Figured I'd post this so others with the same symptoms will find the solution without spending a load of cash. First off, I did a stupid thing: Since I was getting so much blow by, I thought I'd see if it was entering the intake via the valve cover vent, so I blocked off the the hole on the intake side and put a $5 vent filter on the valve cover. Worked great...for a while. Turns out I didn't do my research very well (hindsight is always 20/20). The vent actually pulls air from the intake into the valve cover, not vice-versa. By doing what I did, I caused the engine to run lean. This in-turn, caused my MAF to act funny and, over time, the ECU remapped the fuel/air mixture to run the engine lean. That's no too bad, except that it also causes the exhaust gases to be extreamly hot (I noticed the outside of my car gave off a lot of heat. I had thought it was only due to the 100+ temp outside...). Needless to say, my cat wasn't meant to take that. After about 2,000 miles of this I began to notice a loss of power (felt like about 100 ponies gone!). One day in particular I had absolutly no top end and it felt like the transmission wasn't working right. As some of you may remember, I had the transmission replaced recently so I took it back to the dealer that did the work. They couldn't find anything wrong with the tranny and as we drove it there happened to be a drastic temperature drop outside and a storm came through. The rain cooled the cat enough that it began to act normally again and the problem stopped. Since we couldn't find the problem at that point and had no indication of the cat going bad, we called it a day and I went home. Well, it got worse. I eventually gave in and took it to another dealer in Frederick, MD that, as it happens, does a lot of high-end performance mods to the Z cars. These guys are sharp! They plugged the car in and found that the MAF was acting funny. First they reattached the vent hose to the inlet and got some performance back after re-programming the ecu (I also had them up the timing to 17 degrees). Still acted funny. They then removed the cat and put in a test pipe. Volla! The car ran great. They also checked the pre-cats and luckily those are OK. Now I'm looking for a replacement cat, but for the time being, Cory (Redmax) at www.redlinemax.com is selling me a test pipe to get by. If theres a problem with the fit, instead I will take my cat, punch it out and use it for now. Even with the old cat still in place, however, the car is running pretty darn good. So much so that it feels as good as stock again.
Long story short: DO NOT USE A VENT ON YOUR VALVE COVER! keep the vent hose from the intake to the valve cover. If not, your MAF will overcompensate and think there's more air going into the intake than there actually is. This will cause the engine to run lean. The mechanic also told me that the additional backpressure may have been the cause of some of my blow-by. We'll see how much less oil I lose in the next few weeks. If it takes care of my oil consumption as he claims (I still don't see how, but he swears it will), we know that some of the oil consumption can be fixed by replacing the cat.


Sorry for the dissertation. Had to explain it fully to get the point across.

 

forgot to mention: no SES light during all of this. If the cat goes, I doubt the ecu will figure it out.

 

ECU has no way to monitor the rear cat- there are NO sensors behind that cat. Only Fed Spec 00s have a 02 sensor behind the main cat.

 

Thanks! You answered my next question: if I put on a test pipe, would it throw a SES? No, from the sound of it!


To hell with my stupid cat! It has a date with a long piece of metal and a trash can to empty it into!

 

actually the vent filter grabs unmetered air, which is more air into the engine than the ECM detects causing an initial lean condition, the ECM will compensate by going rich

fuel trims being positive, might kill the cat as in a rich condition, not the heat.

and either way, i think your cat failed due to something else. if it were fuel or even heat that killed the cat, then the pre-cats would have failed first.

glad you got it taken care of

 

I have a Fed spec Maxima, the car will throw a code its only a matter of time. Sometimes it takes 300 miles sometimes it takes 600. I always check and clear the codes with either my friends pocketlogger or at Autozone.

 

RIch conditions cause EGT's to rise not lean.

Also it is almost impossible for the vent line to work the way you describe. air is pulled into the intake from the vent in the valve covers. Not vice versa.

 

I wonder if my CAT is going, massive hot air comes from the sides of my car if u feel under the car from the driver side or passenger side and the other day I got a P0430

 

Actually, it will run lean since the only sensor that detects air flow is the maf and I was NOT pulling air after it like it expected. Hense, more air into the engine than the ECU expected = lean.

 

I realized that after I went through all the problems. Read the post newb!

 

when the ecu sees the maf meter more air, it will richen the a/f to compensate for the additional air. the ecu is setup to run safely which is more rich, not lean.

