Emissions: great HC & CO, poor NOx
10-29-19, 10:55 PM
#1
Emissions: great HC & CO, poor NOx
Took my car for its first CA smog today. Unfortunately it did not pass -- the vehicle was about 5% over NOx allowance at 25mph. See attached emissions test sheet. This came as a bit of a surprise to me as the vehicle has only 27K miles. The HC & CO emissions are also quite good at 16PPM / 0%. Doing some googling and reading on here, it seems as though the air pump doesn't have much to do with NOx, the EGR is next to useless, and that running lean is usually what causes NOx issues?
The car is not modified. What could be causing the high NOx emission on a car with this low of miles? My thoughts are possibly gunked up injectors from sitting around so long, or a shitty old fuel filter. The previous owner had the car in a garage for 18 years, and said we drove it about once every 2 months. He never did the plugs, wires, fuel/air filter, etc. Only oil every year and coolant every two years.
I'm doing a load of maintenance to it weekend after next, but also some reliability/performance mods (DB, CB, air filter, performance fuel pump). I hate to screw up the baseline with modifications before even passing smog, but what can I say, I'm impatient! 
The car is not modified. What could be causing the high NOx emission on a car with this low of miles? My thoughts are possibly gunked up injectors from sitting around so long, or a shitty old fuel filter. The previous owner had the car in a garage for 18 years, and said we drove it about once every 2 months. He never did the plugs, wires, fuel/air filter, etc. Only oil every year and coolant every two years.
I'm doing a load of maintenance to it weekend after next, but also some reliability/performance mods (DB, CB, air filter, performance fuel pump). I hate to screw up the baseline with modifications before even passing smog, but what can I say, I'm impatient!
Last edited by mkd; 10-30-19 at 12:58 AM.
10-30-19, 09:52 AM
#2
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The stock EGR valve's gasket is basically a pinhole and can easily be plugged up with carbon. It may be worth pulling the EGR valve, getting a new gasket, and cleaning the carbon out of the EGR valve.
Typically EGR handles NOx emissions.
Others in emissions testing areas may have better ideas, I've never had to go through an emissions test.
Dale
Typically EGR handles NOx emissions.
Others in emissions testing areas may have better ideas, I've never had to go through an emissions test.
Dale
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IMAGINETHAT (10-30-19)
10-30-19, 10:57 AM
#3
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The stock EGR valve's gasket is basically a pinhole and can easily be plugged up with carbon. It may be worth pulling the EGR valve, getting a new gasket, and cleaning the carbon out of the EGR valve.
Typically EGR handles NOx emissions.
Others in emissions testing areas may have better ideas, I've never had to go through an emissions test.
Dale
Typically EGR handles NOx emissions.
Others in emissions testing areas may have better ideas, I've never had to go through an emissions test.
Dale
10-30-19, 10:58 AM
#4
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i have had a couple fail NOX, and its basically caused by it being too lean. Nox comes up really quickly.
you might look at the o2 sensor, its been the problem for me.
you might look at the o2 sensor, its been the problem for me.
10-30-19, 11:38 AM
#5
Thanks for the tips.
The FSM lists the EGR in the diagram with the text, "EGR SWITCH (CALIFORNIA ONLY)" which to me implies that since this car was not originally sold in CA it may not even have an EGR. I will have to look for it this afternoon.
Dale, I recall reading a thread where you talked about the EGR being nearly useless on rotaries: https://www.rx7club.com/3rd-generation-specific-1993-2002-16/egr-failure-leads-detonation-936685/ Maybe "very worthless" is good enough for a 5% reduction in NOx @ 25 MPH
Looks like the EGR may be a bit of a bitch to get at -- will have to see when I have things apart anyway. Maybe I'll do the tune-up work, and replace the O2 sensor before messing with the EGR. I just think it is strange for an O2 sensor to go bad after 27K miles.
The FSM lists the EGR in the diagram with the text, "EGR SWITCH (CALIFORNIA ONLY)" which to me implies that since this car was not originally sold in CA it may not even have an EGR. I will have to look for it this afternoon.
