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Pre-Cat Engine Killer...things I should have noticed Forums > > Pre-Cat Engine Killer...things I should have noticed Pre-Cat Engine Killer...things I should have noticed
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  #1  
Old 11-21-2008, 08:06 AM
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Arrow Pre-Cat Engine Killer...things I should have noticed

My 03 Altima SE 5spd just had its engine replaced last week at the Nissan shop in town, paid for by Nissan. (6 yrs old, 45kmi).

In hindsight, here are some things I *should have* clued in to:

1. Cold start, strong smell of unburnt fuel.

Since main cat only works when warm, on startup the defective precats let it all through.

2. Blue-grey exhaust smoke upon hard acceleration and/or downshift (particularly on-ramp acceleration to 5k RPM).
Sometimes referred to around my area as an 'Italian tuneup'.

3. Cold engine 'rattle'. Most noticeably on slow speed acceleration immediately after clutch engagement.

Though for sure it was heat shield rattle, turns out it was the timing chain clunking around a nearly empty oil sump.

4. Rapid onset of oil consumption. 12 months ago, no consumption, 6 months ago 1qt, recently nearly 4qts required at oil change (~3kmi only!).

Throughout all this, no engine lights came on. No low oil. No cat efficiency. No codes were thrown that I know of, at least nothing to light a bulb on the dash.

I hope this info saves somebody out there an engine or two.
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  #2  
Old 11-21-2008, 08:15 AM
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Thank you for the information!
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  #3  
Old 12-29-2008, 11:09 AM
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Default UPDATE

After reporting post-engine replacement hesitation on acceleration, the dealer has now diagnosed a faulty Air Flow meter (yeah, the $600 part). This has indirectly cured the cold-start up 'raw fuel stink' and was probably largely responsible for the pre-cat failures in the first place.

If your car stinks on cold day startups...might want to have that checked out sooner rather than later...the $600 is a lot cheaper than the $6000 engine replacement. YMMV
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  #4  
Old 02-11-2009, 03:39 PM
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Quote:
Originally Posted by d0ugmac1 View Post
After reporting post-engine replacement hesitation on acceleration, the dealer has now diagnosed a faulty Air Flow meter (yeah, the $600 part). This has indirectly cured the cold-start up 'raw fuel stink' and was probably largely responsible for the pre-cat failures in the first place.

If your car stinks on cold day startups...might want to have that checked out sooner rather than later...the $600 is a lot cheaper than the $6000 engine replacement. YMMV
So there was no trouble-code stored in the ECU for the faultly mass airflow sensor?

Also, I've heard it's very rare for the 3.5L V6 to have pre-cat failures ... mainly it occurs on the 4 cylinders. Is that true?
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  #5  
Old 02-12-2009, 04:48 AM
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Originally Posted by Altima SSS View Post
So there was no trouble-code stored in the ECU for the faultly mass airflow sensor?

Also, I've heard it's very rare for the 3.5L V6 to have pre-cat failures ... mainly it occurs on the 4 cylinders. Is that true?
Don't know, the dealership doesn't share those codes with me, and I didn't ask since I knew there was a problem, and the MAF replacement fixed it.

Rare? Less common certainly, my dealer fessed up he's doing 2 QR25's a week right now, and this was their first VQ35. Sentra SE-R's are a lot more common than vanilla Sentra's....same engine, same cats....different driver

I'm more of an SE-R style driver, let's just say I don't watch my mileage.

I was told that the dead MAF would cause the engine to run richer than usual (hence the cold start fuel stink, dead cats, dead engine...) I think one of the tech's here referred to it as 'failsafe mode' or similar. Sergey85 has posted a nifty link to use the cheaper, and apparently more robust VQ30 MAF's from 00-01.
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  #6  
Old 02-13-2009, 05:07 PM
dbaby4evr
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How did you get Nissan to replace your engine for free? Is your car still under warranty? I have an 02 and I am having massive oil consumption. I know that a blown engine is inevitable. I bought the car in hopes of keeping it for a long time but it doesn't look like I am going to be able to hang on to it without replacing the engine. And even then, I have heard of people having problems with the second or third engine.
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  #7  
Old 02-14-2009, 06:40 PM
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Quote:
Originally Posted by dbaby4evr View Post
How did you get Nissan to replace your engine for free? Is your car still under warranty? I have an 02 and I am having massive oil consumption. I know that a blown engine is inevitable. I bought the car in hopes of keeping it for a long time but it doesn't look like I am going to be able to hang on to it without replacing the engine. And even then, I have heard of people having problems with the second or third engine.
Under the emissions 8yr/80Kmi warranty. If I'd had to pay the $6K to put an engine in a 6yr old car with 45kmi I would have walked away from both it and Nissan forever. As it is, there are no guarantees it won't happen again, hopefully by that point it will be 12 years old...otherwise, I'm walkin'
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  #8  
Old 02-14-2009, 08:13 PM
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Quote:
Originally Posted by d0ugmac1 View Post
3. Cold engine 'rattle'. Most noticeably on slow speed acceleration immediately after clutch engagement.

Though for sure it was heat shield rattle, turns out it was the timing chain clunking around a nearly empty oil sump.

4. Rapid onset of oil consumption. 12 months ago, no consumption, 6 months ago 1qt, recently nearly 4qts required at oil change (~3kmi only!).
I'm trying to understand the root cause of engine failure due to pre-cats going bad.

