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NOx level high 91 Legacy


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15 replies to this topic

#1 gbrand

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Posted 08 August 2005 - 10:19 PM

Failed inspection tailpipe test today-91 legacy non turbo FWD 2.2L 5 spd.

NOx was 951(limit 970) on low speed (15mph) test but on high speed(25mph) was 1074 (limit 878). HC and CO very low, HC was 18(limit 128) low speed and 9(limiit 128) at high speed. CO was 0.03(limit 0.73) low and 0.01(limit 0.69).

Afer searching the net and finding information such as http://www.autoshop1...m/forms/h64.pdf and others I conclude I am running lean and/or have higher than normal combustion temperatures.

Probabale causes are timng(not adjustable on this car), EGR passages plugged(no EGR on this engine, corrrect?), O2 sensor giving false readings, carbon deposits raising compression, or as last result a bad cat conv.

Looked at O2 sensor with scope, seems to be switching OK, no vacuum leaks in hoses or accessories, did change PCV valve as a little dirty.

Read about a car(93 buick century) that was failing NOx, problem was gunk on the MAF sensor casuing engine to be sucking 25% more air than MAF was telling the ECU, cleaning the MAF brought the A/F ratio back to spec and the NOx dropped dramaticly.

Car runs fine, good performance and MPG's despite high miles. Has anyone cleaned the MAF in this type of car, or have experience corrrecting lean condition or high NOx emissions?

Thanks for your input.......

#2 Steve530

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Posted 09 August 2005 - 12:51 AM

Failed inspection tailpipe test today-91 legacy non turbo FWD 2.2L 5 spd.

NOx was 951(limit 970) on low speed (15mph) test but on high speed(25mph) was 1074 (limit 878). HC and CO very low, HC was 18(limit 128) low speed and 9(limiit 128) at high speed. CO was 0.03(limit 0.73) low and 0.01(limit 0.69).

Afer searching the net and finding information such as http://www.autoshop1...m/forms/h64.pdf and others I conclude I am running lean and/or have higher than normal combustion temperatures.

Probabale causes are timng(not adjustable on this car), EGR passages plugged(no EGR on this engine, corrrect?), O2 sensor giving false readings, carbon deposits raising compression, or as last result a bad cat conv.

Looked at O2 sensor with scope, seems to be switching OK, no vacuum leaks in hoses or accessories, did change PCV valve as a little dirty.

Read about a car(93 buick century) that was failing NOx, problem was gunk on the MAF sensor casuing engine to be sucking 25% more air than MAF was telling the ECU, cleaning the MAF brought the A/F ratio back to spec and the NOx dropped dramaticly.

Car runs fine, good performance and MPG's despite high miles. Has anyone cleaned the MAF in this type of car, or have experience corrrecting lean condition or high NOx emissions?

Thanks for your input.......


I agree that the engine may be running lean or the cat may be bad. I think the low HC and CO tend to rule out the cat, though.

I think I'd check for air leaks downstream of the MAF. You may need to take it to a mechanic with an analyzer to find the problem.

Good Luck.

#3 Legacy777

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Posted 09 August 2005 - 11:39 AM

The fact it has low CO & HC readings doesn't rule out the cat.

At high combustion temps, CO & HC will be low. At low combustion temps, NOx will be low, but CO & HC will be through the roof. There is a medium point at which is the best you can get. That is when the two curves cross. That is however if you do not have a catalyst.

I'd check for air leaks and such. Dirty MAF probably wouldn't cause your problems, but you can clean it if you want. Take the MAF off, take the sensor portion out of the housing. Spray/clean it with brake cleaner. Hold the can a little bit aways so it's more of a mist that hits the sensor. I also took the back plate off of the housing, and cleaned that up as well.

Is the car hard to start when cold? Other then air leaks or MAF sensor, the engine controls the amount of fuel based on the coolant temp sensor. It's a relatively cheap sensor, about 20 bucks from one of the online subie dealers.

If you do all that has been suggested, then it's probably your cat.

Other thing I'd suggest is to make sure the cat is good and warmed up. If you had the car off or it set for a while before they tested it, and the cat isn't 100%, it's very possible it failed because of that. Go drive the piss out of it, and then get it tested.

Also, probably couldn't hurt to run some seafoam through.

#4 Steve530

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Posted 09 August 2005 - 10:55 PM

The fact it has low CO & HC readings doesn't rule out the cat.

At high combustion temps, CO & HC will be low. At low combustion temps, NOx will be low, but CO & HC will be through the roof. There is a medium point at which is the best you can get. That is when the two curves cross. That is however if you do not have a catalyst.


That is correct. When the fuel misture is lean, the engine runs hot and NOX is produced but almost all of the HC is burned and little CO is produced.

