High NOx at 15 MPH smog test

I'm due for a smog test this month on my 1996 Tacoma 2.4L.

The last couple smog checks, the NOx value has been approaching the limit for the 15 MPH test. No problems with the 25 MPH values yet. I used lots of Seafoam in the gas, oil, and vacuum line -- and drove the truck hard -- prior to the 2010 smog check. I also removed the EGR valve and attempted to remove some of the carbon buildup but wasn't very successful.

As you can see, this truck does not get driven much at all... Do I stand a chance of passing the 2012 smog? ... If not, what do you recommend?

FWIW, after the 2010 smog check, the original MAF sensor failed and was replaced with a new one.



Note: Values in red are the maximums allowed.

 

Thanks. By road trip, how many miles are we talking? 100, 250, 500?

So far I have no check engine lights suggesting cat or EGR problems.

 

+1...

Of more concern to me would be that huge jump in HC.

Ordinarily, you could retard timing a bit to lower NOx, but that increases HC, which you don't have a lot of overhead on that.

 

Ahh... and looking back I see CO was way up too.

I'm with you. New cat and she should be good as new.
I put a new cat on my '94... didn't really need it, but I was having a new system put on and figured since they were down there, I may as well have them pop one in.
It brought me down from "I wonder if" to "I'll keep this truck until the new millennium.

Traded it in 2002 on an '03 Tundra.

 

Any idea why the HC was so high in 2002 and 2004? I installed a K&N Air Filter just before the 2006 smog check and sometime around 2004-2005 installed a new radiator because the old one was full of mineral deposits (previous owner used tap water instead of distilled).

I may have cleaned the K&N Filter a few years ago. Should I clean it again?

 

Increase in NOx and drop in HC at the same time normally indicates higher cylinder pressures, normally from timing advance, but can also come from higher temperatures.
Was it running hot (or hotter than "normal") before the new radiator, or did you change it just because it was full of crap?

 

I took it on a road trip in the summer to the Mojave Desert (115° F outside) and the temp needle was pegged at Hot.

The fluid levels appeared fine, but there was poor circulation inside the radiator from all the crap buildup.

 

Were your cats up to temp when you did your smog test? A catalytic converter works best when HOT, not I just drove 10 miles hot, like my engine has been at operating temperature for at least 30 min straight. It also seems that you're not getting a clean combustion b/c of the high HC but that was prolly due to a failed MAF.

Make sure your cats and engine are up to temp, also have at least half a tank of mid grade top tier gasoline in the tank. Anything lower than 8 gal in the tank, the computer senses a lower fuel pressure and leans out the motor by retarding the timing.

Do a tune up 500 miles before testing. Clean the MAF, TB, change the spark plugs, oil, air filter. The 500 miles is to allow your computer to re-adjust back from default A/F ratio to the optimum in your area and driving style.

This should help. This is my field of study. Air pollution chemistry/engineering. I used to smog test cars all day for research data for my supervisor.

 

Right, but I replaced the radiator in 2004/2005. Most of the increase in the NOx has been with the new radiator installed.

 

For the 2010 test, I drove it hard for one hour prior to the smog test, mostly on the freeway at 75 MPH. Prior to that I ran Seafoam through two tanks of gas, Seafoam in the oil, and Seafoam into the vacuum line about three times.

All the other smog tests (2002, 2004, 2006, 2008) I brought the truck in cold without any Seafoam treatment.

 

If you had your MAF replaced that would fix your HC issue, but a NOx issue is your cat. You prolly have to replace it.

 

Before I spend money on a new cat, any chance this is the EGR valve?

 

Like others have said, go get the CATs hot before the test.

They used to sell and additive called "guarenteed to pass" at Kragen when I worked there. I had several customers tell me that they had failed, put that stuff in their tank and tried it again and they got by the second time. Might be worth picking that stuff up if you can get it.

As best as I remember from high school auto shop class: Slightly higher HC is usually a weak CAT. Grossly high HC usually occurs when there is a cylinder missing (unburned fuel). CO is a direct reflection of fuel mixture. High CO means it's running rich. The EGR valve is designed to lower the NOx. It also lowers combustion chamber temps.

If this truck has a vacuum operated EGR, putting vacuum to it at idle should cause the engine to run rough or stall out. If not, something is not right with the EGR. Look very closely at the plumbing going to the EGR, I had one older Toyota that had a carbon buildup right at a 90 degree bend in a pipe going from the intake manifold to the EGR valve. I cleaned that out, truck ran 100% better. It had been knocking under load due to high combustion temperatures.

 

YES! The EGR is designed to lower NOx.

In my experience, a weak CAT results in high HC readings. As in double the allowable amount.

I would not replace that CAT until the check engine light comes on and the ECM has a converter efficiency code in it.

I have a buddy who has been sneaking his Audi S4 past smog for the last six years by getting the weak CATs hot just before going to get it tested.

 

Let's assume I fail with no check engine light, which do I replace first, the EGR valve or the Catalytic Converter?

 

With just high NOx, I'd go right for the EGR valve. After making sure all the passages in the EGR plumbing were clear, and there is vacuum reaching the valve to open it, of course.

If your HC is extremely high, I'd suspect the CAT, but I've never heard of that happening without a check engine light.