question about 1991 Corolla excessive NOx emissions


Old 10-04-06, 06:29 PM
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question about 1991 Corolla excessive NOx emissions

I have a 1991 Corolla. It failed the emission test for NOx. The CO and HC readings were fine and well below the maximum allowed. The "check engine" light has never come on and there are no error codes stored in the computer. The vacuum hoses are all good and fastened tightly and vacuum gauge readings are all normal. Spark plugs, wires, cap/rotor, and timing are all good. I tested the O2 sensor with a meter, and it fluctuates as specified between 0.3 and 0.7 volts indicating that the computer is consistently mixing the correct fuel/air ratio. There is no EGR valve, MAF, or O2 sensor after the converter on this engine (a 4AFE 1.6 liter), so that wouldn't be the problem. The car drives fine with no hesitation upon acceleration, although it idles a little rough at times and I get an amazing 36 mpg on the highway even though the EPA estimate for the highway is 29 mpg. I don't know if this is because of an overly lean conditon, but like I said, the O2 sensor is reading correctly and there are no computer error codes coming up. The car has 55,000 miles on it and this is the first time it hasn't passed the NOx test. (however,it passed 2000 miles ago) Has anyone else had this problem or does anyone know what the problem could be? I'm thinking converter, but I'd like to verify this with someone else who has had the same problem.
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Old 10-04-06, 08:28 PM
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check timing belt. if engine has no EGR valve, may use valve overlap for same effect. if that 15 year old timing belt is stretched, it could cause emission problems without causing major drivability issues.
Old 10-05-06, 04:49 AM
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Don't overlook your cooling system, it's also playing a part of high NOX reading.
Old 10-05-06, 11:39 AM
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Talk to the closest high school chemistry student.

I think they call them 3-way catalysts because they use 3 metals to catalyze 3 reactions. One oxidizes CO to CO2, another splits HC into CO2 and HOH and I think the third is to reduce NOX to N2 and 02. If you can confirm this, keep the cat on your list. Expensive part?

The hot running engine is a good idea, but check more than the radiator. If the block or head water passages are blocked you engine may be running hotter than you think and your cooling system will not be overheating. Most computers and dash gauges take a coolant temp not a head temp. Exceptions include among others, the ’78 vw bus that has a head temp sensor connected to the computer and the ’84 pontiac 6000 that actually has a head temp sensor connected to the dash gauge and a coolant senor connected to the computer. More typical is the ’94 taurus with a single coolant temp sensor. You can run this car with no water and the dash gauge and computer KNOW the car is cold. My wife lost the upper radiator hose on the highway and only pulled over when it started running badly. She said it was not hot, because the gauge said so. I think it started knocking really badly and the oil thinned out so much the lifters got loud. If ford had spent the extra 50 cents to have a head temp sensor connected to the dash she would have pulled over much sooner.

Again, call on the high school chemist. Nitrogen by nature does not want to react very easily. That is why farmer John uses so much of your tax money to by his fertilizer (it is not easy to make) and why you don’t care that every breath is about 60% N2. You need high temps and or pressures to make NOX.

Since you only drove 2000 miles last year, you probably make many short trips. Is your muffler rotted out? I wonder if your car does not get hot enough and your rich warm ups are building up carbon in your combustion chamber and on the cat. This may increase your compression and reduce the effectiveness of the cat. Your car is old enough that it may not have a heated O2 sensor and may not have an O2 sensor after the cat. Without the heated O2 sensor the car runs without the feedback you tested for until it gets hot. If all your trips are a few miles long then most of that driving may be richer than you think, since the o2 sesor not involved.

Take it on a 200- mile highway trip and get it tested that same day.

Another thing to check: Older pollution control schemes included air injection at the exhaust ports or before the cat. Check function of this system if you have it.

Good luck.

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