So did the pre-cat failure cause pieces of it to get sucked back into the cylinders, which in turn caused mechanical engine damage (cylinder wall scraps, ring damage, etc) that then started causing massive engine oil consumption with time? Once it started using oil like mad, the engine probably ran very low on oil at times, thus causing all the noises and mechanical damage to continue until it basicall self destructed.

I didn't think the pre-cats were that close to the exhaust valves on the VQ 3.5L engines to cause pieces to get sucked back into the cylinders during valve overlap ... thought it was just the 2.5L 4 cylinder engines.

Last edited by Altima SSS; 02-14-2009 at 08:16 PM.
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  #9  
Old 02-15-2009, 05:10 AM
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For the most part it is the 2.5L engines that have most of the problems.
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  #10  
Old 02-15-2009, 06:36 AM
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I suggest you google it and form your own conclusions. Yes, the 4cyl with problems are much more common, but that's just because they, or their drivers, encounter those nefarious 'certain operating conditions' more often, causing the cats to fail sooner.

The oil loss is caused by the ceramic powder from the broken down catalytic elements being sucked back into the engine, mixing with the oil, and acting as a grinding abrasive literally polishing your engine so far out of tolerance that the rings don't work and the bearings are all shot. Dead engine. Do not pass Go. I can only imagine the oil starvation issues if the filter gets all plugged up with that dusty gunk.

So you understand what causes the engine failure, where it comes from (the pre-cats) and maybe a root cause (rich mixture that was later leaned out on 07+ models I think...there's a lot of mis-information out there...hence my Google it and form your own conclusions comment).

Given your oil consumption issues I think you're done for. Even if you core your pre-cats or repalce with aftermarket header...the wear is still there, and the abrasive powder is still stuck all over the inside of the engine. If you miss one oil top-up there's a very good chance the engine will seize on you...hopefully not on the freeway and I hope you don't have a manual tranny...
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  #11  
Old 02-15-2009, 06:16 PM
Altima SSS Altima SSS is offline
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I searched the internet for quite awhile and really didn't find anything conclusive about the 3.5L VQ V6 engines suffering from the pre-cat failures. It seemed to be just the 2.5L 4 bangers.

At this point, my conclusion is that the VQ does not melt pre-cats like the 2.5L 4 banger. I'm sure it can happen, but it sounds to be very rare on the VQ engines.


In your case, there must have been some reason for your pre-cat melt down as you discussed in your previous posts, and the pre-cats got super hot and started disintegratin. IMO, a very lean condition will cause the exhaust to run much hotter than a very rich condition. I think the 4 bangers were tuned to run very lean (probably for emissions and gas mileage reasons), which is what caused them to disintegrate the pre-cats. Didn't Nissan somewhere along the line reflash the ECUs on the 4 bangers to help them not run so lean and help prevent pre-cat failures?


UPDATE - I did find this thread which was started just recently. Looks like someone else with a 3.5 VQ has the back pre-cat go. I'm betting the rear bank pre-cat might run hotter than the front one, which is why it goes first. On the VQ, I'm still thinking there has to be an underlying cause that is not very common and a rarity on the VQ 3.5s.

http://www.nissanclub.com/forums/200...haust-mod.html

Last edited by Altima SSS; 02-15-2009 at 07:24 PM.
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  #12  
Old 02-16-2009, 06:35 AM
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Hey guys, interesting topic. I have a problem that I can not resolve and different people telling me different things.

In the morning when I start the car it dies down right away, if I depress gas pedal and start it like that it starts normally and usually fine for the entire day, although I can smell rich exhaust maybe because I depress gas pedal during start?? I dont know. Then... the rest of the day car starts up just fine as long as it does not sit for more than let's say 5 hours. More than 5hours it gets engine gets cold again and same problem occurs. I was told that it's "Idle speed control" that I need to get at a dealership and it will run me about 200-300$ What do you think?

2.5 engine.

There is no blue/gray smoke and I smell gas only when it starts up in the morning..
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  #13  
Old 02-16-2009, 05:27 PM
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Why don't you do the idle air/volume learn yourself and save the two to three hunderd dollars! Just go to the service manual at this site and go to chapter EC. No tools required except for maybe a stop watch.It's not hard to accomplish.
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  #14  
Old 02-17-2009, 01:31 PM
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I was told by the dealer that it was micro-detonations of unburnt fuel within the honeycomb lattice of the catalytic converter. It is NOT heat related, nor the result of a lean mixture. I believe the MAF and or ECU programming were responsible for running too-rich mixtures on my 2003. That's my opinion.
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  #15  
Old 02-17-2009, 02:40 PM
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Quote:
Originally Posted by d0ugmac1 View Post
I was told by the dealer that it was micro-detonations of unburnt fuel within the honeycomb lattice of the catalytic converter. It is NOT heat related, nor the result of a lean mixture. I believe the MAF and or ECU programming were responsible for running too-rich mixtures on my 2003. That's my opinion.


I read that to about over rich causing mini explosions in the pre-cats someplace else just last night - that makes sense.

So did Nissan change the ECU programming on their engines as part of a service bulletin or recall anywhere along the line to help cure the pre-cat failures? Or maybe they changed the ECU programming after 2003 model year? I can't find anything definitive on that.

I thought I heard something about Nissan reflashing the ECUs on the 4 cylinders someplace ... not sure about the 3.5 VQs.

d0ugmac1 - when was your 2003 built? (month/year) ... was it early or late build for that model year?

Last edited by Altima SSS; 02-17-2009 at 02:42 PM.
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