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QR25DE Precat and Butterfly Screw FAQ

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QR25DE Precat Issue FAQ

What is this “precat issue”?

The precat (or pre-catalyst) is a catalytic converter located in the stock exhaust manifold. The QR25DE tends to run a fairly rich air/fuel mixture so it is very likely that not all fuel is being combusted in the cylinders and instead combusted in the precat, which can cause it to break apart into small pieces. These substrate pieces get sucked back into the cylinder and damage the cylinder walls, ultimately causing increased oil consumption. It certainly doesn’t help that the precat sits so close to the engine. To the best of our knowledge all precats from 02-06 are of the same make/construction.

The FSM page below shows the location of the precat:

No one knows for sure what percentage of precats go bad, regardless, whether it's 5% or 50% the problem is a real one, enough to necessitate an official recall by Nissan. It is interesting to note that many more Sentras than Altimas are affected by this problem even though they have identical engines/precats. This leads one to believe that the driving habits of the typical Sentra SE-R owner (hard aggressive driving) ultimately contribute to precat failure, most report it between 30k and 60k miles.

What is the official Nissan description?

"Certain engine operating conditions may cause the pre-catalyst to overheat and damage the catalyst substrate. If pre-catalyst substrate particles enter the combustion chamber, they may score the cylinder walls, resulting in increased oil consumption. If the engine oil level is not checked on a periodic basis as specified in the owner's manual and drops below the low level, and the driver continues to operate the vehicle ignoring noticeable engine noise, engine damage may occur which could result in a fire."

What does the precat look like?

Here is an inside pic of a stock manifold right after the 4 primaries (which were cut off). This is a rare look at the top part of the precat since the primaries usually obscure a clear view. Normally the silver part that you see towards the bottom half of the catalyst is supposed to cover the entire surface, however, damage to the precat has caused it to disintegrate, and small dark gray precat substrate pieces were sucked back into the engine.

So what can I do about it?

1. Remove the precat from the stock exhaust manifold (don't worry, you still have another cat in the midpipe). Take the exhaust manifold off the engine and use a long screwdriver to break the precat into pieces and completely remove it, you're going to have to do this from the bottom of the manifold, it's the only way to access the precat. Once all the big pieces are out, scrape the sides to make sure all the small pieces are gone and use an air blower to ensure all particles are removed. Make sure you wear a 3M surgical mask the entire time so you don't breathe in any catalyst dust.

2. Or you can install a catless aftermarket header. If you get a "shorty" (tri-y or 4-1) like Hotshot or Nismo, you essentially replace the stock exhaust manifold and eliminate the precat but again, you still have that cat in the stock midpipe. If you get a "4-2-1" header like the DC Sports or AEBS it will replace both the exhaust manifold with precat and the stock midpipe cat, resulting in no cats at all.

I removed the precat but I’m getting an SES light!

The QR25DE has two O2 sensors, one before and one after the precat. On ’02 and ’03 models, both O2 sensors are plugged into the stock exhaust manifold. On ‘04+ models the second O2 sensor is further downstream in the midpipe.

After removing the precat or installing a catless aftermarket header, the two sensors are now reading the same exhaust flow. The second O2 sensor must read a different exhaust stream and send that signal to the ECM to avoid a SES light. You can accomplish this three different ways:

1. Relocate the second O2 sensor behind the remaining cat in the stock midpipe, you will need to extend the second O2 sensor wiring and have a bung welded into the beginning of the catback exhaust. One note of caution, if the wiring is too long, the signal may be weakened to point where the ECM will think the second O2 sensor has failed, resulting in an SES light. The pic below shows the O2 sensor relocated to the beginning of a Stromung catback, before the resonator and after the midpipe cat:

2. Use a "ghetto sim” or anti fouler plug adapter to recess the second O2 sensor away from the exhaust stream. This is the easiest and cheapest method, however, it doesn't always work, especially if the second O2 sensor is still close to the engine. The pic below shows a ghetto sim installed on one of the bungs of a Hotshot header:

3. Get a Casper's O2 sim for the second O2 sensor so it sends a different voltage signal to the ECM. This method almost always works but you need to shell out $50 for the sim. Casper just discontinued their O2 sims so you'll have to look for a used one or go with option 1 or 2.

