High NOx Problem SOLVED (86-89 Accord, others) -- Smog Test

My 1987 Honda Accord LX (carb, auto) was failing the california smog test for high NOx emissions (a "gross polluter"). After an exhausting
self-education, I finally solved the problem. Here's the digest:
California tests smog on a dyno at 15 mph and 25 mph, with readings for 5 gases. My readings were:
15mph 1960rpm 12.2% CO2 3.3% O2 16ppm HC 0.00% CO 1355 NOx 25mph 2030rpm 12.2% CO2 3.2% O2 15ppm HC 0.00% CO 1188 NOx
These results say a lot. It failed for the NOx (about 6 times higher than average). The CO2 is very low (should be around 15-17%). The O2 is very high (should be less than 1%). The HC is pretty low (average is around 30 and 20). CO is very low (average is about 0.10%). There's too much air in the mixture; mixture too lean.
First, look at CO2 to see how well the engine is running. CO2 is the biproduct of proper combustion. Ideally it should be 15-17%. The very low CO2 says that the engine is not running well. It's losing 20-40% of its combustion efficiency (far below optimal performance). Given that the CO is low (CO is a product of incomplete combustion), and that the HC is low (HC/hydrocarbons are uncombusted fuel), it says that the engine is running lean. Not enough fuel or too much air in the mix. (If CO and HC were high, it'd be too rich. Either one will cause low CO2; low efficiency.)
The fuel jetted to the carb is not subject to adjustment (unless you mess with reboring the jets). The mixture is adjusted by two vacuum-operated valves (air control valve A & B) which leak air into the manifold to lean the mixture. Feedback Control Solenoid Valve and Frequency Solenoid Valve A, B & C are controlled by the "computer", but all the adjusting is done by comparative vacuum pressure, not by computer.
First, I tested the O2 sensor. Its function is to tell the computer if the exhaust gas has too much oxygen, which causes the computer to reduce the air leaked in, causing the mixture to become richer. The voltage at the sensor lead varies between 0 and 1 volt; representing max lean to max rich. Normal operation (I read) has it hovering between 0 and 0.5 volt, on a cycle of about two seconds. Mine was staying at 0 volts except during acceleration or fluttering the throttle. Replacing the O2 sensor made no change. (Wasted $60) My O2 sensor was saying the same thing the smog failure test said.
Too much air in the mix, so I went looking for vacuum leaks. I took off the air filter assembly (easy) and inspected all the hoses. I replaced a few that were hardened and that helped a little. I inspected the vacuum diagram (available at autozone.com--very helpful free repair manuals online), and indentified all the valves that are connected to manifold. (I knew the carb gasket was not leaking.) Then I applied vacuum to each diaphragm to check that they held it--all were good except for the carb vent bowl diaphragm.
The carb vent valve is located on the front right corner of the carb (looking from bumper), at the top, and is apparently of very bad design, with very important consequences of failure. It sits a couple inches from the carb bowl, and the rubber diaphragm is constantly subject to fuel vapors and some fuel splash. I've found zero documentation on the valve, but after much study, I can tell you its function is to suck the fuel vapor off the bowl at the instant you turn the car off, and then to seal off the bowl. Whenever the car is running, the valve is supposed to be activated (open), with constant suction on line 8. I removed and opened the valve and found that the rubber in mine was transformed into a tar-like goo. The one I saw at the junk yard had the same problem as mine. Thoroughly leaking. I suspect many or most of these diaphragms fail. (This car has extremely low mileage--less than 50k) The design blunder is made worse by the fact that the carb rebuilt kits do not include this susceptible piece of rubber. I was unable to locate this crucial part from anyone (I didn't dare ask the dealer price).
The failure of this valve causes serious problems: 1) it creates a vacuum leak. Worsening the lean condition. 2) it causes the bowl vent valve to stay closed so the bowl is put under vacuum. This fights against proper jetting in the carb, strangling fuel delivery. Causes severe lean. 3) It exposes the whole circuit upstream (line 8) to rubber-disintegrating vapors.
