2000 m6 z28 73k miles

So i went for a smog check today and the car failed on only one part.

the "NO (PPM)" I only failed on the 15 mph test, i passed that section on the 25mph.

The person that did the test suggested that i do some highway driving before i come back because i only really use this car on short trips. do you think this will help??

also right before i left for the test i swapped in my stock lid.

please let me know if you have any suggestions

 

NOX at slow speeds is indicative of an EGR problem. Driving at highway speed won't help, if that's the case. http://en.wikipedia.org/wiki/Exhaust_gas_recirculation

You should have your car scanned to see if you have any EGR codes. If not, you may want to check the EGR for goo/carbon build up, and test the EGR pintle by pressing it in with an eraser. It should come back without sticking. (BTW - You'll need to have a new EGR gasket on hand when you do this.)

 

thanks, i dont have a check engine light right now.

If i remove the egr and its gross can i just clean it somehow? Or will i need to get a new one at this point?

 

Before inspection change your oil and give it a blast down the freeway. Make sure it is nice and hot. I bet you will pass. If it doesn't then proceed with troubleshooting. Don't buy any parts until you are sure they are broken.

 

i changed the oil less than 500 miles ago.....do you still think it will help??

 

The EGR can't really be cleaned, just checked. It's a really expensive part so you'll want to test the spring before you buy a new one. You may have codes for the EGR even though the SES light is not on.

 

ok no codes.

i took off the egr, and the pinsle(sp?) works perfectly. it springs up right after pushing it down.

anything else i should check?

should i spray some brake cleaner in there to remove any build up?

 

Don't "spray" anything in the system. That could gum up the EGR. If you don't have measurable build-up, you should leave it alone.

Do you have a scan tool that shows you the sensors real time? I wonder if something else is causing the EGR to behave badly.

BTW - I believe the factory service manuals have guidance on this stuff, but I'm away from mine until the weekend.

 

no i dont have any kind of real time scanner, is there anything else that i can do to get those numbers down?

 

I'm not sure. You may want to take it to a mechanic to tinker with. The thing with emissions is that it gets really expensive when you start experimenting and swapping out sensors.

I'm sure the folks in CA are better at this kind of stuff than other places. Maybe you can find a dealer/garage that will guarantee their work if you don't pass your smog test?

I understand that the catalytic converter also helps NOX, but that's a really expensive experiment. A dealer should be able to run advanced tests on the EGR and may be able to find issues you can't with normal tools.

 

Make sure you've got a clean air filter, MAFS, and TB.
I would also drive the car for at least 1 day with the factory lid on it, and drive it for a good 30 minutes before taking it to the smog shop.

 

regarding NOx emissions, the EGR is the primary piece of equipment that reduces nox. NOx is created from elevated combustion chamber temps. The EGR (exhaust gas recirculation) introduces various amounts of exhaust gas back into the combustion chamber which reduced combustion chambers temps because the exhaust gas (which does not combust) dilutes the air/fuel mix, thus reducing nox. EGR is not active under full throttle full load settings, and is most active under light load low/part throttle. Generally if the egr is a problem, you have a bad nox number and it's bad all over, not just at one point and just over the limit.

it is important to also look at your HC (hydrocarbon) and CO (carbon monoxide) numbers which can help diagnose the problems. But if those HC & CO numbers are all good and it's just this one NOx reading at 25mph, I sort of wonder if it's a misstest. I'm not familiar with Ka testing, I know here in CT they were all crooks and have been shut down more than once by the state and now all they do is check the pcm for emission related codes. An increase in nox will always happen under any kind of acceleration, was your 15mph test at steady load or was the tester varying the throttle? And how strict are the limits? Are you sure they have the correct limits for your engine- not like limits for a v-6 when you have a v-8?
And you have a 6-speed manual, what gear was he in? If he bogged it out in 4th, i'm assuming the rear wheels are on rollers to simulate a load on the drivetrain, that would put significant load on the engine at low rpms and possibly give out high nox. I don't think it would be the o2 sensor because if it was a problem you generally see it affect the HC and CO numbers- if your HC and CO passed but are just under the limits then it may be the o2 sensors. you're also sure the car was in closed loop when they did the 15mph test? he didn't just start the car and do the 15mpg test within 10 seconds with the car slightly cold, because it can take up to a minute or two before the pcm goes into closed loop. last would be the catalytic converter, which does break apart nox into n2 and o2. the rear 02 sensors monitor cat. converter efficiency but i don't know how nox reduction factors into how they figure cat. converter efficiency and when the pcm throws that specific code. and the cat needs to be hot to work effectively, so if they started the car up on the rollers and immediately did the 15mph test that could be it. hope that helps.

 

thanks for all the help i am going to re-test this week, hopefully it was just a botched test or the car wasnt warm enough.

i could use a new air filter and maf cleaning anyway so i will do that then drive around for a few days

here were the numbers from the 1st test

15mph MAX=419 Measured=723
25mph MAX=706 Measured=520

all the other things were under by a lot.

 

Warm is the key. The EGR sends exhaust in to the combustion chamber to heat things up. No matter how much you drive around, your engine is only going to be as hot as your coolant temperature.

Do you have any mods that would cool down the engine, like a low temp thermostat? (That would be bad for this.) Have you changed any of the other systems, like the AIR pump?

You might even have some better luck by causing the engine to overheat a bit for the test. Maybe put some cardboard over the radiator, etc. for the test. (That's probably not the best thing to do or an elegant way to pass...)

 

yeah i have had the fans reprogramed and a low t-stat. But it passed with this same setup last time i needed a smog check.

could i ask the tech to not use the big fan in front of the car when its on the rollers?

 

HOT HOT HOT, and if you can find it in Cali, get your self a few cans of dry gas, add them to the tank of fuel, go run the **** out of it, and take it to test. When we used to have emmisions tests in MI, I used to do this all the time. I would pass with zeros!!!! all the time. The tech's always looked at me very strange.

 

Ok i took it in today and had the same result, it failed the 15mph test. this time i drove the car for about 30 mins before going in.

his next suggestion was maybe a lazy o2 sensor or to remove my catch can and put the pcv back to stock for the test.

 

Those numbers tell me you need new front O2 sensors, like the smog tech suggested.

Are you otherwise stock...? My NO(PPM) measured zeros.

(I deleted the attached pic after you saw it)

 

is that attachment from right after you changed your 02's?

And could they be not performing well without setting of an SES light?

is it only the 2 before the cats that would need to be replaced right?

i am pretty much stock, but if they are cheaper would o2 simulators work, or are those just for the rear?

 

Yes, right after I changed my front O2's.