Can't pass CA smog - High NOx - '99 5VZ

I'm gonna take the truck into a STAR smog repair tech to get it diagnosed, but I'd like some guidance so I at least have the vocabulary to talk to the tech about it. I've failed 3 times, and replaced my catalytic converter at the advice of a mechanic (not a smog specialist) who claimed everything else checked out so it "had to be the cat". NOx reading was substantially worse after replacing the cat. So I'm out almost $1k and have nothing to show, but I'm sitting on a fix-it ticket for expired tags so it's gotta get handled somehow.

Truck is a 1999 Prerunner, 5VZ-FE. I've read these engines can have an EGR system but I can't visually see anything running from the exhaust manifold to the intake and my smog paperwork says EGR Not applicable. As far as I can tell, O2 sensors have never been replaced (I know that's probably the next step).

Numbers attached:

 

My advice? Move out of California!

I am a Navy brat who was born in NorCal, and I feel that it is my duty as a lifelong car enthusiast to hate that state based purely on the fact that they are such nazis about emissions testing! California is unkind to car guys (and girls)!

 

Some things to check, clean, or change will be the MAF, spark plugs, and vacuum leaks.

One question I have for you is, are you driving straight to the smog place from home or are you driving around town to get the cat as hot as possible before going in for a smog? Also, are you turning the truck off while you wait for your turn to smog or are you letting the truck idle while you wait?

 

I'll leave eventually when I'm looking to buy property, I don't think I'll ever be able to afford to own anything here.

But I have no beef with the emissions standards. There are 40 million people in this state, and 20 million of them live in the 5 big counties of SoCal (LA, OC, Riv, SBn, SD). If we deregulate emissions we're cooked. It's inconvenient and frustrating for someone with an older vehicle, and I admit it's unfair if you live in a rural county that's less populated, but Socal would be ruined if we could all dump whatever we wanted out of our tailpipes. I enjoy the quality of life I have here enough to deal with making my truck compliant.

 

1st test on 11/23/24 I drove straight from home, pretty much no warm-up, about 1.5 miles to the smog station. Shut the car off for about 20 minutes while I waited.

2nd test on 01/09/25 I drove about 40 miles (stopped at a different station along my commute). I think I let the truck idle while I waited, can't remember for certain.

3rd test on 02/27/25 I drove about 40 miles (same deal) the day after I got my cat replaced. Again, can't remember if I let it idle or not while I waited.

What's the optimal strategy in terms of running temp/cat temp? Should I get it nice and warm and make sure I'm idling before the test?

 

Because you don't have an EGR valve I would suggest cleaning your MAF and throttle body as there's a possibility you don't have a sufficient air fuel mixture leading to higher combustion chamber temps (cause of NOx).

 

NO or NOx is a direct result of high combustion temeratures, EGR is used to introduce a tiny bit of exhaust gas into the intake to reduce combustion temperatures. There is a work around... I am sending you a PM.

 

I had a similar problem. Ended up running a can of seafoam through the tank, drove it an hour on the highway to make sure it’s good and hot, and airing up my rear tires to ~50psi to reduce rolling resistance on the rollers. I let it idle while waiting for my test. The test tech told me to keep it rev’d around 2500 while I waited to make sure it was hot. Was able to pass on my second try.

Now I live in a cali county without a smog requirement because it’s too rural.

 

Greats tips, I appreciate the advice!

 

LMFOL!
so true

got a race friend that moved from MA to CA
he has a similar 98 Impreza with an engine swap like mine
went to this 'smog inspection' place(rookie's 1st trip)
inspector there busted out a picture book with every manufacturer's Stock Engine Bay!!!!!
(all years/makes/models)
of course did not even pass the 'eyeball' test

i couldnt believe it

 

My aunt and uncle have lived in Monterey for more than forty years. My uncle has two Volkswagen Buses, a 1971 and a 1977. The 1971 is exempt from emissions due to age, but the 1977 unfortunately is not. The 1977, a Westfalia camper, was originally equipped with a fuel injected motor and a catalytic converter.

Early fuel-injected VW motors are notoriously finicky, and the previous owned replaced the original motor with no catalytic converter. Most states wouldn’t give a shit. The smog station refused to touch based simply on the fact that the VIN says it was originally fuel-injected, and said it wouldn’t pass anyway because it doesn’t have cats.

The Westfalia runs fine and is hardly a smoke-belching piece of shit. My uncle simply registers it at his brother’s house in Texas!

