failed smog test only again

[blazinbg]

So I replaced my cat and egr and failed my test only again for the sme reason the nox were to high. so I went to a mechanic I was referred to and he said there is not enough back pressure to make the egr open fully.
Is this a correct diagnosis? if so what is happening to not create enough pressure. the truck runs great.
350 tbi

 

[NorCal_Chris]

you check all these?

high nox can be caused by the following:
Misfire condition
Malfunctioning or improperly adjusted EGR valve
Failed oxygen sensor
Leak in exhaust tubing upstream of converter
Excessive carbon deposits in combustion chamber
Improper spark advance
Blocked coolant passage
Overly lean air-fuel mixture
Damaged cold air duct
Failed or malfunctioning catalytic converter
Corroded or damaged engine sensor electrical connections

 

[blazinbg]

all the electrical connections were good to the sensors I was asked if I had the right thermostat I am running a 160 but stock is supposed to have 195 will this affect the nox I replaced the ect it looked kind of corroded with a delco (the egr is a also a Delco) when you mean leak in the exhaust upstream of the converter is this before the cat ar after the cat

Malfunctioning or improperly adjusted EGR valve-replaced
Failed oxygen sensor -checked
Leak in exhaust tubing upstream of converter
Excessive carbon deposits in combustion chamber-how do I check
Improper spark advance-timing was fine
Blocked coolant passage-how do I check
Overly lean air-fuel mixture-how do i check
Damaged cold air duct-no air duct but never been a prob. in years past
Failed or malfunctioning catalytic converter-replaced
Corroded or damaged engine sensor electrical connections-checked

 

[K5Jimmy]

Put in a 195* tstat...it makes a huge difference to the sniffer....someone will post up here with a tech description of open loop and closed loop, which will affect your fuel mixture...I have an 89 TBI and dealt with the sniffer nazis in Dallas....I moved to Arkansas...NO inspections...ever...also, make sure your timing is set to 0* (stock setting)...I bump mine to run, but dialed it back on inspection day...

 

[sweetk30]

t-stat huge thing in computer controled stuff. it tells the computer when its up to temp to add or remove fuel tables to make it run rich or lean. kind of like leaving a man choke on when motor is all warm. wounder were the extra fuel went and black tail pipe came from .

years ago i had a truck with fuel injection. had a bad t-stat opening at 135-140. no check engine light figured no problems. but had black tail pipe and 1hr trip took 3/4 tank of gas. had it checked out and installed new t-stat and bam no more black pipe and went from 3/4 tank of gas to just under 1/4 tank for same trip .

 

[blazinbg]

will replace the thermostat tomorrow.keep the advice coming fellas I need to register my truck.

 

[mpio13]

i literally JUST put the book down I'm studying to become emissions repair certified: which is really a bunch of crap, but I am reading and studying like crazy and here is the deal with NOx:

NOx are always present in the combustion process, I could get into the exact chemical content and chemical reactions that take place but I wont bore you.

The thing about NOx is that they increase rapidally when the combustion chamber temp reaches over 2500 degrees: and is reduced in a properly working catylitic converter, by the metal rhodium.
Yes, the EGR is the main component that controls NOx, but the term "backpressure" doesn't really apply to an EGR. The EGR is controlled by manifold vacuum, or electronically. Make sure its getting vacuum (around 18"Hg) at the diaphram, or being commanded on, if it is electric.
Also make sure the ports in which it is bolted over are clear of carbon, that can clog them up, so even with a working EGR, the goal of putting exhaust gas back into the intake is not being met.
Speaking of carbon: if you have carbon build up in the combustion chambers, that can cause high NOx.
Also: the tstat really shouldn't effect NOx, like I said, it gets really high with more heat, so actually you should keep the engine cool. (but not too cool that you fail for CO or HC's)
Also: NOx can increase if your air/fuel mixture is too lean. Possibly bias rich O2 sensor, or vacuum leak (*which could also effect the EGR performance)
And yes, retarded timing can cause high NOx, so make sure its set to 0 degrees.
Good luck!

 

[blazinbg]

how would I go about cleaning the carbon build up in the combustion chamber?

 

[NorCal_Chris]

how did you check the 02 sensor was good? i would be replacing it.

seafoam is known to do wonders for carbon buildup.

 

[K5Jimmy]

t-stat huge thing in computer controled stuff. it tells the computer when its up to temp to add or remove fuel tables to make it run rich or lean. kind of like leaving a man choke on when motor is all warm. wounder were the extra fuel went and black tail pipe came from.

X2....KISS= Keep It Simple Stupid...the tstat is a blatant issue, so fix it first...it's cheaper than an O2...on the O2 issue, I have had issues with Bosch O2 not working right out of the box, so I use Delco only,

I tend to believe that cheap stuff fails first. Along that line of thought, I would even lean in the direction that your Cat and EGR might not have been an issue if you had the correct Tstat in the first place. You didn't post your numbers, so you have people kinda swinging at generalities here. Did changing the Cat and EGR help the numbers? If so, by how much? Sometime we have a tendency to make things harder than they have to be and try to fix them by just throwing money at it .

 

[PWagon]

So I replaced my cat and egr and failed my test only again for the sme reason the nox were to high. so I went to a mechanic I was referred to and he said there is not enough back pressure to make the egr open fully.
Is this a correct diagnosis? if so what is happening to not create enough pressure. the truck runs great.
350 tbi

You might try running your gas tank down to near empty, then putting in 5 or 10 gallons of Ethanol. Ethanol burns A LOT cleaner. You're likely to pass your inspection with flying colors. I wouldn't leave Ethanol sitting in the gas tank too long though. After you pass your inspection, drive around and use up the Ethanol in your tank. They say it'll eat rubber over time, but it shouldn't hurt anything if it's used within a couple of days. Beside, they are already adding 10% Ethanol to my gasoline today.

 

[4xcrazy]

He's LIKELY to pass with the CORRECT thermostat installed as well...

and also get better fuel mileage, especially after the engine warms up to where it's suppose to be, and leans out the mixture,,,I believe when you're runnin them that cold, the computer always reads it as cold, like just started,, thus burning more fuel, like a choke mode setting.

I know my old TPI Camaro used a bit more when I installed a 180* t-stat... Never tried it with the Burb, as it already sucks with fuel mileage.