Aside from failing the smog test, I was having drivability problems. Forcing the bowl vent valve open did a lot to solve them. (I tried RTV and gasket-maker to fix the diaphragm but it only held for hours and soon melted to goo.) The fuel starvation caused stuttering on acceleration and severe hesitation from a stop. (And I had no idea how much more power this little car has) The vacuum at the carb bowl was preventing the jets from sucking in the proper amount of gas. Before, I had to flutter the pedal (squirting gas) to get going from a stop. Fixing the leak helped dramatically. But I was still failing smog from too much NOx.
NOx are caused by excess heat and pressure during combustion. At excess pressure-temperatures, the oxygen will combine with nitrogen, forming oxides of nitrogen (NOx; NO1, NO2, etc). EGR problems are a leading cause of high nox. EGR reduces cylinder temperature by displacing some incoming oxygen with recirculated (non-combustible; inert) exhaust gases.
I tested the egr valve by applying vacuum to it at idle and verifying that the car stumbled hard or dies--the valve worked fine. I also testing the vacuum applied to it by reading the vacuum on revving the engine to 3000rpm--it only needs to get up to about 6 in Hg, and mine was fine.
The final problem was that the timing was too advanced. It was 21 degrees BTDC and should be 15 degrees. Timing that's too advanced means that the mixture is igniting while the piston is still compressing, and this causes high pressure and temperature (high NOx) as the expanding gas is compressed. Retarding the timing to 15 degrees caused the nox to drop down to sub normal, and I PASSED easily.
Final readings: 15mph 1940rpm 13.8 CO2 1.0% O2 26ppm HC 0.49% CO 168ppm NOx 25mph 2010rpm 13.8 CO2 0.9% O2 29ppm HC 0.49% CO 243ppm NOx
NOx is now about average (passing easily), but CO is five times higher than average and HC about 50% higher than average at 25 mph. My exhaust is within limits, and CO2 is better, but still far below optimum.
For reference, I have old readings from 1994 (when the car was 7 years old and had only about 20k miles). At that time, the tests didn't measure NOx, nor did they put the car under load; they just ran at 1000 rpm and 2500 rpm. Here are the like-new readings:
1150rpm 16.2% CO2 0.0% O2 2ppm HC 0.00% CO 2500rpm 17.3% CO2 0.0% O2 0ppm HC 0.00% CO
Now that's CLEAN! So I'm far from optimal. Since O2 and HC are high now, this means that there's uncombusted fuel and air going out the exhaust. That suggests that the timing is too retarded, leaving some air and fuel unignited. And confirming this, in adjusting the distributor before, I had to swing it almost all the way up.
On further inspection, I found that there's a slight but very effective vacuum on line 25 (secondary vacuum advance). When the car is hot, there shouldn't be any vac on line 25. The result is that the distributor is excessively advanced at idle when it's adjusted to 15 degrees, which means that as the manifold vacuum decreases (wider throttle) the advance decreases far below what it should be and timing becomes retarded. The wider the throttle, the more over-retarded the timing. As suspected, when I disconnect and plug both vacuum lines, the timing is a few degrees AFTER TDC. This retarded base timing reduces NOx, but it robs performance.
I'm still tracking down this problem. When I find out why there's vac on line 25 and fix it, I'm expecting 20% or more improvement in performance. The car already runs almost like new.
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Good work, Greg! It sure reminds me why I hate feedback carburetors. Sure, EFI drives me crazy from time to time but not like that.
BTW, although I don't think you wasted $60 on the O2 sensor - the original was probably getting pretty wimpy by now - the indications you had suggested it was responding to the leanness, rather than causing it. If the O2 sensor output were that sluggish and low (2 seconds is about 1/10th the normal response rate) the result should drive the mixture richer. But if you had the $60 to put toward the cause I would have recommended replacing the sensor on GPs.
Mike
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Greg wrote:

good work!
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Great post..
I'll reread it later for covering the basics is so important and you covered it well; I can defatilly learn something from this.
--
Stephen W. Hansen
ASE Certified Master Automobile Technician
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