Anyone's bad cat converter ever allow high NOx?

[supraguy@aol]

Now every year, I normally have to do a bit of tweaking to pass emissions, and it's usually with the EGR valve. And as usual, I had high NOx when I tried this year- roughly 1800ppm vs. the allowed ~950ppm. The HC's(and everything else) were easily 20% under max limits. Well, I checked the EGR system as per the TSRM, and verified that the VSV, EGR valve and EGR regulator are all working. I even removed the EGR and verified that it was clean and flowed well. However, after 4 tries now, I still have high NOx levels.
My catalytic converter is about 2 years old, and was installed to replace a blocked cat. I'm just wondering if you guys think that my NOx could be the result of a worn out cat. I know that the cat is not blocked up, as I have no boost problems like I did with the other one, I'm just wondering if I might have worn out the substrate to the point were it's not reacting with the NOx emissions. If anyone has any experience with this, please chime in.

PS- my car is an 88'- how much longer till I'm exempt from emissions testing?!?
:aigo:

PPS- How the hell do some of you guys pass emissions without a cat????

 

[skipbarber]

high nox is a result of high cylinder temperatures. over-advanced timing, lean a/f mixtures, etc. how long are you waiting in line to get your car on the rollers at the emissions test facility? usually when you sit idling for long periods of time, the cat can cool off and literally not do its job. how old is the o2 sensor in your exhaust? make sure you dont have vacuum leaks, or excessive oil in your intake..

 

[Nick M]

You have an aftermarket downpipe with out a pre-cat? That is a big piece of the puzzle. As is the EGR, and engine timing.

What are the rest of the numbers? You need HC's and CO to get rid of NOx. In the cat that is.

 

[supraguy@aol]

Engine timing is at 10btc. O2 sensor is 1 yr. old and operating normally.
Time from driving till rollers was always less than 5 minutes. Unsure if the mixture is running lean at all, but it's set at the same setting on MAFT as last year's test. Maybe I'll bump up the part-throttle mixture a bit, and see if the temps drop without killing the HC readings.

PS- I ordered a new 3" cat- was only $50 shipped. If the fuel richening does the trick, I'll just shelf the new cat till I need it.

 

[jdub]

Did you check the EGR gas channel through the intake manifold to under the TB was clear?
I've seen them clog up with crud over time.

 

[supraguy@aol]

No, I didn't check it, but I did check to make sure that the EGR itself was feeding the intake- I applied suction directly to the valve, and the engine began to stumble and die- as per the TSRM. I assume that shows there is sufficient flow into the intake manifold, but I might just take the EGR back off, and check the flow thru the intake directly.

 

[jdub]

You can use a piece of 3/8" wire rope (fray the end a bit) and an electric drill to clean out the passage. Just use a shop vac to suck the crap out of the passage after you're done.

 

[supraguy@aol]

You can use a piece of 3/8" wire rope (fray the end a bit) and an electric drill to clean out the passage. Just use a shop vac to suck the crap out of the passage after you're done.

LOL-
I've done it exactly that way before- I 'roto-rootered' the EGR itself as well.

PS- I was thinking about the intake blockage issue a bit more last night, wondering if maybe there's just enough EGR flow at idle to cause stumbling, but not enough flow at speed, due to the possible restriction of the intake you mentioned. That could certainly be possible, and wouldn't necessarily be revealed by the TSRM diagnostic.

 

[3p141592654]

Can you post up your numbers for HC, CO, CO2, O2 and NOx? Then we can at least calculate what you mixture is.

 

[Rennat]

i had similar issues, EGR SEEMED to be working, car would stumble when you unhooked the one or however you do it...

i just ended up getting new spark plugs and a new cat and reset the timing to make sure it was in spec. i might have retarded it a few degrees too... i cant remember.

but a new cat got me to pass. wasnt clean... but it passed.

 

[Nick M]

You have to pull the valve open with vacuum, you can't just unhook it. That only creates a ported vacuum leak.

 

[supraguy@aol]

Well, here's the results-
I replaced the cat(2 yrs old and no blockage) with a $50 3" cat off of Ebay.
Going into the test, my HC was around 108 out of 140 max, and the NOx was 1851 out of 984 max. After the cat, my HC went down to 64/ 140 max, and the NOx went to 675/ 984 max. SO, it appears that the cat was the culprit afterall. I asked the tech what the cat looked like when he got it off, and he said that the core looked fine to him. I'm just wondering if maybe the core got shellacked or coated to the point of being unreactive to the exhaust gases. Anyway, it only took 6 tries, but my emissions are finally clean.