I have these codes. I reset them but they go back after 30-50 miles. I also tried to use seafoam and catcleaner for this; in these cases the codes came back after 80-100 miles of driving. Seems, that the main problem here is dirty or damaged cat. The second problem here is fuel with ethanol (I was not able to find pure gasoline in my area) which also may affect on th3 cat efficiency. There are two questions here. How it is dangerous to drive with these codes? I heard about ceramic dust that may damage engine. Is it true? And how can I resolve this problem without replacement the cats? The car drives well.

The second moment which is partially connected to the problem above is misfire. I see that 2, 4 and 6 cylinders have 1-2 misfires in each driving cycle. What may be a reason for this? A month ago I replaced the valve cover near the firewall becouse of oil leack, installed all 6 iridium spark plugs and also installed new three ignition coils from that side. The same job is planning for the front head (valve cover and coils) but I am interested to find the reason of misfire at first. What does it make sense to check? I also found channels from intake to cylinders were yellowish or even a bit brown color, intakes for 2,4 and 6 cylinders looked more dirty. I also checked all pipes to intake, all of them were clean and in normal state, thus there should not be leaks.

QuestMan wrote: ↑

Wed Jul 20, 2022 6:50 am

How it is dangerous to drive with these codes? I heard about ceramic dust that may damage engine. Is it true?

Very dangerous. Nissan engines all use Miller-cycling to avoid needing an EGR Valve, which is great unless a cat melts and disintegrates. If that happens, the engine inhales 2500F debris through the exhaust valves and ends up in the JY. Cat inhalation is 100% fatal. To see if your cats are actually working or not, shoot an IR thermometer at the inlet and outlet end of each cat with the car fully warm, preferably after a drive. The outlet should be significantly warmer than the inlet. If it isn't, that cat is dead and watch out.

QuestMan wrote: ↑

Wed Jul 20, 2022 6:50 am

And how can I resolve this problem without replacement the cats? The car drives well.

If the cats are actually dead, you can't. If they're still working but not working well, spacers on the rear O2's may coax a bit more life out of them.

QuestMan wrote: ↑

Wed Jul 20, 2022 6:50 am

The second moment which is partially connected to the problem above is misfire. I see that 2, 4 and 6 cylinders have 1-2 misfires in each driving cycle. What may be a reason for this? A month ago I replaced the valve cover near the firewall becouse of oil leack, installed all 6 iridium spark plugs and also installed new three ignition coils from that side. The same job is planning for the front head (valve cover and coils) but I am interested to find the reason of misfire at first. What does it make sense to check? I also found channels from intake to cylinders were yellowish or even a bit brown color, intakes for 2,4 and 6 cylinders looked more dirty. I also checked all pipes to intake, all of them were clean and in normal state, thus there should not be leaks.

Three things come to mind.
1) The cat on bank2 may already be partially melted and blocked, so the exhaust is somewhat restricted
2) You might have a bad coil ground to bank2
3) You might have an issue in the PCV plumbing
Since you already have dead-cat codes, I'd tend toward #1 as my first guess

VStar650CL wrote: ↑

Wed Jul 20, 2022 9:35 am

Nissan engines all use Miller-cycling to avoid needing an EGR Valve, which is great unless a cat melts and disintegrates.

Wait, but Nissan vq35DE has an EGR valve.

Only the early ones, and they all still use IVT to manipulate the intake cam, which is what causes inhalation.

you are frustrating me.....

considering that I need to add oil every 500 miles, the fatal inhalation was been done.....

This post on another site has some great pics of what "cat inhalation" did to a QR25 Altima:

https://grassrootsmotorsports.com/forum ... 366/page1/

I have found a pyrometer and will take the test today. I can measure bank 2 but I cant figure out how to carry out this for bank 1.

I know, it's rough without either lifting the car and squirming, or removing the cowl. But unless a cat is already clogged, there's no other good way to check it.

