Quote:
Originally Posted by welfare
from what i've researched, this issue can often times be attributed to the close location of the pre-cats. apparently on start up the valve overlap allows tiny bits of the substrate to enter the cylinders through the exhaust. this causes severe wear to the rings and excessive cylinder taper. not just a problem with the 3.5, but also the 2.5 i4. makes transferring a used engine quite the gamble, since a lot of them out there have this problem.
i had this issue with my murano. luckily i found a very low k engine to swap in. deleted the cats for insurance. they had failed prior to this. which can also be a good indicator that this may be the cause of your oil consumption.
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On the QR25, there is no EGR, so what they did is timed the exhaust valves to open slightly during the intake cycle which sucks back some of the exhaust gases to reduce emissions.
Like you said, the pre-cat is too close to the motor and this intake cycle sucks back in some of the cat material which damages the cylinder walls and causes the motor to burn oil.
My motor failed but I was lucky enough to have it covered under warranty. Once I got the new motor, I drove the car home and pulled off the stock exhaust manifold and installed one with a gutted pre-cat.
When I removed the stock manifold, you could clearly see all the fine powder from the cat that was ready to be sucked back into the motor.