Umrswimr

The car *barely* passed the high-speed NOX test, but failed the low-speed test. By a LOT. By Texas law, I can have it retested for free after I fix it, assuming it passes. If it fails, I have to pay again. So "just run some Seafoam through it" isn't going to work here.

The guy at Autozone told me it's most likely the cat at that level of failure, so it's going in to the muffler place for a test.

Question:
1) What causes high NOX levels? The Autozone guy told me that it was most likely NOT the O2 sensor or a filter
2) If I replace the cat, I'm considering having them lop off the entire resonator in the center of the car. Anyone done this?

Edit:
I found this interesting little article on the web about emissions:
http://www.autoshop101.com/forms/h56.pdf

Here are my values:
High speed (25 MPH)
Pollutant, "normal" value, my value
HC (ppm), 117, 71
CO(%), 0.65, 0.45
CO2(%), 14
O2(%), 0.7
NOX(ppm), 821,820

All pass. However, the low speed fails:
Here are my values:
High speed (25 MPH)
Pollutant, "normal" value, my value
HC (ppm), 120, 114
CO(%), 0.67, 0.67
CO2(%), 13.8
O2(%), 0.7
NOX(ppm), 907,1659

VetteSlave

I'm not so sure about what the people at Auto Zone told you. The first thing I would do is replace the oxygen sensor(s). If your car has 100K + it might be a catcon but usually when a cat starts plugging you can smell the change in exhaust scent and the car will be a little harder to start and feel like it's missing some power under throttle. Remember the oxy sensor causes the computer to set mix and if it's fouled/blackened from age it sends wrong info on start up that stays in the computer through the entire run cycle.
Best of luck your way!

Umrswimr

Stopped by a muffler place and they told me the cat is definately "clogged". However, they also indicated that it would be best to have a diagnostic done before I replace the cats because it could be something else that CAUSED the cats to fail.

He did mention that I can get a full Magnaflow exhaust added for about $300 at the same time.

VetteSlave

Just saw the numbers you added. How many miles on said car? In reality, a "clogged" or real bad cat would most certainly have tripped the oxy sensor if it's working and you would be looking at a service engine light on the dash. I think the muffler guy wants to sell you some stuff!
The Magnaflow for fourth gens is a great looking and sounding system for the money!

jdubois77354

I barely passed the smog on my '86 last year, after replacing the EGR and O2 sensor. Very high NOX, just a couple of points under the max at both high and low speed. This year I decided to check out the cat. When I pulled it from the car I was surprised to find it a completely empty can. I bought a new center cat from Midwest for about $279.....putting it in was no big deal, but it was not a perfect fit...I did not change out the "pre-cats" but NOX was close to zero on both low and high speed tests.....

You may also want to check your water temperature at full operating temps. My termostat had failed open and the car was not getting over about 150 degrees back when I did the O2 sensor and EGR. NOX level dropped also when I put the "correct" 180 degree thermostat in...

Just my results, your mileage may vary.....

MK 82

How do you "trip" an O2 sensor? The EGR system is designed to reduce Oxides of Nitrogen.

vetteblack

I would think if you had a clogged cat, your car would run like crap, like no power, and no high rpms either.

95Torch

I had that problem last month..., before you go and spend a bunch of money, heres what I did. The NOX emissions that failed my initial test were at 961. The person doing the inspection told me that it was probably because I did not drive it regularly and that it needed to blow out all the carbon. I gave this info to shop that does all my high end maintenance, And they sold me this stuff called "104 Emission Control", it's a fuel additive. I added to a tank of gas...ran it all out, put some fresh Shell 93. When I returned for the retake, my NOX reading went from 961 to 119. the inspector could not believe the drop! I did a search online for the product, but could not find anything. What part of Texas are you in, If you are in Houston, then go check out the Corvette Center on 610 and Main. Talk to guy named Calvin, tell him James with the red 95 sent you.

Good luck.

VetteSlave

Lack of oxygen at start up............

MK 82

Explain to me how "lack of oxygen at startup" has anything to do with O2 sensors.

VetteSlave

I had a cat problem on my '88 back when it was still under warranty. In that case it was a "clogged" converter! I got a light upon start up one day and it stayed on, by the time I got it to the dealer a few days later the car was hard to start and barely running. It was explained to me at that time by the dealer that on start up the computer system checks the O2 sensor for "flow" and that the reason the light was on days before was because the O2sensor was detecting a diminishing exhaust flow on start up. I've never heard anything to the contrary when I've explained this story to others, so I believe it to be the way it works.

Umrswimr

I thought the EGR system only ran at startup?

The car does occasionally idle poorly when the AIR pump is running. Otherwise the car runs fine.

Umrswimr

I pulled the codes:

1 (CCM) C12. What is this?
4 (PCM) H21, H25, H72. I assume "H" means history, so I'm not worried.
9 (ABS) ---
A (Airbag) ---

So what is CCM code 12?

SunCr

Scanning it would take the guess work out of it and you should clear those History Codes. 50 ignition cycles usually gets rid of them, so relatively speaking, these problems didn't occur that long ago (and you just might want to see if they reset). It is a bit rich - particularly at idle and all of these numbers can appear with a dead Cat. You want the scan to verify fuel delivery and that EGR is being commanded at cruise (and absent a code, it probably is and it's working the way it should be). Assuming that fuel delivery is deadon and a working EGR, then it's the CAT, but you want to verify that before you start throwing money at it. And finding that it doesn't run well with the Air Pump running points to something else - possibly a bad header check valve - which could make it run rich. I'd remove the check valves and inspect them. See if a scan shows a rich or lean condition on one side or the other. Repost with your scan results.

VetteSlave

I just asked a real guru I know and gave him the run down. He told me a bunch of stuff, but based on what you've described about what you're reading off the computer, long and short, he thinks your converter is plugged too!

Umrswimr

What the heck is a "header check valve"?

It's only 1/25 times that it stumbles a bit when the AIR pump is running. I suppose that could be a result of a clogged cat- the air being pumped in has nowhere to go...

The muffler guy did use a heat probe to test it- it read hotter before the cat than after which he indicates is a sign of a bad cat. He clearly wasn't just trying to "sell me something" as he's the one that told me to go get it diagnosed before throwing money at it.

MK 82

O2 sensors don't know diddly about flow. They sense unburnt oxygen and generate a corresponding voltage that is interpreted by the PCM. When the engine is in open loop after startup, they are not used by the PCM. After a period of time when they are hot and functioning, the PCM enters closed loop and uses the O2 sensor to adjust fuel delivery.

Consider this your contradiction. Don't listen to the person that told you that fairy tale.

VetteSlave

You got me to wondering. I just pulled out the paper work from 1990 and it reads that the "tech" pulled code #13 which I believe to be the O2 sensor? It goes on to say after pulling code tested sensor voltage...OK. Then Found converter to be defective. Paperwork seems to say what I said. Why did the car give a code #13 which apparently led them to the diagnosis????..........

MK 82

The EGR system is prohibited when:
1. Trans is in Park or Neutral.
2. Throttle Angle is less than a predetermined value.
3. Wide Open Throttle.

The PCM uses engine coolant temp (ECT) and intake air temp (IAT) to determine when to enable the EGR system. If IAT is above 59F at startup, EGR will be enabled at 140F.

MK 82

What vehicle are we talking about? The codes in OBDI are different from OBDII. The OP has a 94 OBDI.