1990 LT5 emissions fail

[Franke]

Can anyone tell me why a 1990 LT5 would fail emissions with 3.9ppm Nox? The car has 55,000 miles on it and runs great. The max Nox limit in this state is 3.5 ppm. The HC and CO are well within limits. Some emissions people tell me to check the EGR but I don't think the LT5 has a EGR. The local Chevy dealer thinks the cats are failing but can't go any farther. Any help would be appreciated.

[Racinfan83]

Not trying to be a smartarse - but are you sure you even have to pass emissions with a 1990 model car?? We have emissions in the Metro area here - but in Mo the car has to be a 95 and up. The test here is also an OBD test that plugs into the ECM to see if there are any codes, they don't do the "run test" anymore. You might want to check and see if you are exempt???
And I have also seen cars with ZERO emissions equipment on them that are well tuned pass a run test with lower emissions then a car with a ECM and full emissions. I proved it to one of my professors in College. SO - I guess what I'm saying is that it may not be running "great"....

[Franke]

Thanks Racinfan. In Colorado's front range counties the requirement is that cars newer than 1982 have to pass the IM240 enhanced emission test. Perhaps i need to run it out for awhile and then use new fuel to see if there is a dif. I can't believe the cats are bad at 55,00.

[mike100]

NOx is very sensitive to ign timing, combustion temps (EGR help this), coolant temp, and fuel mixture. The air/fuel ratio at which NOx is minimized is a very narrow range and the oem calibration strives to keep it there during light loads and cruise conditions.

On mine, I found that changing out both of my 22 yr old O2 sensors did the trick and lowered the NOx significantly- like nearly half as much output.

[Franke]

Thanks Mike, I thought about the O2 sensors as well. I might try that before going through the beating i will take on the cat converters.

[SoCalZR1]

Are you sure it is 3.5 ppm. I just ran my 90 through California smog, no small accomplishment, and it passed NO. We are on a "dyno" and it was 388 PPM at 15 MPH and 306 at 25 MPH. Max allowed is 541 and 498 respectively.

I did swap O2 sensors last time through. Also make sure they have a fan running through the radiator if it is a hot day or your NO will shoot up. And the usual clean tank of gas, of course. I squeaked by on HC right at max, by the way.

Mike

[Franke]

Hi SoCalZR1,
The standards out of the book from Colorado is 2.000 HC gpm, 20.000 CO gpm, and 3.5 NOX gpm. I will run out this tank of gas, replace the O2 sensors and refill before I retest again. I will also run this car for 10 to 15 miles before taking it in. O2 sensors are alot cheaper than cats. AutoZone sells them for $44.95 and they are mfg by Bosch.

[SoCalZR1]

I see, gpm vs. ppm is all. Just different units of measure.

Sounds like a plan. One sensor is easy and the other, I forget which, is a PITA. But like you said, cheap swap of old parts. Good luck!

Mike

[Schrade]

Welcome onto the boards there...

Hows the burn temperature? Hotter burn means higher NO's, so you want it to burn a little cooler. This will also boost HC's however; what was the HC count? WAY under limit???

You're only 10% over limit on NO's; if you can bring down the burn temp a little, you should be ok on it...

[Franke]

Thnx you for the welcome. The HC is 1.2913 and the limit is 2.000. Thats approx 1/2 the limit. CO was 15.1407 and the limit is 20.0000. So i'm good on that too. Some history on this car is that i've owned it for 5 years and the nox has been slowly creeping up. In 2007 with 53389 miles on it the nox was 1.282 and the state standard was 6.0000. In 2009 with 55294 miles on it the nox was 2.6949 and the standard was 3.5000. CO State changed the standard sometime between those two years. In May of 2011 with 55933 mi on it nox was 3.2886 and the standard was 3.5000. today it failed with 3.9183 nox.