besides the cat ,what can cause the nox part of an emmissions test to be to high ?

Question

my 95 f150 with an inline 6 has failed the nox part of the smog test 6 times ,i have replaced the cat, plugs , wires cap ,rotor and changed the oil each time before running it through , what else can cause the nox reading to be in exess ?

Answer 1

I BARELY passed the NOx limit 2 years ago, and I've got another test coming up in a few months... so I'm asking myself the same question.

Here's the info I've got from the research I've done so far:

First, your vehicle's EGR system is specifically designed to reduce NOx emissions, that's it's only purpose. There is an EGR valve, and EGR ports, both can become clogged with deposits. On MY car the valve is relatively easy to clean, but the ports are not. I'm going to try cleaning just the valve and see if that is enough.

As IronHand mentioned (his whole post was very good, he knows what he's talking about), a high engine temperature is what creates NOx in the first place. So you need to check for things which will make your engine run hot. I don't know the details, but I've heard there are a few things that could make a difference here...

* Spark plugs - proper gap, torque, and temperature rating. Use Ford recommended plugs and make sure they're adjusted to Ford specs. It's another tangent, but carbon deposits can increase temp = NOx.
* Fuel/Air ratio - too lean = too hot = NOx.
* Coolant system - make sure you've got proper coolant/water mix, no air in the system, and a working thermostat and cooling fan.
* Hot weather = higher engine temperatures = NOx.
* The Oxygen sensor is what passes the information about the air part of the fuel/air mix to the computer/ECU. Oxygen sensors can go bad, or can become biased toward a too high or too low reading with age. If yours is old and biased low, it will tell the computer to cut back on the gas, reducing the gas/air mix = too lean = NOx.
* Cold weather = higher air density. If your truck's computer (ECU) doesn't properly compensate for this, it creates too lean a fuel air mix, so... NOx.
* The valves are what allow an engine to "breathe", bad valve timing/adjustment can result in higher NOx.
* I've read in several places that the catalytic converter can be a "fix-all" for passing emissions... a new cat will suck enough pollutants out of the exhaust before it gets to the tailpipe to mask problems upstream of it... but only if the cat is adequate for the application. Apparently some catalytic converters have something called a "reduction bed", which helps with NOx, some don't. Also, some non-OEM cats simply don't do the job, or are undersized. Some states have laws about not replacing the car's original cat unless the technician somehow certifies that it's gone bad, and certifies that an adequate replacement is being used. I suppose it's possible to use an "oversized" the cat for your truck, but that's off the top of my head, I haven't seen it mentioned anywhere.
* And as another poster mentioned, a vacuum leak will cause a problem also.

IronHand said "If you will E-Mail ALL the results of the test, with all of the values and the timing, I will run the calculation for what is called "lambda" to help determine if all of the failure is in engine and fuel control, or if the cat may be the main cause." - I would take him up on that, it's a very cool of him, and will maybe help point you in the right direction. However, not all states show timing, so you might not be able to give him the info he needs.

Good luck...

Answer 2

There is NO additive that you are going to add to the fuel or the oil that will lower NOx.
What were the other gasses like? Any others elevated?
EMISSIONS 101
HC - Unburned fuel. Fuel that went into the engine and came right back out and never got burned.
CO - Partially burned fuel. Fuel went in, got lit up, but the fire went out before complete combustion.
CO2 - One of the by-products of complete combustion. A good gas as far as auto emissions is concerned.
O2 - Oxygen that is unused. A high level is not good.
NOx - Oxides of nitrogen. Results from high temperatures (>2500° F) and pressure (compression.)
Your little pick-up has a device for the control of NOx. It is the EGR valve. It's purpose is to recirculate exhaust gasses back into the engine to reduce the efficiency and thus reduce temperatures. Exhaust gas has no appreciable oxygen or fuel, so it is just a place-holder. There is nothing there to support combustion.
Check that the EGR valve is functioning, and that the ignition timing is not excessively advanced, that the fuel isn't too lean, and that the cooling system is in good condition, and that the clutch fan is engaged.
You may want to invest in a repair manual for the proper testing procedures for the EGR valve and the related systems. Use the manual for the instructions for testing the ignition timing related to the SPOUT connector.
If all of this is good, you may want to retrieve the DTCs from the PCM. That manual can illustrate the procedure.
If you will E-Mail ALL the results of the test, with all of the values and the timing, I will run the calculation for what is called "lambda" to help determine if all of the failure is in engine and fuel control, or if the cat may be the main cause.

Good Luck & Merry Christmas

Answer 3

The EGR valve is responsible for the NOx and you probably need to replace it.
Ironhand already mentioned that Nox is formed by extremely high temperatures. He was right on the money and sounds like he knows what he's talking about which is rare these days.

If your engine is running too hot, check to see if your spark plugs are gapped correctly. If the gap is too narrow, this might cause it. Other than that, I wouldn't know.

Answer 4

Well, one thing to do is to get your car to about an eighth of a tank of gas. Then pour about 3 or 4 bottles of rubbing alcohol into the gas tank and then drive it to the testing place. Make sure you drive it a couple of miles to get the alcohol and gas mixed and so that the alcohol gets in the engine. I've seen it work a few times to pass a NOX test.

Answer 5

what were the other numbers like on the test..the convertor is a big part of the system..but..there is also oxygen sensors and the egr valve involved in this portion...check for a vacuum leak as well

Answer 6

Go to any auto parts store, the all sell an additive that will lower the emissions for the test. Although it's not the cure...it'll get you a sticker.

Answer 7

well if your not passing emissions its because your not doing something you should be or you are doing something you shouldnt be. so either knock it off or get it done, sheese its not rocket science...you try noxing on wood? hahaha just a little emissions humor

Answer 8

go to a local autoparts store and buy a can of seafoam. Use it because it works.

P.S. it will cause it to smoke a good amount but that is normal. I've done it before its ok