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Old 08-07-2010, 01:27 AM   #1
Linnie
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1994 SC2
Question 94 SC2 failed CA Smog NOx High only at 15 Mph

Hello,

I have a 1994 SC2 1.9L DOHC (automatic) with approx 128,000 miles (odometer stopped at 124,525). I only put about 1500 miles a year on it.

It failed CA Smog Test only on the NOx at 15 Mph. My test results were:

15 Mph: RPM 1991 CO2 14.6 O2 0.1
HC Max 77 Ave 39 Meas 63
CO Max 0.39 Ave 0.05 Meas 0.12
NO Max 504 Ave 184 Meas 792 FAIL

25 Mph: RPM 1993 CO2 14.5 O2 0.1
HC Max 41 Ave 23 Meas 40
CO Max 0.39 Ave 0.05 Meas 0.07
NO Max 542 Ave 169 Meas 420 PASS

All Visual Inspections and Functional Checks Passed.

I have been told to go to a test and repair facility to have it fixed and then back to original Smog station for retest.

I am looking for advice on what would cause it to fail on only one speed, and/or what to have checked (my father usually helps me with these things).

If any further info is needed, please advise.

Thank you for any help you can give.

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Old 08-07-2010, 01:39 AM   #2
quantumech
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2000 SL1
Default Re: 94 SC2 failed CA Smog NOx High only at 15 Mph

I believe the speed they do that at is about the same place where the transmission shifts from 1st to 2nd or 2nd to 3rd or something like that. Haven't experienced the issue, but I've seen other have it. The solution is to put it into 2nd, thereby preventing it from shifting to the "bad gear", to do the test.

...
Sasha: 2000 SL1, Manual, cracked head Sept. 2014. Donated Dec. 2014

I learn from my experiences. If I screw up, I'm sorry.

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Old 08-08-2010, 03:18 PM   #3
adventureoflink
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1997 SL2
Default Re: 94 SC2 failed CA Smog NOx High only at 15 Mph

Suprised the ECTS and thermostat weren't mentioned yet...

did you try changing that? Instructions are in the how-to library.

...
97 SL2
DOB: 3/19/97
Date Obtained: 5/30/07
Status: Alive, 1/2 exhaust

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Old 08-08-2010, 05:10 PM   #4
acman
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1999 SC1
Default Re: 94 SC2 failed CA Smog NOx High only at 15 Mph

NOx is controlled by a number of things with the EGR system being at the top of the list.
Based on the smog report numbers, it appears as though the EGR system is working correctly.
A lean running engine will increase NOx. A restricted fuel filter does not allow the fuel
injectors to receive sufficient fuel and causes the engine to run lean. Based
on your numbers, the fuel filter looks ok but replace it if it has been in service
for over 30K miles. Excessive engine operating temperatures, usually due to a
restricted radiator, or thermostat that fails to fully open, will increase NOx.
That does not appear to be the case here. A slow O2 sensor has a profound
influence on controlling NOx. Given that the the NOx is high at low speed, a slow O2 sensor
is a possibility. The CO at low speed is also elevated which could be due to
an old, or slow O2 sensor. Assessing O2 sensor operation at home is
difficult though not impossible. If you want information on how to test,
please ask. When the engine coolant temperature sensor is defective these engines
run pig rich and have very low NOx due to the cooling effect of the extra
fuel. I don't think the ECT sensor is an issue here. Which brings us to the
catalytic convertor. At 128K miles it is probably on the weak side and more
than likely is the source of the elevated NOx. After the cat gets
sufficently hot it works fine as seen with the 25 mph emissions numbers.

Here is what to do (after checking fuel filter and maybe replacing O2
sensor). At the smog station, keep the engine running at 2500 - 3000 RPMs until the
mechanic takes the car. They will ask you for the key when they
write the service order, so have an extra one with you. Sit in the car
with your foot on the gas pedal until they ask for the car. By keeping
the cat hot, it will probably pass inspection. Also, give the smog
mechanic a copy of the last smog report, it will help him enter the
information more quickly and get your car rolling on the dynomometer
where tailpipe emissions are measured.

...
1997 SL2, 5 speed
1999 SL1, Automatic

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Old 08-08-2010, 08:31 PM   #5
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1992 SL2
1997 SL2
Default Re: 94 SC2 failed CA Smog NOx High only at 15 Mph

A decarbon service can lower the NOX---DID THEY HAVE A FAN RUNNING IN FRONT OF YOUR CAR WHEN THEY DID THE TEST!!!!!!!! When they tested my car the car failed. I took the car back to work and it passed. I have go to a test only with my 1992 Satrun. I cant do it at work.--You could add Redline water wetter to your coolant that will lower your temp and my make it pass and do a decarbon service.

