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Help with Emissions Test

Having a little trouble with emissions. Seem to pass everything no problem except for NOx. Test#1 limit 450, my reading 790 Fail. Test#2 limit 450, my reading 773 Fail. Car warmed up.

After mechanic added gasline antifreeze and a few tweaks I did Test#3 failed at 1133. After I changed the catalytic converter I did Test#4 failed at 840. Car was never warmed up at all in the last 2 tests and tested at a different facility than the first 2.

What should I do next. Thanks

'93 Oldsmobile Cutlass Ciera, 3.3L V6

High combustion temps can cause a high NOX level. The EGR valve may need to be cleaned or replaced. Also check for any intake vacuum leaks which would cause a lean mixture and higher NOX levels.

I agree. Check and/or clean the EGR and passages, and drive the car for 15-20 mins before testing to ensure it is warmed up. A cold engine will never pass an emissions check that uses a tail-pipe analyser.

Catalytic converters don’t go bad by themselves, with rare exceptions. Their failure is caused by poorly running engines which send a lot of unburned fuel into them, and from contaminants (such as antifreeze, silicone, oil). If the engine isn’t fully fixed, a new catalytic converter will go bad.
I would like to hear, “The mechanic used the appropriate repair manual to test and clean/repair/replace the EGR Valve. Now, all is happy!”.

When the OP stated the " the mechanic added gasline antifreeze"what was he saying? Is this a knowm technique to lower NOX or just extra information unrelated to the concern?

Who told you to replace the catalytic converter? It has nothing to do with NOx control (only CO and unburned hydrocarbons). As others have mentioned, the problem is excessively high combustion temperatures, for a variety of causes.

Thanks for your replies. Finally this is all starting to make sense. My mechanic had no idea really how to diagnose the Nox reading on my test so he suggested we add gasline antifreeze which I have also read somewhere on the net that the alcohol in it burns cleaner(which I will never do again…I think it’s not good for my car) and he said to change the catalytic converter. The new cat brought the CO and other readings down to almost 0.

(Current Test Results)
HC ppm- Limit 60 - My Reading 9 (PASS)
CO% - Limit 0.33- My Reading 0.00 (PASS)
NO ppm -Limit 450 -My Reading 779 (FAIL)
Dilution: 14.9-VALID

Curb Idle Test:
HC ppm- Limit 200 - My Reading 5 (PASS)
CO% - Limit 1.00- My Reading 0.00 (PASS)
NO ppm -Limit N/A -My Reading N/A
Dilution: 15.0-VALID

My engine has been running quite rough and sluggish also it is eating gas and hesitating and surging slightly also my heat works sometimes and other times not at all…at times I can feel heat from the vent but the fan doesn’t work(same as air conditioner). Other than that my car seems to be working quite fine.

So next I am going to have my mechanic put in a new O2 sensor, new spark plugs as well as test and clean the EGR valve. I am a little peeved that my mechanic never suggested these things in the first place. If I make these changes to the 02, plugs and clean egr do you think it will lower the nox and get me to pass?


I would do it in the opposite order: EGR first, then plugs, then O2 sensor. The good CO and HC #'s likely rule out the plugs and O2 sensor IMHO.

Get rid of the car. It is a GM product, and GM hasn’t made a reliable product since the 70s. Get rid of the car even if you have to be dependant on public transportation. And this emissions test stuff is nothing more then a money makeing scheme of dishonest politicians and (Or) tree hugging, glassey-eyed hippies. Vehicles have been made to put out the least amount of pollutants for years in any condition the engine components are in. The amount of pollutants a car puts out is very insignificant. The only thing wrong with the quality of air is the politicians. For example, how many polutants are put into the environment by the utility companies who service Al Gore’s mansion? How many pollutants are put into the environment because of him learing around the world telling the citizens of other countries how screwed up Americans are? That is the real inconvenient truth!

Very helpful, norm.

I agree with the others, the primary control of NOX is provided by the EGR. Start there.

First, let’s look at where NOx comes from. The air that’s drawn into the combustion chamber consists of roughly 77% nitrogen and 22% oxygen. The balance is argon and misc.

When these are compressed in the combustion chamber and subjected to excess heat, the nitrogen and the oxygen begin to bond at a prodigeous rate.

There are three primary means of controlling NOx.

The first is maintaining a rich enough fuel mixture that combustion chamber temperatures do not run away. Just like a bellows in a fireplace, if you add too much oxygen the hydrocarbon molecules begin to tear apart into hydrogen and carbon and bond to the oxygen prodigeously and the burn gets hotter…and NOx begins to form/

The second is the Exhaust Gas Recirculation (EGR) system. When the engine is under load, the EGR system allows a little exhaust in the intake stream to displace a bit of the oxygen, again keeping the combustion chamber temperatures fro m skyrocketing. This also helps prevent preignition, but the discussion here is about NOx.

The third is the catalytic converter. The first stage of the catalytic converter contains a ceramic substrate coated with Platinum-Palladium, a Rhodium on the elemental chart. When the platinum-paladium is heated above 400 degrees F, it will break the bond and cause NOx that comes in contact with it to seperate, becoming nitrogen and oxygen again.

Additionally, if the system is not fully warmed up the ECU will byppass the upstream oxygen sensor, the engine will run rich, and the catalytic converter will not cause th nitrogen and the oxygen atoms to seperate. The engine should ALWAYS be fully warmed up when doing an emissions test.

So, allow me to summarize: the vehicle should be properly in tune with good components, and the engine should be fully warmed up before doing any emissions testing. And, you need a new mechanic. Even a putz like me knows this basic stuff.

Thanks again…now here’s the update. Great!!! Just as I was on my way to work this morning with my brand new shiny catalytic converter the car slowed right away and stopped dead. My dash lit up like a Christmas tree…lol (good thing I still have my humor) The battery, oil, engine, Temp lights came on at once and the steering wheel locked.

Luckily I wasn’t on the highway instead it was a school zone. CAA came and tested my car…it would not start. It sounded like a car that ran out of gas. He took one of the wires off the spark plugs and had me try to turn over the car and then he said there was no spark and he felt it was the coil or the module.

So now my car is in the shop. Well Norm…looks like I’ll be taking the bus after all…lol

ive got a 91 ford van and i replaced the egr and the o2 sensor, and passed with flying colors.