Dale, I recall reading a thread where you talked about the EGR being nearly useless on rotaries: https://www.rx7club.com/3rd-generation-specific-1993-2002-16/egr-failure-leads-detonation-936685/ Maybe "very worthless" is good enough for a 5% reduction in NOx @ 25 MPH
Looks like the EGR may be a bit of a bitch to get at -- will have to see when I have things apart anyway. Maybe I'll do the tune-up work, and replace the O2 sensor before messing with the EGR. I just think it is strange for an O2 sensor to go bad after 27K miles.
Last edited by mkd; 10-30-19 at 12:55 PM.
10-30-19, 12:44 PM
#6
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I don't recall EGR being TOO bad. I don't think you have to pull the turbos, but I could be wrong.
All US cars had an EGR valve, it was vacuum actuated. The California spec cars had an EGR switch - there was a switch on the EGR valve that when it moved from the vacuum it tripped to tell the ECU "yes I did open". The valve itself is the same, just the switch is different.
O2 sensor could be fouled/corroded/who knows. Worth looking at. O2 sensors are pretty cheap.
The EGR has a small pin hole that the gasses go through, kind of like Mazda figured it would just carbon up anyhow. But, a small amount could help. Again, I know the principle of this but not the practice.
Dale
All US cars had an EGR valve, it was vacuum actuated. The California spec cars had an EGR switch - there was a switch on the EGR valve that when it moved from the vacuum it tripped to tell the ECU "yes I did open". The valve itself is the same, just the switch is different.
O2 sensor could be fouled/corroded/who knows. Worth looking at. O2 sensors are pretty cheap.
The EGR has a small pin hole that the gasses go through, kind of like Mazda figured it would just carbon up anyhow. But, a small amount could help. Again, I know the principle of this but not the practice.
Dale
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10-30-19, 01:26 PM
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1995 cars do not have EGR.
1994 cars all have the EGR with the Switch
1993 all cars have EGR, only California gets the valve with the switch
1994 cars all have the EGR with the Switch
1993 all cars have EGR, only California gets the valve with the switch
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IMAGINETHAT (11-08-19)
10-30-19, 01:56 PM
#9
The high NOx is saying you're running lean and also results in the squeaky clean CO and HC. #'s.
Change the oxygen sensor and spray carb cleaner around the manifold to check for vacuum leaks. Primary injector flow would also be suspect, but isn't easily checked.
If you're going the EGR route, you'd need to clean the passages through the manifold, not just the valve and the opening there. It will easily pass without an EGR so don't bother..
Change the oxygen sensor and spray carb cleaner around the manifold to check for vacuum leaks. Primary injector flow would also be suspect, but isn't easily checked.
If you're going the EGR route, you'd need to clean the passages through the manifold, not just the valve and the opening there. It will easily pass without an EGR so don't bother..
10-30-19, 02:10 PM
#10
Highly likely to be the O2 sensor. The 25mph zone is where O2 feedback is most active in those steady state tests. EGR is a red herring as others have stated.
Old OBD 1 cars do not have a sophisticated oxygen sensor diagnostic monitor. It can only detect an open or short circuit basically. Anything 1996 or newer has a rear sensor to help diagnose the main sensor.
Old OBD 1 cars do not have a sophisticated oxygen sensor diagnostic monitor. It can only detect an open or short circuit basically. Anything 1996 or newer has a rear sensor to help diagnose the main sensor.
10-30-19, 02:43 PM
#12
You're looking for air leaks after the throttle plate. Intake leaks will allow more air in than your ECU is expecting and with a MAP based system the ECU won't ever know the difference.
Though typically I would expect this to bump up the idle a bit, what is the car idling at?
11-07-19, 10:27 AM
#16
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11-07-19, 09:20 PM
#17
California smog is just silly, or it was the last time I heard about it. Newer cars get a 'computer test' which just asks the ECU to report any OBD faults. People with reflashed ECUs can adjust the ECU so it reports 'everything is fine here' and then the car passes even if all the O2 sensors have been deleted. Older cars get a sniffer up the tailpipe, and a dyno test running at bizarrely irrelevant speeds like 15mph and 30mph. Older cars also get a visual inspection that will fail you for nearly anything that isn't stock, even if the sniffer says the engine runs clean. They're even supposed to fail you if you've replace the original cat converter with a newer better one, because the car was 'designed to work with the Original Equipment only'.