 

How could more air into the valve cover area get into the combustion chamber?

I too have not ever read a cat going bad by getting lean. Rich, yes. Lean, no.

And although the maf measures the mass of air into the engine, the ecu will trim the a/f ratios via the 02 sensor if that sensor sees a lean/rich condition. Regardless of what the maf reading was.

 

I agree, but the hose I blocked off was AFTER the maf, not before. Trust me, it was lean (I saw my CAT: it was melted.).

 

Update: SES light showed up on Sunday. Got a new racing cat on Monday off Redlinemax.com. Installed it Tuesday: First off, the quality of the cat was FAR better than the stocker. Everything about it! Secondly, once installed I noticed a) a lot more power (chirped the tires last night at the 80 mph shift point!). Thirdly, I had some hesitation during acceleration for the last 40k miles: Gone now!. Fourthly, my gas milage is back to 26 mpg. Fifth: SES light is now gone. Finally, my oil consumption problem has been reduced to negligable ammounts! I guess the mechanic was right when he told me that the excessive back-pressure was causing some blow-by and thus the oil consumption. One little part, so many problems...


Oh, and BTW: Nissan wanted $750 for the cat. Redmax was quite a bit less. if you have to replace a cat, call Cory and tell him you want one like mine!

 

It didn't. The air the maf thought was going into the valve cover was redirected to my intake because the hose was blocked off behind the MAF.
Mine actually was melted and clogged. Probably more from the 125k miles than the A/F ratio...
100% sure? I don't know about that one. I'm just relaying what the mechanic told me. He seems to have been correct on the rest.

 

I don't know about this. I see oil spots from my hose that leads into the midpipe. If air was going the other way, I wouldn't see this. This also sorta defeats the purpose of that system doesn't it? Aka... this system is supposed to vent vapors that accumulate in the valve covers etc.. and burn them up in the engine.

I don't doubt it was bad. But cats are supposed to live well beyond 125k.

That's what the 02 sensor does. Fine tune the preset a/f maps in the ecu

 

Actually, that would be the EGR. The vent tube from the intake to the valve cover is there to allow air to enter the valve cover to equalize the vaccume, not the reverse. You could also be getting blow-by as well. I noticed the same thing which led me to where I am now. Actually, I saw Irish's catch-can post (I had thought about doing that as well long before he did, but I'm too lazy to do it to this day) and thought, hey why not do something simular on the other end? When I inspected the hose between the valve cover and intake I saw the oil and started on the filter....

The more i think about it, I doubt the problems I had were even related to the hose / filter mod. I think the bad cat caused the ECU to act up and remap the A/F. That's why it ran so bad (that and the Cat failing causing the backpressure which caused the ECU screw-up). The more I think about it, the more I see that the cat was probably shot for a long time (I have had hesitation and slowly lost power for 40k miles now). If you have oil in that tube, it's worth looking into the cat (to figure it out quick and cheap, unplug the exhaust b-4 the cat, clean out the breather tube, and run it for a while. Check the breather tube again: dry=bad cat, wet= something else).

 

PCV: Postive Crankcase Ventilation function:

http://www.filtercouncil.org/techdata/tsbs/94-2R1.pdf

EGR Exhaust Gas Recirulation: Exhaust gas not vent breather vapor

How EGR systems work The EGR valve recirculates exhaust into the intake stream. Exhaust gases have already combusted, so they do not burn again when they are recirculated. These gases displace some of the normal intake charge. This chemically slows and cools the combustion process by several hundred degrees, thus reducing NOx formation.

http://www.asashop.org/autoinc/nov97/gas.htm

Two totally different/seperate systems

I don't doubt the cat was bad and that new ones would make your car run much better. I also agree that maybe the bad cat was the cause of the other problems. I just question what made the cat go bad. Could be random. Point being, you car is fixed!

 

That's fine, but I wasn't talking about the PCV. It's the vent hose that goes from the valve cover to the airbox. On the 5.5 gens, the PCV is on the back side of the engine. The vent hose is on the driver's side. Completely different hose. It's not even the EGR valve. Note: the EGR system was developed in the 70's to reduce emmisions. It does, in fact, contain non-combusted combustables. A portion of the exhause is recirculated in order to be re-burnt thereby reducing the emmisions levels. Trust me, I'm a redneck from Indiana with a degree in mechanical engineering. Since we don't have a Nissan engineer on board, I'm probably the next best thing to explaining the theories. Maybe not so much the practice, but the theories.