What about the official Nissan recall?

When you take your Nissan in for the official precat recall, here’s what the dealer will do:

- Reprogram the Engine Control Module (likely to lean out the air/fuel mixture)
- Test the pre-catalyst to determine if it needs to be replaced
- Inspect the pre-catalyst to determine if more extensive repairs are needed
- Install new heat shields
- Change your oil/filter

IMPORTANT: There are numerous instances of precats failing even AFTER the official Nissan recall procedure. The only 100% proven and effective solution is to remove the precat entirely.

What about state emissions testing if I remove the precat?

Catalytic converters work best at high temperatures, this is why the precat sits so close to the engine, it reaches its optimal temperature quicker, thus cleaning your exhaust for the first few minutes after startup. However, the QR25DE comes equipped with a second catalytic converter located in the midpipe. This midpipe cat alone will still allow you to pass emissions after the car has warmed up. If you have a visual inspection in your state (like California) an aftermarket header will obviously cause you to fail, so your best bet is to either use the stock manifold without the precat, or just swap in the stock manifold for the inspection.

QR25DE “Butterfly Screw Issue” FAQ

What is this “butterfly screw issue”?

All QR25DE intake manifolds are “dual stage” and come equipped with two runners in each primary. There are four butterfly valves (or power valves) in the lower half of the intake manifold, one in each runner. Below 5000rpm the butterfly valves remain closed to optimize torque, above 5000rpm the valves open to allow more air into the engine. For each butterfly valve there are two screws that hold them in place. These screws can come loose and get sucked into the engine, causing damage to the head, cylinder walls and/or precat. This issue affects all model year QR25DE engines except '06 which have redesigned screws.

Official Nissan description

"Nissan has determined that on some model year 2004 Nissan Sentra and 2004-2005 Nissan Altima vehicles equipped with the 2.5 Liter engine, there is a possibility that power valve screws located in the intake manifold may become loose. If this condition occurs the malfunction indicator lamp (MIL) may illuminate and unstable engine idling or power loss may occur. To prevent this from occurring, Nissan is conducting a Service Campaign to inspect and replace the power valve screws with new ones. In a limited number of cases engine damage may have already occurred; and, if so, more extensive repairs including repair or replacement of the engine or catalytic converter may be required."

What do the butterfly screws look like?

First is a pic of the butterfly valves with all eight screws in place:

Here is a pic of an unfortunate owner who found that one screw was missing:

Finally, certain '05 and all '06 model QR25DE engines have new revised screws:

So what can I do about it?

The fix is relatively straightforward: take apart the intake manifold and tighten the screws along with a very small drop of loctite on the threads of each screw. Here is a great writeup by JoelAZ:


Below is a diagram from the FSM which includes torque specs for all the bolts and screws:

What about the Nissan Voluntary Service Bulletin?

Nissan will install new butterfly screws for you, but this only applies to certain ’04 and ’05 models even though it has been proven that ’02 and ’03 models have the same issue. ’06 models already have redesigned screws installed from the factory, only time will tell how effective they are.
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I have received permission from the mods on this forum to turn this thread into a sticky. There are numerous new threads regarding these two common issues with the QR25DE, so an FAQ makes sense.

If you see any errors or anything that needs to be added, please reply to this thread and I will include it in the first post.
Jesus god damn christ Steve, getting a lot of work done today huh? lmfao! ;)

Awesome thread though! :D

Now noobies have no excuse...this thread should be required reading for any new b15 (qr25de powered) member.

