pre-cats

[blackdog]

hi, is there a mod a person can do to get rid of the pre-cats, i need to put in a another engine and don't want the same thing to happen again with the oil consumption. a nissan tech admitted to me that oil consumption due to pre-cats is a known problem, nissan plans to do nothing

[njjoe]

bd-

You could drill them out, but you would end up with error codes from the downstream O2 sensors. Take the sensors out of the loop and the engine management computer will not be able to properly manage the air/fuel ratio, and you'll still have a ton of error codes.

Is the problem a known problem? Yes. Does it affect 99.999% of the MOs? No. You were one of the 0.001 percenters.

This is not just a MO or Nissan problem. Precats need to be close to the combustion changer to operate efficiently. Their close proximity to the exhaust ports makes it possible (not probable) for debris from a defective cat to ride the pulse waves back into the combustion chamber where they can damage the rings and cylinder walls.

Again, you experienced a very rare occurrence.

-njjoe

[blackdog]

yeah, my local dealership knows of three locally as well as myself , it is growing.

[blackdog]

just thinking, if this problem is caused by pre-cats then when i replace the engine i will have to replace the pre-cats as well, this is getting costly.

[njjoe]

Originally Posted by blackdog

just thinking, if this problem is caused by pre-cats then when i replace the engine i will have to replace the pre-cats as well, this is getting costly.

Definitely.

When a catalytic converter fails, depending on the type of failure, the substrate can become loose and travel into the combustion chamber where it can play hell with the sealing surfaces.

Out of curiosity, why do you assume the pre-cat caused the oil consumption problem? Usually when a pre-cat fails you will get an error code because the O2 will be out of whack.

-njjoe

[blackdog]

yes you are right why do i think it's the pre-cat ? why would my engine start using a liter every 1000 km's when i have honestly done all maintenance and not abused it.

[Pilgrim]

There have been some discussions here in which it was asserted that loose material from failing pre-cats was pulled back into the motor and the resulting abrasion caused excessive oil consumption.

This is something that more than one person has reported, but it's the first time in 40 years of working on cars that I've ever heard of such a thing. If it's true, then as far as I'm concerned it's a major design flaw to put the pre-cats close enough to the exhaust that this can happen.

[njjoe]

bd-

Just curious... Other than the increased oil consumption has your engine given you any other trouble? Did the SES throw any codes? Compression test show anything out of the ordinary? Blows blue smoke at start-up?

-njjoe

[blackdog]

blows a little blue at start up but only if revved up. no check engine light.
had someone follow me all over town , up hills etc, no blue no leaks , comp test is hard to do and you have to remove a manifold.

[njjoe]

Originally Posted by Pilgrim

If it's true, then as far as I'm concerned it's a major design flaw to put the pre-cats close enough to the exhaust that this can happen.

In order for three-way catalytic converters (pre-cats) to function efficiently they need to heat up quickly and remain as hot as possible. If you were to put a three-way cat at the end of your exhaust pipe it would not function properly because the exhaust gases will have cooled somewhat.

By placing the cat immediately downstream of the exhaust manifold it is assured of being fed the hottest stream possible.

When I first read about a cat's substrate being sucked into an engine I thought it was pure bunk. Somewhere in the archives are several of my posts were I (unsuccessfully) argued with Homer (hfelknor) about the validity of this phenomenon. I just could not see how the substrate could defy gravity, travel against the fast-moving exhaust stream and enter the combustion chamber. Homer strongly debated () the point and set me straight. He was right, as usual, and clearly explained how the substrate could travel into the combustion chamber by riding the exhaust's pressure waves, similar to how an expansion chamber is used to send a pressure pulse into a two-cycle engine to aid compression.

I miss Homer. He has likely forgotten more about automotive theory and troubleshooting than I will ever know.

-njjoe

[Pilgrim]

Yes, I read that discussion. It still seems to me like an engineer/designer with that in mind could have found a way to prevent it from happening.

[nafddur]

Originally Posted by Pilgrim

Yes, I read that discussion. It still seems to me like an engineer/designer with that in mind could have found a way to prevent it from happening.

You're right, Pilgrim. This post of yours and Joe's previous one got me to thinking about my old kart racing days. Way back in the mid 60's, the 2-stroke engines (mostly McCullochs) switched from noise filtering mufflers bolted onto the exhaust port, to expansion chambers. Those tuned exhausts (as we used to refer to them) not only attenuated the noise from those engines, but could also be moved relative to the exhaust port to help scavenge the exhaust - thereby improving engine performance.

Thinking about this principle, it's hard to believe that in depth engineering of the Mos Y-pipe, and more specifically the size and positioning of the 2 cats in this pipe, couldn't preclude sucking pieces back into the exhaust ports.