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93 Nissan 240SX smog check help

I have a 1993 S13 Hatchback with 24K miles.

The previous owner failed the smog check yesterday.

The result:

M1. 15MPH

HC: MAX 88 AVE 21 MEAS 28

CO(%): MAX 0.52 AVE 0.06 MEAS 0.08

NO(PPM): MAX 704 AVE 150 MEAS 775

M1. 25MPH

HC: MAX 53 AVE 13 MEAS 24

CO(%): MAX 0.50 AVE 0.05 MEAS 0.13

NO(PPM): MAX 738 AVE 136 MEAS 507

My friend help me to do the regular maintenance services after I brought the car.

We did:

Change engine oil and oil filter

Flush coolant and fill in 50/50 coolant

Clean fuel and change fuel filter

Replace air filter

fill up 91 Octane

I just smogged the car today and it failed.

M1. 15MPH

HC: MAX 88 AVE 21 MEAS 41

CO(%): MAX 0.52 AVE 0.06 MEAS 0.12

NO(PPM): MAX 704 AVE 150 MEAS 784

M1. 25MPH

HC: MAX 53 AVE 13 MEAS 31

CO(%): MAX 0.50 AVE 0.05 MEAS 0.10

NO(PPM): MAX 738 AVE 136 MEAS 432

How do I pass the NO(PPM) with 15mph? Any suggestion? Replace O2 sensor, catalytic converter?






The EGR valve may not be functioning properly. It helps keep combustion chamber temps down, which helps reduce the production of oxides of nitrogen (NOx). Since your other numbers are good, I think your cat and oxygen sensor are good.

Cat converter. Check the EGR system too while you’re there.

The EGR system prevents runaway formation of NOx, the cat converter takes what NOx comes in and seperates it into nitrogen and oxygen. On a car this age that’s clearly running well as evidenced by the good carbon monoxide and unburned hydrocarbon readings, I’d bet on the converter as being due for replacement.

Make sure that the exhaust system is fully warmed up before the test. In preparation for a test I usually change the oil, check the air filter condition or change it, check the gas cap, and drive it 10 or more miles at freeway speeds before getting to the test station as soon as possible. It helps to have a reservation or go at a time you will not have to wait long. Also you might ask the tech to full warm up the exhaust system before they do the test, i.e. run the car on the chassis dyno at the upper speed until he is sure the cat is up to temperature.

Maybe that will help.

“The EGR valve diaphragm should lift up and down without restriction.”

How does EGR valve diaphragm look like? Is it like a gold-colored, round-shape metal plate?

I checked last night, but I am not sure how high will the valve diaphragm lift up?

I need details on how to clean EGR. I have a carb cleaner. If replacing it will be easier, I might just buy one for $120.


2 days ago, when my friend and I were under car doing regular maintenance services, we saw the whole exhaust has rust and a red color. My friend told me that could be high temperature from somewhere in the engine. He remembered that his friend’s new car has this red exhaust system problem, and they brought the car back to the dealer. And the dealer person told them that the cause of the red exhaust system problem is from high temperature from the engine or some sort.

If that is the case, what should I do?

I don’t how old is the whole exhaust system. I think it is at least 5 years old because of the rust and stock looking.

Assuming, the previous owners never replace the exhaust system. I believe I should just replace the cat.

Would replacing the muffler help the emission?


The ‘red exhaust system problem’ mentioned is actually the exhaust pipe glowing red hot while the engine is running – not necessarily just rusty red cold. A glowing red exhaust pipe is an indication of an engine problem that would probably have melted the cat element.

Yes it might be best to just replace the catalytic converter if the rest of the exhaust system is still solid and intact. The muffler does not do anything for the emissions but can cause a problem for the engine oxygen sensor(s) if is leaking. So make sure the exhaust system is gas tight once all the work is done.

Hope that helps

Agree. And that engine problem just might be that previsouly mentioned EGR system. When the cylinder temps get too high one of the effects is that the nitrogen and oxygen atoms begin to bond much more readily creating that NOx you’re struggling with. The EGR system prevents this by allowing a bit of inert exhaust gas to mix with the intake air displacing some of the oxygen and reducing the combustion temperature.

It’s sort of the reverse of when you get the fireplace fire glowing hotter by pushing in added oxygen with a bellows.

Do you think using seafoam would help passing smog check in my case?

