failed smog test; NOx too high. why?


Old 04-17-04, 09:37 PM
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Question failed smog test; NOx too high. why?

After suffering a leaky head gasket several months ago, I just finished rebuilding and reinstalling the head on my 1989 Chrysler Lebaron 4-cylinder, 2.5 liter, single-point fuel injected, non-turbo engine. The vehicle has approx. 200K miles. (the odometer is broken) It seems to start and run fine, but it failed to pass a California smog inspection. The NO emission readings of 1181 ppm (parts per million) at 15mph(1646 rpm) and 1124 ppm at 25 mph(1869 rpm) were above the respective limits of 799 and 738 and way above the average readings of 237 and 199.

Rather than pay a repair shop, I am attempting to diagnose which of the emission control components that affect NO levels is having a problem. The Haynes brand repair manual I bought states that the emission control systems on this vehicle include: catalytic converter, evaporative emission control system, exhaust gas recirculation (ECR) system, heated inlet air system, oxygen sensor, positive crankcase ventilation (PCV) valve, and Single Module Engine Controller (SMEC) information and self-diagnosis system. In addition to these components, the manual points out that there are numerous switches and sensors which provide feedback to the SMEC, such as the A/C cut-out relay, coolant temperature sensor, Manifold Absolute Pressure (MAP) sensor, transmission neutral safety switch, A/C switch, speed control switch, brake light switch, vehicle distance(speed) sensor, throttle position sensor, etc.

At the time the SMOG check was run, the vehicle was unbeknownst to me missing a heated inlet air system connector tube to transfer heated air from the carburetor air heater chamber back to the air cleaner. The technician indicated that the missing tube must be replaced, but implied that this alone would not likely account for the high NO readings. I have since replaced this missing aluminum-coated cardboard, slinky-like, tube.

The only component which the manual specifically mentions affecting NO emissions is the ECR system. I followed the procedure in the manual to determine whether the vertical shaft of the ECR valve moved up and down during engine RPM changes. I put a dab of white-out on the shaft to make it easier to detect motion. Testing seemed to show that the shaft did open during sudden RPM increases, but the upward motion was slight. I am unsure how much motion is normal. A state vehicle inspection website stated, “If the EGR valve is inoperative, a 5-gas analyzer displays high NOx, high HC, low CO, low CO2, and low O2.” The only emission that was above the limit during the smog test was NO, however both HC and CO measurements were about halfway between “average” and maximum. A 2nd website said, “High NOX emissions are almost always due to a defective EGR valve (or some component that controls the operation of the EGR valve). A related symptom that usually occurs when EGR is lost is spark knock (detonation) during acceleration.” Because the EGR valve moved and since the HC level was not excessive and because the engine does not exhibit spark knock, I tend to think the EGR is working.

The website also said that, “A lean air-fuel ratio results in high cylinder temperature and excessive NOx emissions…A lean air-fuel ratio may be caused by low fuel pump pressure, partially plugged injectors, a vacuum leak, or defective O2, MAP, ECT, or IAT sensors. A lean air-fuel ratio causes high NOx, high HC, high O2, and low CO and CO2.” Because the CO and CO2 were both low during testing, I tend to think that the problem is not one of the above sensors.

I realize that high NO emissions occur when combustion chamber temperatures rise above 2500 degrees F. This car has a fairly new radiator and brand new coolant and thermostat and has always run in the bottom 1/3 of the temperature gauge.

A friend who helped me to disassemble and reassemble the engine said that once after blowing a head gasket he had to replace his catalytic converter due to damage caused by all the water vapor entering the exhaust system. When I check the manual, it only mentioned HC and CO gases being reduced by the catalytic converter; however, the website stated, “A defective three-way catalytic converter may result in high NOx emissions.” Another website stated that 3-way catalytic converters began in 1991, which I hope means that the CC on this 1989 model has not been damaged. How would I test for this?

I am wondering if anyone has any ideas what else might potentially adversely affect NO emissions. I would really appreciate some assistance. Thanks.

Cameron Hess
Santa Ana, California
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Old 04-17-04, 11:13 PM
mike from nj
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WOW, i think we found someone who spends more time on the internet than me.

you seriously did your homework here. (we could use you in my dealership and replace some serious deadwood floating around, like 3 people).

here's another test for you i didn't see on your post. with the vehicle idling, take a small screwdriver and pull the egr stem up(your white mark), and see if the engine stumbles severely or stalls right out. if it doesn't....the egr passage is clogged. (while your there, lift the valve a few times and let is snap closed. this will seat the valve and remove any carbon that might be holding it slightly open---rough idle)

another quick thing to do, is to use a handheld vacuum pump to apply vac to the egr diaphragm and see if it holds vacuum. if it doesn't.....the diaphragm is leaking.

headgaskets blow a few different ways. if you had the huge white cloud coming out the tailpipe, then this definitely would affect the cat converter. the water has no affect on the internals, but the antifreeze does an excellent job of coating everything inside there....rendering it useless.

it's tough to tell how much a cat is working(pre 96), i usually only replace it as a last resort, after checking every other system out thoroughly. on the few i did replace, the emissions came down to near zero on the dyno.

an oxygen sensor....mainly for HC and CO, but it will also have a slight affect on NOx too. it could have also suffered the same fate as the cat, antifreeze will coat it too and make it sluggish or completely dead. it's also good maintanance to replace it every 100,000 miles (you might pick up 1-2 mpg too)

all the other sensors you mentioned, would affect HC and CO more so than NOx, but you would need a scan tool, a voltmeter, and a thermometer to check them all.

when all else fails, there are a few tricks to getting a car to pass on the dyno, when the NOx is too high.

set the ignition timing back, like in the 2-4 BTDC range(timing is always set with the coolant sensor unplugged)
remove the thermostat
wire the fan to be on all the time
(also be positive that your timing belt marks line up correctly)

this all should get you through with ease.

let us know either way.
Old 04-18-04, 08:41 AM
Join Date: Feb 2004
Location: So Cal
Posts: 248

Some causes for excess NOx:

1. Overadvanced ignition timing. Set it to factory specs.

2. EGR not functioning. Make sure it holds vacuum and stalls engine or drops RPM when activated. Also make sure you've got a functional vacuum source to operate the EGR.