When the misture is rich, some of the HC is not burned and CO is produced. The engine runs cool because of the excess HC and little NOX is produced.

The catalytic converter oxidizes CO to CO2, HC to water and CO2, and reduces NOX to N2 and O2. A properly opeating catalytic converter can mask problems, though.

Anyway, after looking at the results again, it seems that your engine is running lean. The catalytic converter ma or may not be bad. Considereing the cost of a converter, I'd have the exhaust checked with a gas analyzer before I replaced the converter.

#5 gbrand

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Posted 10 August 2005 - 07:18 PM

Thanks all-I had Seafoam in the tank, will prolly get some and run through the intake to try to clear carbon from combustion chambers. Car always starts and runs fine. I cleaned the MAF using contact cleaner, didn't see any obvious gunk, maybe a little dust.

Now for the strange part-put a timing light on it tonight just for grins. Assumed little "V" in front of front of pulley was timing mark, and the spec 20B mark was just a little clockwise of 12 Oclock facing the pulley. Now the number one cylinder the timing mark showed to be at about the 3-4 Oclock position, as well as the other front cylinder. The back cylinders showed the opposite position, about 9-10 Oclock. Almost as if timing mark on pulley was 90 degrees clockwise from where it should be? Again the car starts and runs flawlessly so don't see how the timing could be out this far out but couldn't see anything else that looked like a timing mark.

I will look at some other stuff tomorrow too, and make sure it is very hot upon retest. I tried to find the coolant sensor today while waiting for the DSL technician to show up, is it the 2 wire sensor under the intake manifold on the pass side of the car toward the rear of the engine? I saw a one wire and a two wire sensor in that area.

Thanks again...will post results good and bad.....

#6 rweddy

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Posted 10 August 2005 - 09:10 PM

Thanks all-I had Seafoam in the tank, will prolly get some and run through the intake to try to clear carbon from combustion chambers. Car always starts and runs fine. I cleaned the MAF using contact cleaner, didn't see any obvious gunk, maybe a little dust.

Now for the strange part-put a timing light on it tonight just for grins. Assumed little "V" in front of front of pulley was timing mark, and the spec 20B mark was just a little clockwise of 12 Oclock facing the pulley. Now the number one cylinder the timing mark showed to be at about the 3-4 Oclock position, as well as the other front cylinder. The back cylinders showed the opposite position, about 9-10 Oclock. Almost as if timing mark on pulley was 90 degrees clockwise from where it should be? Again the car starts and runs flawlessly so don't see how the timing could be out this far out but couldn't see anything else that looked like a timing mark.

I will look at some other stuff tomorrow too, and make sure it is very hot upon retest. I tried to find the coolant sensor today while waiting for the DSL technician to show up, is it the 2 wire sensor under the intake manifold on the pass side of the car toward the rear of the engine? I saw a one wire and a two wire sensor in that area.

Thanks again...will post results good and bad.....


Another way to pass is to have your tank at about 1/4 full and put 3-4 cans of heet into your tank. You will pass with flying colors.

#7 Gnuman

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Posted 10 August 2005 - 10:00 PM

I tried to find the coolant sensor today while waiting for the DSL technician to show up, is it the 2 wire sensor under the intake manifold on the pass side of the car toward the rear of the engine? I saw a one wire and a two wire sensor in that area.

Thanks again...will post results good and bad.....


the single wire is the sensor for the guage in the dash, and the two wire one is the one that Josh (Legacy777) recomended replacing. It is the sensor that the ECU reads.

#8 Legacy777

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Posted 11 August 2005 - 11:45 AM

Personally, don't even bother looking at what the timing mark on the pulley is....it really doesn't matter. As long as you aligned everything up properly on the crank gear (using the mark in the back of the gear, near the tooth, and not the front mark), and the cam sprockets, you should be good.

#9 gbrand

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Posted 11 August 2005 - 02:35 PM

OK, here is my update for today.

Timing marks- I pulled the #1 plug, rotated the crank with a screwdriver in the hole and found what I felt to be TDC. Marked pulley, fired up engine, my new mark lined right up with the 20BDC so I dont know that other mark signifies but know the idle timing is good.

Collant sensor-I pulled it, a moderate PIA, after removing air hose assy's for PCV I was able to get a 3/4" deep well on it and get it out.

Read 5.45K @ 30F, 2.16K @ 72, 790 O @ 122, 220 O @ 180

Specs in my Chiltons were 7-11.5K @ 14F, 2-3K @ 68, 700-1000 O @ 122

But specs in Autozone online repair manuals, normally a good source of infor, listed 133.9-178.9 @ 122F, 47.5-56.8 @ 176, 26.2-29.3 @ 212 , 14.9-17.3 @ 248.

My sensor seemed to follow the specs in Chilton, so figure it is good and reinstalled it.