But there is no safety inspection, for any reason, ever. You can drive in and out of the smog inspection station with the brakes completely worn down to the metal and grinding on the rotors. Tires, shocks, and suspension don't keep the air clean so the DMV isn't interested in checking up on them. I don't disagree with the concept of clean air and I can vouch that California air quality is better than it was in the 1980's. But some of the specifics could really be improved. For instance, I don't think semi trucks get any testing which is dumb since they have huge engines and operate at heavy loads for 8+ hours every day.
But there is no safety inspection, for any reason, ever. You can drive in and out of the smog inspection station with the brakes completely worn down to the metal and grinding on the rotors. Tires, shocks, and suspension don't keep the air clean so the DMV isn't interested in checking up on them. I don't disagree with the concept of clean air and I can vouch that California air quality is better than it was in the 1980's. But some of the specifics could really be improved. For instance, I don't think semi trucks get any testing which is dumb since they have huge engines and operate at heavy loads for 8+ hours every day.
11-13-19, 11:31 AM
#19
Given that DMV gave me a 30-day pink temporary reg, I felt pressure to get this done quickly, so took it to a shop. When I first went in, they told me that since it was an out-of-state car it needed an "enhanced" smog check (on a dyno, with NOx -- the same as the original I posted). I told them to just replace the O2 sensor (I already had a new one to give them) and check/clean the EGR. [I did the DP since and it turns out changing the O2 sensor is quite easy -- live and learn!]. Before picking the car up the shop called and said that, hey, guess what, we CAN do a "regular"/non-enhanced smog on this car (seems like maybe since it has been title transferred to CA this worked? -- not sure. I may follow up with them). They did the smog and it easily passed. The down side is, with the non-enhanced smog, they don't check NOx emissions, so we can't see a before and after 
Anyways passing results attached. Thanks for the suggestions.
Anyways passing results attached. Thanks for the suggestions.
Last edited by mkd; 11-13-19 at 11:32 AM. Reason: additional detail
11-13-19, 11:47 AM
#20
Just got off the phone with the shop and got some additional clarification.
Apparently in CA there are three levels of smog checks:
1. Regular
2. STAR
3. Enhanced (dyno, NOx)
The county your vehicle is registered in, and its age, determine which type of test it requires.
Turns out in my county, a 93 only needs STAR level (without NOx). If it had been registered in the neighboring county of Los Gatos, for example, it would have required Enhanced.
The shop wasn't 100% sure why the first smog station I took it to did an enhanced check. Two possibilities are:
1. They are incompetent and/or intentionally overcharging.
2. Since when I went to the first shop, the car was not titled or registered in CA, so it defaulted to the most stringent check. And since after the first test, I took it to DMV and titled it + got a temp reg to my county address in CA, the second shop only had to apply my county's regulations (a STAR check).
I'm leaning towards #2.
Hope this info helps someone...
Apparently in CA there are three levels of smog checks:
1. Regular
2. STAR
3. Enhanced (dyno, NOx)
The county your vehicle is registered in, and its age, determine which type of test it requires.
Turns out in my county, a 93 only needs STAR level (without NOx). If it had been registered in the neighboring county of Los Gatos, for example, it would have required Enhanced.
The shop wasn't 100% sure why the first smog station I took it to did an enhanced check. Two possibilities are:
1. They are incompetent and/or intentionally overcharging.
2. Since when I went to the first shop, the car was not titled or registered in CA, so it defaulted to the most stringent check. And since after the first test, I took it to DMV and titled it + got a temp reg to my county address in CA, the second shop only had to apply my county's regulations (a STAR check).
I'm leaning towards #2.
Hope this info helps someone...
Last edited by mkd; 11-13-19 at 02:06 PM. Reason: MORE SPELLING
11-13-19, 12:38 PM
#22
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11-13-19, 01:33 PM
#23
2. Since when I went to the first shop, the car was not titled or registered in CA, so it defaulted to the most stringent check. And since after the first test, I took it to DMV and titled it + got a temp reg to my county address in CA, the second shop only had to apply my county's regulations (a STAR check).
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