3. Overly lean fuel condition. You can check this by using a DSO to check the amplitude (range) of the O2 sensor between rich and lean. If you didn't replace the sensor during the head gasket swap, do it now. Note: If the sensor fails lean it can cause VERY high NOx. A new sensor in my Honda dropped it from 1800 to 474.

4. Increased combustion chamber temps. If the cylinder head has been machined too much, it can change compression resulting in higher temps and higher NOx.

5. Malfunctioning cooling system. Higher engine temps = higher NOx. Make sure the cooling fan is working.

6. Defective catalytic converter. Oil and especially coolant can poison a "cat" rather quickly. Plain water won't hurt it. No real DIY tests for a bad cat. I usually perform a "cranking CO2 test" to measure its efficiency. Requires a gas analyzer to do this however.

The TAC (or pre-heat) tube will have no effect on the NOx.

Here's what I'd do if you brought your car to me:

1. Check for fuel control by watching O2 sensor operation.
2. Check the EGR system for proper operation.
3. Perform a catalytic converter efficiency test.
4. Repair as required.

I'm a CA Smog Tech, so post or PM me if you need additional help or direction.


Old 04-18-04, 08:46 AM
Desi501's Avatar
Join Date: Oct 2003
Location: Boynton Beach Florida
Posts: 2,207

Sounds like you just repeated what Mike said. I guess it must be true, huh?
Old 04-18-04, 08:50 AM
Join Date: Feb 2004
Location: So Cal
Posts: 248
Yep, more or less. LOL!
Old 05-08-04, 04:46 PM
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Thumbs up Too high NOx problem resolved

I would first like to thank Mike and Matt for their replies.

It took several weeks of sporadic spare time repair work, but I was FINALLY able to get my 1989 LeBaron to pass the California SMOG test…and WITHOUT paying a mechanic to diagnose it for me. I did, however, have the help of a very knowledgeable amateur mechanic friend.

After I replaced a missing heated inlet air system connector tube (which transfers heated air from the carburetor air heater chamber back to the air cleaner), my friend and I used his vacuum pump/gauge to verify that the EGR valve was both receiving the proper amount of vacuum and responding appropriately. Next we checked out the voltage and impedance on the four connectors on the oxygen sensor connector and found the output voltage to be too low, so we replaced the valve, bought from for $39.75 w/ free shipping (A word of caution- we had to heat up the engine a bit in order to expand the female threads in the exhaust manifold to release the oxygen sensor threads and then had to use a special split deep socket that allows the sensor wires to exit through the split in the socket while the sensor was being rotated and tightened into place.) Hoping that these two corrections would resolve the high NO levels, I then took the car back in for my one free smog retest. The good news was that the NO levels were not as high as before. The bad news was that the NO levels were still too high and the car failed the retest. Having saved the most expensive remedy for last, I bit the bullet and had my old 2-way catalytic converted replaced with a new 3-way one for $140 parts/labor and took the car back in and paid for a new smog inspection. This time the CO2 levels rose slightly (apparently still within spec), but all the other test emissions were lower, with NO being dramatically lower. The third time was the charm and I passed. Here are the results of my 3 smog tests:

1- after rebuilding the head but before doing any emissions systems repair
2- after replacing the air inlet heater tube and oxygen sensor
3- after replacing the catalytic converter

MPH/ %CO2 %O2 HC(ppm) CO (%) NO (ppm) RESULTS
1- 15/1646 14.6 0.2 118 31 86 .75 .10 .32 799 237 1181 FAIL
25/1869 14.7 0.2 93 20 73 .63 .09 .27 738 199 1124 FAIL

2- 15/1612 13.8 0.9 118 31 87 .75 .10 .37 799 237 1033 FAIL
25/1826 13.9 0.9 93 20 73 .73 .09 .32 738 199 874 FAIL

3- 15/1563 14.6 0.2 118 31 14 .75 .10 .05 799 237 87 PASS
25/1867 14.6 0.1 93 20 9 .63 .09 .04 738 199 112 PASS

I guess the moral to the story is, if your engine develops a head gasket leak that enables coolant to get into the combustion chamber and you have antifreeze in your cooling system, your catalytic converter could very well be damaged beyond repair.
Old 07-18-11, 03:59 PM
Join Date: Jul 2011
Location: usa
Posts: 3
i have a 2001 nissan frontier pick up that i drive daily to work 180 miles round trip so it has 295,000 miles on it still run great,but i have a problem with a check engine light,and a p0304 code cylinder 4 miss fire,so with that i have swapped the injectors around,i changed the egr,the back pressure valve,both cats,both o2 sensors,all the plugs wires cap rotor,did a compression test,smoke test,leak down test,but still have a check engine light and high knocks on the smog test,i did also change the konck sensor dont know what else to do,can someone please help me!!!!!!!!!!!!!!!!!!!!!!!!!!

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