 

Sorry: didn't see the last paragraph. I got wrapped up in the details above it. Anyway, here's a list of the symptoms that I had that went away when the cat was replaced:
Hesitation during acceleration
reduced gas milage
reduced power
oil consumption
oil in the vent tube mentioned above several hundred times
black smoke during WOT runs at the time of the upshift
got my a$$ handed to me by a Honda
excessive heat coming out from under the car after it warmed up.
LOUD intake (much quieter now for some reason)
weird automatic shifting (wanted to down-shift excessivly)
hard shifts in the auto

I'll list more as I think of them.

 

ACtually both right and wrong. It IS part of the pcv system. But it works like you mention:

 

Thanks. That would have saved me about $5 in parts and about three weeks of hassle! Actually, that diagram is pretty darn useful in explaining how the system works.

 

Hey GBAUER,
Thanks for this thread, Im going to purchase a new cat early next week. My symptoms are almost exactly the same. I am worried however that my precats are f*$*ked up as well since the car has run "rich"

Gene

 

It's possible that the pre-cats are shot, but, as I said mine aren't. Make sure you go to Redlinemax.com. Trust me: the quality of the cat Redmax sold me is well worth the cash. Tell him I sent you: 301-218-7336. He can sell you the same one he sold me (NOT the one shown on the site).

Side note: he sells about anything, but the site isn't the most up-to-date. Call him for stuff not shown.

 

This is incorrect. Turbo cars run rich to cool the combustion temperatures down.

 

You are correct. But in this particular case, he might have meant "cause the cat temperature to rise". That is fairly common AND will cause the cat to clog.

 

Yep. The cat combusts any remaining combustables. If theres excessive combustables, the cat heats up further, get more build-up, and eventually fails.

Mine, on the other hand, looked pretty much gunk free, it was just outright melted. I think the exhaust gases were VERY hot.

 

If you block the mid pipe and add a breather to the valve cover then the outside (unmetered) air will enter the vavle cover. then throught the PCV it will enter the intake mainfold. plain and simple. So a very small amount of unmetered air could cause a lean condition during cold idle and at WOT.

Since an engine creates vacuum most of the time it would appear that oil could not be blown out the valve cover hose, BUT under WOT the vac is close to zero and the intake could suck the oil through the mid pipe hose. thats why the factory mid pipe is actually called the oil separator. I know my hose has oil in it.

On a side note: if you put a filter on the mid pipe hose after the MAF then you will also suck in more air than metered, again leaning out the mix.

 

which is exactly how I ended up with a fried cat. KFC anyone?

 

Now that you've fixed this issue with the new cat and returning the valve cover hose to its correct position, what is your new fuel economy?

I am thinking I might have a bad cat but I regularly get around 21mpg . . . Just really interested to know what was "good" and what was "bad" fuel economy for you.

Regards,

Andrew

 

BAHHH! the PCV valve backseats during WOT, there is no flow. not only that, even if there was, the HUGE amount of air that is metered would dwarf a small leak, and certainly wouldnt cause a lean condition, especially under power enrichment.



you get oil from mist over a period of time, its caused by the surge between when the car goes WOT and the PCV valve backseats. going WOT for a period of time wont blow oil from the vent hose.

this is further reduced by those who replace the PCV valve regularly.

 

I agree, a rich condition will kill the cat, o2 sensors and cause the extreme heat. Would suggest you look at your o2 sensors to confirm...if they are black, you had a rich condition.

 

Adding a filtered vent to the valve cover will not cause a lean condition. The only reason that hose is their is to prevent sludge build-up from the constant misting of the oil in the valve cover. The O2 sensors constantly detect the a/f ratio and adjust it to optimum, that is the point of having O2 sensors, they let the ECU know what the a/f ratio is so small fuel adjustments can be made. The ECU has a preset amount of fuel it injects according the the amount of air metered by the MAF, and throttle position. That is only the first 2 steps. The O2 system is the follow up, to make adjustments to that fuel amount, in order to maintain 14.7 afr. You are not going to suck air back into that vent. I also am not argueing that your cat is bad, but the reasoning behind does not hold any weight. Nissan tech's are notorious for not knowing what they hell they are talking about. Slapping on a few performance parts on a car by following the instructions does not make anyone an expert troubleshooter. It seems you believe whatever they tell you, but I would still research why the cat failed, or it will most likely happen again.