Accelerated Ring Wear 4

[Elihu]

A local Nissan dealer just told a lady friend that her 2005 V-6 w/96000 miles had suddenly begun to burn oil at a high rate because the catalytic converters were disintegrating. They claimed that when she went downhill and lifted her foot the pieces were being drawn up into the combustion chamber causing the sudden ring failure. They’d done all of her service work since purchase - so she believed them.

I need some replies to help her see the light.

 

[Tmoose]

Elihu's last login was on Wednesday, May 21, 2014. I guess I'll never learn what excessive oil consumption was.

 

[Elihu]

Thank you all for your comments on this problem. For those fixated on the rate of oil burning, the car is burning a quart in 500 miles. Others asked if the car was smoking. No, modern (cat-equipped) cars don’t smoke when they burn oil. This oil consumption is unacceptable for a dealer-maintained car with 90,000 miles, but that wasn’t the question. The car’s oil consumption changed from 1200-1500 miles/Qt. to 500 miles/Qt. in less than 3000 miles. This engine was suddenly self-destructing, and my initial post gives the Nissan dealer’s explanation. Could Nissan be telling the truth?

Turns out that they were.

I’m an old car guy, got my BME in ’61. Later on I was the lab instructor in an IC engine course while in grad school. Cats weren’t invented until decades later – and they went down under the car with the muffler. My (obsolete) common sense was that the Nissan dealer’s story was nonsense.

I knew nothing.

The pre-cats are practically inside the exhaust manifold, to get them hot ASAP. (Yes, there’s another down under the car too.) Numerous posters had it right – Nissan used a large valve overlap in their emission control in lieu of the usual EGR valve strategy. Periods of sustained high intake manifold vacuum results in exhaust gas being drawn back into the cylinder. When the cat begins to fail (as they all do eventually), bits of the media (and perhaps the baffling itself) enter the cylinder with disastrous results.

This unfortunate lady lives 35 miles away from her Santa Barbara dealer (who is at sea level). Her house is at ~500’ above sea level – but in between is a 3000 foot mountain. Each trip into town provided a steep, fast descent with her foot off the gas, and the car rolling up to 80 mph. It was likely years of making these descents that contributed to her trouble. Worse, the disintegrating cat still worked, so the oxygen sensor did not detect any problem. Her motor is now destroyed, yet she still doesn’t have a check engine light!

Nissan is apologetic, but otherwise unhelpful. She’s car shopping now, for other brands.

 

[tbuelna]

The pre-cats are sometimes located close to the exhaust ports, and the ceramic matrix they use can begin to break down over time. But personally I find it hard to believe that debris from the ceramic matrix could work its way back up the exhaust manifold, past the exhaust valves and into the cylinder. If this was indeed occurring, it would be easy to diagnose with an analysis of the engine oil. The oil would contain particles of the ceramic matrix material from the pre-cat.