into the combustion chamber, where is mixes with the fresh air that passes by the intake valve into the combustion chamber. As many Altima 2.5L owners know, this can lead to problems if the catalytic converter starts to break down. What happens is that particles of the catalytic converter substrate get sucked into the combustion chamber along with the exhaust gas and the score the cylinder walls until they create a poor sealing condition between the cylinder walls and the piston rings. The fix is replacing one or both of the upstream catalytic converters and replacing the engine or short-block assy., or overhauling the engine. Used VQ40DE engines usually show up on Ebay for around $2500.
The other issue with VQ-engines is something I think doesn't affect the 4.0L and is more of a problem with the earlier Pathfinders with the VQ35DE engine. The VQ engines apparently have a very slight taper to the cylinder bores. The oil control rings would stick in the piston grooves and not seal during the full stroke of the piston and cause an oil burning issue. From what I understand, Nissan re-designed the ring set to prevent this from happening. The problem, of course, is that you need to teardown the entire engine to re-ring it. I doubt that this is your problem because I believe this issue was addressed before the VQ40DE production.
Leaking valve seals could cause oil burning, but I haven't heard of any cases of bad valve seals on VQ-series engines. If the PCV system is working, then a compression test would be in order...and cylinder leakdown test, if needed.