The EGR Valve, itself, may be fine. It’s very easy to get to, and easy to test (if you have a vacuum hand pump). Click on this Auto Zone site for instructions and pictures showing how to work on the EGR Valve:

To clean the carbon from the EGR Valve pintle, and passages in it, and to clean the carbon from the passages that lead to, and from, the EGR Valve use Carb/Throttle Body Cleaner, round wire brushes, screwdrivers, etc. You can blow through the passages to ensure that they are open.

After you have worked on the EGR Valve, have the emissions test done, again. If the engine fails the emissions test (high NOx), you can assume that the catalytic converter is defective.

THis is what we did last night:

  1. We tried to remove the EGR valve entirely, but we don’t know exactly how. Basically, we just removed the EGR valve vacuum hose and spray some Pryroil Carb&Choke Cleaner in it. We also sticked out finger in the bottom of the EGR valve and lift up the diaphragm while the engine is running. The engine stumbled, so we believe the EGR valve is working. Is there a easy way to remove the EGR valve from the car entirely? I want to clean the inside parts of the EGR valve. What nuts do I have to remove first? And what other parts do I have to remove first?

  2. Then we seafoamed the car. We followed his steps:

33% gas tank, 33% crankcase. 33% intake manifold through brake booster line. I have a question here. Why the brake booster line will connect to the intake manifold? What is the purpose? And why there are other vacuum lines connect to the intake manifold?

Compare with other cars on youtube, my car doesn’t have alot of smog coming out from the exhaust. Do we need to pour one more bottle of Seaform? If we decided to pour 1/2 bottle to intake manifold and 1/2 bottle to gas tank, do we have to do oil change and change the oil filter again? My friend said we don’t have to change the oil and oil filter, but I argued that the seafoam will make the engine oil thinning. So, changing the oil and oil filter is needed after each seafoam regardless where we pour into.

That is all we did.

  1. "ok, do this, add a bottle of isopropyl alcohol to about a half filled gas tank, drive it around a bit to get it mixed and in the fuel line. alcohol lowers combustion temperatures thus NOx levels will be lower. also, nox resolved from running lean which is one of the symptoms. get the highest percent isopropyl alcohol and try again, get around 90% because the other 10 percent is water, though water is not combustable, it will dilute enough to not cause any harm. also, try retarding your timing a few degrees. that may help. these are just cheap/money free possibilities. the alcohol wont hurt your engine, just dont use more than 1 bottle…about 16 ounces

Let me know if this works for you. i bet it will. pm me or something"

Do you thing pouring alcohol in the gas tank will help emission?

Reading your posts about what you have done so far, I have to warn you that you may have already done some damage to your engine. You are way off the track by pouring oil into the passage after removing the vacuum line from the EGR valve. You should go to a mechanic and have it changed because it will stop working soon if it wasn’t already shot. Try not to do any more work by yourself. You can’t imagine the damage you could do.

My 240SX actually has 240K miles instead of 24K miles.

You may have damaged the diaphragm in the EGR valve with the SeaFoam poured into the EGR vacuum port. The SeaFoam should go into the openings which have the carbon deposits. The carbon deposits are what you want to remove with whatever won’t damage the EGR valve.
You need to let the engine and its computer lift the EGR diaphragm. Follow the instructions in the repair manual. Let the engine get hot. Watch that the EGR vacuum diaphragm is lifting when the manual says it should.

I had a 1989 Mazda 626 which failed NOx emissions test. I checked the EGR system and the indications didn’t match the repair manual findings. From theory of engine operation, I tried something. I retarded the timing a little bit (maybe 5 degrees) by turning the distributor a little toward Retard. I did nothing else. Upon retest, the emissions levels were GREAT!

  1. Will high octane gas and octane booster help emission in my case?

  2. Will retard ignition time help emission? How does it work? And how do I adjust the timing?


  1. Changing the octane won’t help. It could even hurt the engine performance.

2.The timing on your car CAN’T be adjusted. The engine computer controls the timing. You can’t adjust the engine computer, either.

The NOx is controlled by the amount of exhaust gases the EGR Valve lets into the engine. The more exhaust gases that go in, the less NOx (to a limit, of course). The carbon deposits should be removed from the holes coming to the EGR Valve, and the holes going into the intake manifold.
You need to determine if the engine vacuum (and computer?) are opening the EGR Valve. You may be able to observe the diaphragm moving with a mirror (I don’t know if it’s observable this way). Use your repair manual for guidance.