 

Freakin mechanical fuel injection!!!!
The worst……ask me how I know

good for your uncle

Funny, that is exactly what my friend did to “by some time “
MA renewed plates/registration…..coming up on his 2yrs(?) thou
He is a “by the book” kinda guy

to be legal and pass smog…..
Owner can do engine swap, but every part of the engine HAS to be from the swapped engine. He bought a donor, had to swap fuel tank/charcoal canister system/filler tube/gas cap/some EGR crap in engine bay among other things
Lucky, all the above parts bolt in to the Subaru.
Has been chasing an evap code for months
He is 100% more technical than me when comes to these things…..that’s why I call him all the time when I have a Subie problem
Smoke tested every single inch of the swapped system, in sections even

I just went to his wedding in May
After 4months of chasing…..
He said threw in the towel, has the GC8 at a specialist now

so much work to have/own/drive an old car
In CA.

 

If you can contribute to op’s issue besides telling op to move out of CA, don’t derail the thread with nonsense. Simply move on and create your own “why you don’t live in CA” thread.

OP, vacuum leak might be the culprit. As stated, get the cat hot by driving on the highway for a length of time and let the truck idle while you wait at the smog place. A cat is designed to function when hot.

 

Here in colorado I had the same condition. My limits were 1.5 on the nox and I barely passed with a 1.49. I put a K and n filter back in for more airflow and went with regular unleaded with no ethonal. I also waiting in line turned my truck off this time. I passed easily with .90. Mine is 2002 tacoma 3.4. Our trucks werent really designed to run on these new fuels since they are older. For the record they changed the limits to be lower from 3.0. They dont want us to run these older vehicles in these states.

 

Had an interesting conversation with a guy at All Toyotafest in Long Beach this past weekend. He said he worked as a Toyota engineer for years. He had a 3rd gen 4runner in the show with a 5VZ, he was telling me how he swapped in a 3VZ crank because certain years they're forged, which I've heard of, but also he used the 3VZ exhaust cams.

He explained that the timing of the cams in the 5VZ is designed to have a "natural" EGR effect, so there's some overlap between intake and exhaust or vice versa to reintroduce some exhaust. 3VZ exhaust cams eliminate this, according to him, so great if you want better performance at higher RPMs. But since the EGR effect is supposed to be intergrated on these engines he was really surprised mine couldn't pass because of NOx.

Might be a stretch but I'm wondering if the shop that did my timing belt last summer aligned my cams properly. I'm not throwing any codes and the truck runs great, so I've always assumed the job was done right, but if they timed it in a way that eliminated this "natural EGR" effect, it could be why my numbers have been so high.

Wish I asked him about this while I was there. I'm not a mechanic so maybe that's a crazy/impossible hunch.

He recommended just replacing my MAF since it's not an expensive part, along with O2/AFR sensors.

 

NOx is highest when running a bit lean vs every other pollutant being higher when running rich, can't hurt to replace the upstream O2 just incase something is wrong with it and it has been biasing your mixture towards lean. If you're not getting a code for the downstream O2 then there's no reason to replace that one though, it is just a nanny for the catalyst performance, it doesn't contribute to mixture control.

 

Thanks for the advice. I commute about 90 miles once a week and I have a smog station about halfway that I can use so that should ensure the cat is hot enough.

Anytime I mention I live in CA on here I expect some anti-CA rabble. It's just a part of the deal being a Californian on the internet lol

 

This may or may not be relevant... but before you start throwing parts at it... make sure all your monitors are ready. You need an OBDII reader for this.

I failed smog a couple months back. I had the battery unplugged for 2 months while I was traveling in Thailand. When I went in for the smog... I failed. My Cat monitor failed. I was told to drive it on the freeway for 30 minutes at 55MPH to reset it. Followed those directions went back in and failed again! Did a little research and learned that there is a very specific drive cycle that you need to do. I followed this video and it reset my monitor and I passed. Worth a look.

https://www.youtube.com/watch?v=EKfEr2LP2Nw&lc=UgzJutMt2fyvYlB1F6R4AaABAg

 

I’m glad you moved out of my state. I wish more would.

 

Gosh darn AI says the 99s have EGRs, I'm sure a Yota SME here can list out what VINs did and did not have it. That and what @Speedytech7 said, maybe replace the primary o2 sensor as that is what really controls air fuel ratio accuracy. After that if still no-go you can google the assortment of brand x, y, z etc additives at your typical AutoZone that profess to help pass and I think I've read there is some mix/ratio of E85/normie gas that will throw much better numbers (truck will auto adjust for the eth content).

Damn, totally missed this event. I've been going to the Honda headquarters Cars, Bikes and Coffee they host every other month and might start taking my truck there, there is a growing 4x4 scene at that event.

I both receive and give this lol. Gotta live where the work is.

As did I (for visual of the TRD blower, story in the S/C thread), but -

Interest peaked haha. I was in Hua Hin at Sitjaopho for some training. I'm not in that shape anymore but crave going back.