I know, it's rough without either lifting the car and squirming, or removing the cowl. But unless a cat is already clogged, there's no other good way to check it.

VStar650CL wrote: ↑

Wed Jul 20, 2022 12:12 pm

I know, it's rough without either lifting the car and squirming, or removing the cowl. But unless a cat is already clogged, there's no other good way to check it.

In accordance with the odb data, bank 2 has worster metrics then tbank 1. I measured bank 2. The cat is covered with thermal screen, thus I used points near the sensors in the pipe for measurements.
I drove 15 miles, parked a car, visited pharmacy and then remembered that forgot to measure temperature. Thus, after 10 minutes on parking the temperature near two sensors looks similar, 138 near the top sensor and 144 near the bottom one. After 1 mile way to home, the temperature near the top sensor was 180 C, while neat the bottom it was ~220C. The tendency looks obvious.

That one is still working, so you could try spacers on the rear O2's. It won't work forever, but in most cases it will stop the ECM throwing DTC's over a cat that's inefficient but not yet dead.

VStar650CL wrote: ↑

Wed Jul 20, 2022 4:49 pm

That one is still working, so you could try spacers on the rear O2's. It won't work forever, but in most cases it will stop the ECM throwing DTC's over a cat that's inefficient but not yet dead.

How long does it usually take the way from low efficiency to dead?

You might get another 25~30K, or you might get a lot less. It depends on a lot of other factors, fuel, engine health, driving habits, climate, etc. There isn't really a "usually".

P0420/30 codes are the result of the processes that are going in the wrong way inside the engine. Thus, as I decided to replace the catalytic converter, I also would like to be sure that everything goes in the correct way. I also ordered the endoscope camera to investigate the cylinder walls for the presence/absence of different kinds of defects.
As I noted before, I have a problem with oil consumption. The typical value is close to 1 liter to 1000 miles. I replaced the oil cooler o-ring, crankshaft sensors o-rings, and valve cover from the firewall side (the back side of the engine oily). After that, I still have oil consumption. I do not see oil spots under the car and I do not see the white smoke from the pipe. Even when the engine is cold and I accelerate it to 6000 rpm rapidly, I do not see the white smoke (on a parking lot, not in motion). Where the problem can be? I use Penzoil 5w-30, if it is important.
I saw that the intake plenum looks browny inside (like a glazed) but all pipes were looking dry inside. I do not exclude that it is a result of the previous issue(s) that killed the cats. I also have a photo of the spark plug after 3500 miles. Is that black deposit normal or it is a result of oil burning?

If the intake was brown and oily, the chief suspect would be the PCV Valve or the plumbing for it. That can also cause "invisible" oil consumption.

I bought a small endoscope camera for evaluating the internal condition of the engine (cylinders walls and pistons) to investigate the oil consumption problem. I am frustrated. Cylinder 6 has a mirror on the wall (I see the piston reflection), the piston N2 looks partially destroyed! Now I do not know what to do. I am not sure that it makes sense to replace the cats...

Can't get to your pics, the redirect is giving a Fortinet error on all my devices. Try downsizing them under 1M and post them as attachments. Hit the "Full Editor and Preview Button" on this page, then go to the "Attachments" tab underneath the main text-entry box and use the "Add Files" button.

VStar650CL wrote: ↑

Thu Aug 04, 2022 6:12 am

Can't get to your pics, the redirect is giving a Fortinet error on all my devices. Try downsizing them under 1M and post them as attachments. Hit the "Full Editor and Preview Button" on this page, then go to the "Attachments" tab underneath the main text-entry box and use the "Add Files" button.

Done!
And also I have added a spark plug photo.

That just looks like a crack in the carbon, not a hole in the piston. You always get a layer of carbon built up on any piston crown. In fact, a shiny crown is usually bad news, generally a blown head gasket. That carbon layer is thick, but usually that just means you've been driving like grandma or the cylinder is running rich. Your plug doesn't look super rich, so my diagnosis is your engine needs a serious de-carboning. If the crowns are that thick, I'll hate seeing the backs of your intake valves. They're probably 1/4" thick with carbon deposits.