Checked vacuum, good, did some spraying around intake maifold to see if there was leak, nothing detected.

Ran 1/3 can of Seafoam throught he PCV valve, then ran the snot out of it.

more to follow

#10 Legacy777

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Posted 11 August 2005 - 08:48 PM

The coolant temp sensor can read ok, but still not be working the way it should. I can't say why, but myself and others have run into this. I've found that it's less hassle to just replace the damn thing since it's only 20 bucks.

#11 gbrand

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Posted 14 August 2005 - 09:07 PM

Has anyone ever sued this stuff http://www.rxp.com/ esp in context of reduciing NOX emissions. Normally have pretty dim view these types of aproducts but test results , both listed and in comments from users, appears to have some favorable results. Also surpiresed to see Ed Wallace, who I feel is a no BS automotive columist I have read for years recomends the stuff.

#12 NorthWet

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Posted 14 August 2005 - 09:35 PM

The coolant temp sensor can read ok, but still not be working the way it should. I can't say why, but myself and others have run into this. I've found that it's less hassle to just replace the damn thing since it's only 20 bucks.

In the Old Gen cars, the CTS usually has problems not with the sensor itself, but with the connectin between the sensor and the wires; they get corroded, gunked-up, whatever. At least one of our folk has taken to soldering leads to the CTS and having the connector further out where it stays cleaner and is easier to maintain.

Regarding cats, not sure (yet) how they package them on the Legs, but they use 2 separate technologies in the cats: One is an oxidization catalyst that oxidizes HC and CO, and a reduction catalyst that reduces NOx (pretty much as Steve530 wrote). In the Old Gens, these are 2 physically separate cat-cons. In most newer cat-cons, they are still 2 separate "bricks" inside a common case. So, just because HC and CO is being controlled doesn't mean that the cat-con(s) is(are) OK.

And engines run warmer when running lean due to rejected heat, but the flame is actually cooler. If all they are doing is running it up to RPM with no load, I would not expect the combustion peak temps to climb very high; if part load, I would still not expect high peaks. If your engine has EGR, that would be my first suspect (but you say it doesn't); my second choice would be the reduction cat-con.

#13 gbrand

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Posted 19 August 2005 - 10:35 AM

Just got back from retest-PASSED, NOx down, HC and CO up but within limits

NOx was 944(limit 970) on low speed (15mph) test but on high speed where I failed previously (25mph) was 832 (limit 878). HC was 105(limit 128) low speed and 49(limiit 128) at high speed. CO was 0.21(limit 0.71) low and 0.10(limit 0.69).

I did use the RXP in about 8 gallons of gas and ran to near empty, then filled up with about 4 gallons before the test and made sure the car was FULLY warmed up inclduing idling in the parking lot for 20 minutes waiting my turn for testing.

Next year, if the car is still around, I will do the SeaFoam, RXP, etc. before the first test.

Thanks for everyones input, BTW the connectors on the CTS looked pristine, gold colored not usual corroded copper color that I expected.

#14 gbrand

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Posted 05 August 2006 - 05:21 PM

OK, a year later-got ready for inspection this year by running RXP in about 10 gallons of fuel, new air filter, cleaned MAF sensor wires, and then ran 1/3 can seafoam thru power brake line connection. After that changed out plugs, and this morning took out for 20 minutes on freeway to fully heat up cat convertors and everything else (was already at least 90 outside and climbing) and took into inspection station(left running to keep cat hot)

After waiting to recalibrate machine, they ran the ASM tailpipe test-same station as last year, same machine, even the same test procedure-but imagine my shock not to see it pass first time out, but, to see on the printout that the NOx level was not even tested! other results great:

High speed test HC was 17, limit 220 CO was .04, limit 1.2
Low speed test HC was 53, limit 220 CO was .05, limit 1.2

So was running a bit lean if anything, as it was on first (failed) test of last year-but with NOX test would have passed with flying colors last time on the first test. Im not going to complain, won't worry about it till next year.

BTW turned 265000 miles this week-well on way to 300K

#15 gbrand

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Posted 05 August 2006 - 05:58 PM

OK, went back and re-read test reports from 2005 and 2006 line by line-and discoved the difference. 2005 was an ASM(dynomometer) test and 2006 was a TSI(two speed idle test). If the engine is not under load, regardless of engine speed, then NOX is not produced so is proabably not monitored for the TSI test. Still dont know why the test spec for the car was changed, but hope they continue to run it on TSI. We'll see enst year.

#16 gbrand

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Posted 25 August 2007 - 11:33 AM

2007 inspection PASSED, first time out, they did the Two Speed Idle test and HC and CO came in WAY lower than the limits.....and NOX not looked at at all, so at 282K+ is good to go for another year.




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