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Old 08-08-2015, 09:51 PM   #6
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Default Re: 94 SC2 failed CA Smog NOx High only at 15 Mph

Reviving an old thread. Thanks for all the good info on here. I have a 1994 SL1. I failed NOx emissions recently:

HC: 0.31 CO: 3.24 CO2: 301.40 NOX: 4.12

I flushed the coolant system, replaced the ECTS, drove 10 miles on the freeway just before the test and my results were:

HC: 0.34 CO: 7.96 CO2: ? NOX: 2.85

I still failed NOX (limit is 2.00) but those changes did improve things.

I plan to next replace the fuel filter (hasn't been replaced ever AFAIK) and O2 sensor. I don't really know what or why I'm doing things other than following the great advice on here.

If anyone happens to read this and thinks that replacing the fuel filter and O2 sensor isn't the right direction to go, I'd love to hear other suggestions.

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Old 08-08-2015, 10:32 PM   #7
RobertGary1
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1996 SC2
2006 VUE 3.5L
Default Re: 94 SC2 failed CA Smog NOx High only at 15 Mph

Clean the EGR.

-Robert

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Old 08-10-2015, 12:21 AM   #8
TomM96
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Default Re: 94 SC2 failed CA Smog NOx High only at 15 Mph

I'm not familiar with '94...

Some threads of possible interest:

http://www.saturnfans.com/forums/sho...d.php?t=208281
1997 SW2 failed smog by 6ppm @ 15mph
http://www.saturnfans.com/forums/sho...d.php?t=193979

Exhaust vs emissions ; CO / HC vs NOX :
http://www.saturnfans.com/forums/sho...d.php?t=208882

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Old 08-10-2015, 12:39 AM   #9
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2005 VUE 2.2L
Default Re: 94 SC2 failed CA Smog NOx High only at 15 Mph

Some EGR valves need backpressure to work properly.


With my 86 Toyota truck (22RE) I used to have a problem with just barely squeaking thru on NOX...


I fixed it buy putting this behind the cat for the test:


...
Ed

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Old 08-15-2015, 03:03 PM   #10
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Default Re: 94 SC2 failed CA Smog NOx High only at 15 Mph

I was able to find the EGR on my 94 SL1 but am having issues removing the bracket holding it in place. The two upper bolts were easy to remove (13mm) but for the lower arm of the bracket I can't figure out how to get it off and visibility from under the hood isn't great. I used a cell phone to take a picture but still can't figure out how it's connected.

Anyone familiar with how this is held on?
Attached Images
File Type: jpg 20150815_115238_resized - Copy.jpg (197.3 KB, 6 views)

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Old 08-15-2015, 03:10 PM   #11
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Default Re: 94 SC2 failed CA Smog NOx High only at 15 Mph

The EGR valve is up on top of the engine, just to the left (driver side) of the cylinder head; that picture doesn't pertain to it.

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Old 08-15-2015, 03:19 PM   #12
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Default Re: 94 SC2 failed CA Smog NOx High only at 15 Mph

That picture was a side/bottom view of where the lower part of the EGR bracket is connected, location 'C' in the pictures here.

I can't figure out how to get the bracket off of what feels like a threaded bolt at position C. The bracket, whose upper bolts are at locations A and B, come down in a V-shape about 4 inches to where the two sides of the bracket meet and are secured at point 'C' by something that I can't figure out how to remove.
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File Type: jpg 20150815_121243_resized.jpg (217.2 KB, 6 views)

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Old 08-15-2015, 03:25 PM   #13
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Default Re: 94 SC2 failed CA Smog NOx High only at 15 Mph

I'm guessing this wasn't originally a Calif car; in '94 the Calif. ones had the electric EGR valve, all up on top. Sorry, can't help you, never seen one like that.

Edit: Oh, wait, do you have and SL1, not the '94 SC2 of the subject line? It adds to the confusing when you scab-on to an old thread...

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Old 08-15-2015, 03:31 PM   #14
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Default Re: 94 SC2 failed CA Smog NOx High only at 15 Mph

sorry, yes, this is an SL1 I'm asking about.

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Old 08-15-2015, 04:41 PM   #15
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Default Re: 94 SC2 failed CA Smog NOx High only at 15 Mph

in case someone else stumbles across this thread looking for 94 SL1 EGR info, this other thread I finally found is a better place to look for info:

http://www.saturnfans.com/forums/sho...d.php?t=114636

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