PS - De-carboning requires exactly 2 tablespoons of any transmission fluid. Warm the engine thoroughly. Shut it down momentarily, disconnect the brake booster hose and put your thumb over it. Have an assistant restart the engine and hold it at 3500~4000 RPM. Take your thumb off and squirt the tranny fluid into the hose. Bye-bye carbon.

VStar650CL wrote: ↑

Thu Aug 04, 2022 7:52 am

That just looks like a crack in the carbon, not a hole in the piston. You always get a layer of carbon built up on any piston crown. In fact, a shiny crown is usually bad news, generally a blown head gasket. That carbon layer is thick, but usually that just means you've been driving like grandma or the cylinder is running rich. Your plug doesn't look super rich, so my diagnosis is your engine needs a serious de-carboning. If the crowns are that thick, I'll hate seeing the backs of your intake valves. They're probably 1/4" thick with carbon deposits.

I am driving it just last 4K miles. The previous owner was using 89 gas, which is allowed by the user manual. I tried to use it once and the engine sounds horrible, not smooth. I know about the carbon problem because every time I see that an oil stick is covered with a thin layer of carbon. I use 93 gas only for the last 2-3k miles and also add Seafoam for decarbonization (each second or third refuel).
I also sometimes use L mode for holding higher RPM (somehow simulation of the sport mode). Yep, in a drive mode, the RPMs are between 1200-2500 values. In reality, I am trying to give the best care to this car that I can.

SeaFoam is great, but it will take forever to get rid of carbon which is that thick. You need explosives, which is basically what that little bit of ATF will do to the carbon. The only risk is that sometimes a big chunk off a valve can get squashed between a spark plug and the piston crown, which will ruin the plug. So have a spare one on hand.

Small block Chevys and Fords used to be infamous for that sort of carbon buildup. It would get so thick that a valve wouldn't close, and grandpa would come in with a dead miss on one cylinder. In the days before cats you'd get dime and quarter sized chunks blowing out the tailpipe as soon as the ATF hit it. The occasional customer who watched would usually turn white and mumble something like, "Holy crap, that was in my engine?"

Thank you.
I have never heared about using ATF for this purpose. After a small investigation on the internet, I have found a few mentions about this. Ok, the main idea is infection an ATF into the intake manifold.
I have read that Marvell mystery oil is a mix of ATF with cleaner. Will it have the same effect if I add a few tablespoons of the ATF into the gas?

Yes, but gradually, like SeaFoam. Introducing it straight into the intake stream coats the carbon and causes a temperature differential that cracks the weak bonds between the carbon and the underlying metal, basically splintering it. Almost any vaporized liquid will have the same effect, which is why a blown head gasket will result in shiny pistons. ATF is just safer than water or glycol, water can cause miniature steam explosions and glycol vapor actually has a very low flash point and can cause an actual explosion. Mineral oil doesn't burn readily and is the most harmless thing you can employ.

Is decarbonization a good idea as a maintenance procedure and if yes at what intervals? Besides the sparkplug fouling are there any other possible problems that may result? Should the oil be changed after doing it?

schuylkill wrote: ↑

Sat Aug 20, 2022 1:51 am

Is decarbonization a good idea as a maintenance procedure and if yes at what intervals? Besides the sparkplug fouling are there any other possible problems that may result? Should the oil be changed after doing it?

I recommend a tablespoon of ATF with every air filter change for my regular maintenance customers. That way bigtime carbon never gets a foothold and crushed electrodes are never an issue. It only takes a minute and many of my "not mechanically inclined" customers aren't even aware that I do it for them. It's more critical on DGI engines than EFI, a DGI vehicle with a grandma driver can build meaningful carbon on the intake valves in just 20~25K miles. Carbon doesn't affect the rings, just the crown and valves, so the oil isn't affected by it.