5 stars broseph. :cool:
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you are a good man. A gentleman and a scholar.
02 Sensor

What's the easiest and least expensive thing to do to not have an SES when the pre-cat goes away?
MrCritical said:
What's the easiest and least expensive thing to do to not have an SES when the pre-cat goes away?
By far the easiest is to use the "ghetto sim" or anti-fouler plug. It's like an adapter for the O2 sensor and recesses it away from the exhaust stream. If one doesn't work you can even try two of them to recess the sensor even farther away.

Here's an aftermarket one from http://www.voodooboost.com/performance_accessories/O2_Simulator/

You can also buy one at your local auto parts store, they cost less than $10 and you'll need to use dremel for the O2 sensor to fit properly.
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nice faq...hopefully all the newbie QR owners look at it
Dangling Fury said:

You may have to take a dremel and slightly modify the adapter to make it fit correctly.
or what i did which would be easier and faster... take a drill bit (1/2") or if you can just see what bit fits down in it but make sure its the same diamiter as the anti fouler inside or the o2 wont slide into it...
Steve, you are my hero. Excellent FAQ. :eek:

Now how about adding the head gasket replacement to that list. ;) :D
piknockout said:
Now how about adding the head gasket replacement to that list. ;) :D
I thought about adding that but I've never done it myself nor have I seen a good writeup on this site. There doesn't really need to be a FAQ on it because it's a pretty straightforward issue: head gasket goes bad and you need to replace it. It's definitely not a procedure for the beginning mechanic.

More importantly there's really nothing you can do to prevent head gasket failure. The NPM thermostat equalizer mod *in theory* should help but there's no hard proof that it's 100% effective. The culprit could be the head gasket design itself.

It is worth mentioning though that symptoms of head gasket failure include coolant consumption, hard starts, hard shut offs, green color in the oil and white smoke coming out of the exhaust. Shining a flashlight down the spark plug hole can confirm coolant in the cylinder. It can happen to any year QR25DE and usually happens between 40k-60k miles.
Dangling Fury said:
More importantly there's really nothing you can do to prevent head gasket failure. The NPM thermostat equalizer mod *in theory* should help but there's no hard proof that it's 100% effective.
This is a temporary fix to a permanent problem. Just replace the thing ASAP.
fluid1 said:
This is a temporary fix to a permanent problem. Just replace the thing ASAP.
The NPM mod is not really a fix, but rather a preventitive measure (*in theory*) to avoid the head gasket from failing.

Everyone thinks its the factory head gasket itself causing the problems, especially when compared to a nice aftermarket one:

If anyone is interested, robbie2883 has a group buy going over at the Vboard for these.
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I have read that an aftermarket replacement gasket that was slightly thicker would raise the compression a little for a tweek more power as well...
Thank you SO much for posting this.
Yes, thanks A LOT. I only have one question, I have an '06 model. You said they changed the screws. So I just have the pre-cat issue to take care of then? I shouldn't worry about the butterfly screws, or should I? Thanks a lot.
I specifically remember seeing a pic of an '06 model that had revised butterfly screws, it looked like there as a washer attached to the head of the screw. It looks like they fixed the problem but of course I can't tell with 100% certainty, just to play it safe I would check them myself after say 30k miles.
Ok I found it, here's a pic by seasick over at the Vboard, it shows the butterfly screws in his '06 QR25DE:

I also read that at least one '05 owner reported that he did NOT have these revised screws, so it looks like Nissan only started putting these in the '06 models. As far as I can tell these new screws are available for purchase from the dealer and are likely included in the package they use for the Voluntary Service Bulletin.
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A couple more useful threads on butterfly screws

QR25DE Sentras (02-06+ SE-R / SE-R spec V)
1. Butterfly screw failure / i'm burning oil and my spark plug is mangled.
How to Loctite butterfly screws: http://www.b15sentra.net/forums/showthread.php?t=78320
My spark plug had dents in it: http://www.b15sentra.net/forums/showthread.php?t=51299 (this is one of the original ones)
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