Ok, First i got the car to pass after 6 times ut what i had to do to get it to pass is bugging me. This car passed with flying colors 3 years ago and it was even in need of a tune up back then. It was as it was from the dealer though since then i have changed things.

Its a 1988 Trans am, 305TPI Mods are TB Bypass, SLP y-pipe , 3inch catco cat, 3inc hooker aerochamber catback, MSD wires + coil + 6A, timeing at 8deg BTDC, Accell 19# Injectors,

The first reading was
Allowed Actual
Nox 1100 2400
HC 149 98
CO 0.89 0.43

Since the egr valve is the big one in the nox reading i changed it. I had the same problem on my old iroc so i figured thats it. WHen i went back after that i got the same readings. I checked the vacume lines all around the motor. Everythign was workign and connected properly. so i got to thinking. This test was taken after idleing for about 10 mins in line so the car was around 215 deg or so at the time of the test. Since NOx comes from high chamber temp, i wedged the fans to stay on. The next test was taken the same way but with the fans stuck on and a 180 deg stat.

Allowed Actual
Nox 1100 1700
HC 149 70
CO 0.89 0.5

Then somone said to me if i was to richen the mixture it could cool the chamber temp, so i plugged the vacume line at the fpr and left the fans on.

Allowed Actual
Nox 1100 1600
HC 149 35
CO 0.89 0.07

This puzzled me, I richened the mixture and co and hc dropped ! I changed the 02 sensor, fresh spark plugs (r45ts ac delcos gaped to 35) and got verry close to those readings, I checked the TPS and it was at 0.60 at closed throttle. I then killed the timeing , Dropped base to 10 deg AFTER TDC, HC and CO didnot go up mutch at all but NOx came down to 900 and that passed me.

SO... Why did i have to do this crap. The largest mod on the car is the exhaust. Since i changed everythign from the manifolds back it woke up greatly but it still has the manifolds.

Reaserching the egr valve i found it needs vacume at the top AND pressure at the bottom to open , would freeing up the exhaust as i have drop that pressure to a point where the valve couldnot open enough for the system to funcion properly?

Also when i richened the mixture by increasing the pressure at the rails why did HC and CO drop... is the low HC and CO readings a sighn this car is running lean?

Thanks for the help guys.
-Stell

 

I also live in NJ and have had a similar problem. My car failed for the same reason except not as bad as yours, it was barely over.
When it failed it was in the summer and the car was close to overheating, with the temp creeping into the red.
The second time I went was in the winter and the car was nice and cold and it passed easily. Dont have any clue why but someone told me that being too hot before caused a bad reading.

 

It's real simple, NOx gases are from too hot of a combustion chamber. It isn't a bad thing but lots of reasons for this. Since you seem to have room for HC I'd run the car cooler. I passed the 330ho motor with TBI and an ANLU eprom (lean) without the air injection and without EGR. Only thing it had was a CAT which had over 150k miles and again, no air going into it to help. I failed for NOx too but it was because my thermostat was broken (bent) and wasn't closing so the temp would creep up slowely. Next morning I took it through inspection with the temp down at 190 and it passed no problem, just a little high on the HC.

 

Hey... Let me ask you.. do you keep your car running when your in line? if so, the NOx can build up in the exhaust and can throw it off... it happend in my taurass. I did it somwhere else, and i passed after they let it sit and then start it up and test it. it passed well under. i sugest you turn your car off and then turn it back on when you have to start it... just to give you some help

Also its good to see more NJ memebrs... Were having a meet in Toms River on May 24th... its on rt 37 at the super stop and shop across from Downs Ford (errrr!)

 

I'll be there Nahh i dotn leave it running, i left it off and rolled in line, it happens to be downhill so i didnot need to run the car just kinda rock it forward and it will go I was bringing the motor to like 2500 for a moment when i got next in line to clear out everything, Also when the guys ran the dyno if it was gonan fail they would void the test run the car at high rpms and try again and still fail.

What i am wonderign is what could cause it to be failing when left as it is with the mods. There are no internal mods to the car so it kinda throws me thaughts as to why

-Stell

 

BTT

 

The 2 major things for nox is egr and convertor.Mayslanding inspection lines can be long sometimes,shut it off and let it sit,oh and bring a good book.

 

Did you pop off your EGR? Check for excess carbon buildup. you can check your EGR by checking the vacuum pressure. If it doesn't hold the pressure, you probably need to replace it. If you have TPI, you have some work to do.

 

OK I finaly passed , my question was why was i failing....

The car has a New egr, New 02 , New cat, new injectors, Fresh tuneup , Mods listed above., The manifold came off and was cleaned with a brush through all the holes..

After i did all that the car still was pushing 1800 nox, where it should have been 1110

To get it to pass i had to kill the timeing to 10 deg AFTER TDC , through a few bottles of drygas in, unplug the FPR to richen the mixture at low rpms, and advance the TPS to .7, this got the car to blow 900 nox........

My question is why did i have to do all this crap to get it to pass, when the same car passes 2 years ago bone stock and all i added was verry minor bolt ons...

-Stell