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#4450908 - 07/05/17 11:30 PM Exhaust Reversion as an Engine Threat.
Ducked Offline


Registered: 10/25/12
Posts: 4378
Loc: Taiwan
Originally Posted By: skyactiv
And if the compression is on the lower side, the cat may be slowly falling apart and pieces of it getting sucked into the engine via reversion, scratching the bores up.


Thought this might be worth its own thread, since its slightly OT in the original.

I wonder how likely this is.

Car in the OP is a Nissan Altima. Dunno what model but 2.5L engine was mentioned. This might be a picture of the upper exhaust manifold and catalyst (the squid-head thing).

http://i.ebayimg.com/images/i/131110816600-0-1/s-l1000.jpg

I assume this is a potential issue when both valves are open.

The manifold seems to be for a four stroke engine, so IF say both valves are open for 10% of the downstroke, it could potentially suck-back about 60 cc's. Though the catalyst is fairly close to the exhaust ports, the intervening volume looks to be more than that. If a failing catalyst was likely to wreck an engine that'd be a fairly widespread concern and I'd have expected to have heard of it, but I might have missed it.

I don't have a catalyst but have re-invented the potential problem due to my practice of making oversized aluminium exhaust gaskets and allowing the exhaust gases to "burn them to size". The melted aluminium in the exhaust is probably a good thing, but in the cylinder, not so much.

I also sometimes put cement and aluminium in the downpipe, but that's further away from the exhaust ports and so probably less of a threat.



Edited by Ducked (07/05/17 11:35 PM)

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#4450946 - 07/06/17 02:31 AM Re: Exhaust Reversion as an Engine Threat. [Re: Ducked]
d00df00d Online   content


Registered: 10/20/05
Posts: 11213
Loc: PA
I've definitely heard about this happening on Nissan 4-cylinders with cats so close to the exhaust ports. Altima and Sentra IIRC. This was years ago.
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#4450949 - 07/06/17 03:14 AM Re: Exhaust Reversion as an Engine Threat. [Re: Ducked]
Silk Offline


Registered: 07/26/03
Posts: 4565
Loc: New Zealand
Back in the mid '70's we had a 6 cyl Thames Trader (English Ford) in, it had broken the bottom off an exhaust valve guide. When we pulled the head the piece of guide had visited every cyl. No damage done, new guides and it was good to go.
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#4450982 - 07/06/17 05:44 AM Re: Exhaust Reversion as an Engine Threat. [Re: Ducked]
Char Baby Offline


Registered: 05/25/05
Posts: 10028
Loc: ROCHESTER, NY
The issue was with 2002-2006 Altima 2.5L.
I had an '04 without any issues however, I know others who did!
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#4450997 - 07/06/17 06:43 AM Re: Exhaust Reversion as an Engine Threat. [Re: Ducked]
mobilaltima Offline


Registered: 02/03/11
Posts: 1149
Loc: Golden Meadow, LA
I had an '02 Altima 2.5 with this problem. I fixed it before it had a chance to happen by installing a header with no cat. Ran great and sold it 12k miles later
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#4451021 - 07/06/17 07:25 AM Re: Exhaust Reversion as an Engine Threat. [Re: Ducked]
exranger06 Offline


Registered: 01/25/08
Posts: 3909
Loc: Guilford, CT
It affected Sentras with the 2.5 too. I used to work with a guy who had a Sentra with this problem. Very common issue for those engines. Nissan issued a TSB for it.
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#4451059 - 07/06/17 08:07 AM Re: Exhaust Reversion as an Engine Threat. [Re: Ducked]
Ducked Offline


Registered: 10/25/12
Posts: 4378
Loc: Taiwan
OK, I'd better not do that self-forming gasket thing again then, since its probably a comparable distance from the exhaust ports.

Can't work out if its likely to be a problem with my engine, but can't see an obvious reason why not.

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#4451190 - 07/06/17 11:17 AM Re: Exhaust Reversion as an Engine Threat. [Re: Ducked]
ARCOgraphite Offline


Registered: 05/17/09
Posts: 11970
Loc: N.H, U.S.A.
Lead Engineer at work has an oil burning accord - looks to be same problem as Nissan - with scored bores - though they dance around it in the tsb.
Yes there is a TSB for popular honda engines failing. Imagine that. Now get over it smile
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#4451643 - 07/06/17 07:06 PM Re: Exhaust Reversion as an Engine Threat. [Re: ARCOgraphite]
Ducked Offline


Registered: 10/25/12
Posts: 4378
Loc: Taiwan
Any educated guesses/thumb rules as to what would be a safe distance (or perhaps volume, relative to the cylinder volume) from the exhaust ports to avoid this kind of sick-swallowing?

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