OK, so my car has failed 3 straight times this month. And now I'm completely stumped on what the problem could be. I've replaced the O2 sensor with a new Denso one (basically OEM), and a brand new cat.

I'm starting to think that it's either the header itself (even though it's CARB legal), or the location of the O2 sensor with the extension harness (O2 located right before the cat on the collector).

Car Specs:
- 1995 Honda Civic EX Coupe
- Basically stock and Original D16Z6 with 125,000 miles
- Only modifications to the actual engine is an AEM short ram intake and CARB Legal JG Edelbrock Header.

For the 5 years I've owned the car, I have never touched anything internally, as far as timing, a cam, etc. I've done just the intake and header, and 2 1/4 piping and ES oval muffler, but I highly doubt that does anything.

Here are the three test results...

Test #1. Car had what I'm assuming the original cat (bolts/flanges were rusted together), and a "lazy" o2 sensor (but no CEL). Failed NOx @ 15mph



Test #2. Replaced o2 sensor to fix failed NOx. Car failed everything and did extremely worse than the first time.



Test #3. Replaced original cat with a brand new one. Car is now running on brand new cat and o2 sensor. Passed everything, but still failed NOx just like the first test, this time at both 15mph and 25mph.



Anybody know what the problem could be?

Test #3 was just taken today (11/20). I took the car to a friend of my dad's, and he retarded the timing to 14btdc.

 

Vtec shouldn't engage during a smog test.

NOx pollution is caused by high combustion temps. If the EGR (if the car has one) is clogged or otherwise non-functional, NOx will rise. Lean burning can also raise NOx, as can an ignition advanced too far.

Check the EGR and timing. Cleaning the injectors might help, if one clogged and causing lean burn. If all those are good, just cheat the test with some ISO Heet (or "Guaranteed to Pass") in the gas tank, since both of those use alcohol and burn cooler (reducing NOx).

 

I figured VTEC shouldn't be engaging during a test.

The technician did say something about the car running lean. And as far as timing, it is set to factory specs.

But I will check on the injectors.

 

check injectors, plugs, cap/rotor, maybe retard your timing a lil on the dizzy (just till you pass, then reset it where it should be), or, like tunerN00B said, "cheat" it...

 

yeah, the injectors i'm sure are the original ones, so those might have gone bad seeing as i've never touched those in the 5 years i've owned the car.

spark plugs, cap, and rotor are basically new (about a year old)

and i had the timing adjusted right after today's test. off topic, but after i had the timing adjusted, my car felt sluggish when accelerating. would the sluggish feel be caused by the timing?

 

well, if your timing was advanced before and you set it back to factory specs, then that could be a reason for it feeling slower than it was...

 

True, but it shouldn't hesitate or otherwise drive poorly with the specified timing.

However, running lean can make the car feel sluggish. Advancing the timing on a lean running engine can make it feel better, but shoots the NOx through the roof.

I've had a car test at over 5,000 PPM NOx before, and it was running very lean (stupid side draft carburetor) with massively advanced timing and a broken EGR. It also recorded 5 PPM HC that test though, and since the car lacked a cat (at the time) its was obviously the lean burn causing the issue.

 

the timing wasn't advanced before. it has never been touched until i had the mechanic adjust it after the smog test today.

 

I had a similar problem with my car, when I bought it the numbers were high, but still passed, after two years I figure it would be worse. I got some advice to change the oil, put some thicker oil in, (My problem was excess oil in the combustion chamber), do a full tune-up especially the air filter and then finally beat the crap out of the car on the way to the place to heat up the cat a lot.

 

yeah the mechanic that adjusted the timing asked me what oil i use, in which i use mobil 1 full synthetic 10w-30.

as far as warming up the car, i drove it around for 30 minutes before i went to the station, and i had the car running for almost an hour before he started to smog it.

i will check out all the things associated with a tune up though just to make sure.

 

i think it might be ur headers. my boy with dc headers fails smog even tho their CARB legal. aftermarket headers get hella hot and sometimes cause emissions to go up. thats what i heard atleast. do you have the stock exhaust manifold?

 

yeah i've been really thinking that the culprit is the header. i do still have the stock header, so i might just put that back on, along with cleaning the injectors, oil change, some iso heet, etc. just to make sure.

 

x2

NOx is caused by high temps in the combustion chambers. check the EGR (if equipped) and also check for leaks.
some causes maybe:

vacuum leak, head gasket leak, advanced timing,lean mixture, lower octane fuel, decreased or no EGR flow

 

might want to change your fuel filter. might have a clog causing a lean condition.

 

The motor is a D16Z6, so I believe there's no EGR. I'm also suspecting some sort of leak so I'll have to look for that.

I'm suspecting a clogged fuel filter or fuel injector/s also. The fuel filter though is still fairly new since I replaced it in October of last year in which I've put roughly 8,000 miles since I replaced it.

Do you guys suggest I try to either just clean the fuel filter and/or injectors, or just replace them?

 

Okay so far I've put my stock header back on just to make sure it'll pass compared to with the edelbrock header. I also just put a bottle of CRC Guaranteed to Pass in a full tank of gas and I'm going to run that until the tank is empty. The timing is also retarded to 14btdc.

If that doesn't help pass, then I'd assume that there's a vacuum leak somewhere..?

Is there anything else that I can check before I take it in for it's 4th smog test next week?

 

If you don't mind paying the extra, you can ask for a "pre-test" instead of the actual smog test. They do a full test, the only difference is that the results aren't submitted to the state. This way, if you fail, it isn't recorded (again). If it passes, you can ask for them to do the test immediately after.

 

Just by reading your second smog check result, you were running very lean with 9% and 8% oxygen content. Seems like a a leak after you replaced your oxygen sensor. Make sure your oxygen sensor is on tight. Second thing, high nox is caused by high combustion temperatures. What temperature is your spark plugs? Are they NGK's or another brand. Factory plug is a temp. 5 on the sale for NGK's. Another thing can be the thermostat being a high temp unit vs the factory temp unit. Its your CAT a CA model with a carb number? None CA cats have less material to rid nox.

 

I was reading about a pre-test in another thread and think I just may do that

I can't remember what temp my spark plugs are, but they are Denso spark plugs, and the spark plugs model number is equivalent to the NGK's that should be on the car, based on the model numbers on the timing belt cover. And I'm sure that the cat is a CA model because I saw some numbers on it when I looked at it.

 

use sea foam.. but do it the correct way. We used a trickle system that snap on sells and sucks it into your engine slowly so you dont burn your cat and o2 sensors. Works great if used properly and not all at once took like 15min for the whole process keeping the idle around 1.5k and no more than 2k idle.

 

I had this exact issue passing my tests
I ran one tank of premuim gas all the way through
then added sea foam then added more premuim gas
and drove my car around for a while to warm it up then go in to get tested
if you go in cold your results arent going to come out well.
this is how i was ha