Failed emissions, high HC and NOX
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Junior Member
Joined: Jan 2014
Posts: 4
Likes: 0
From: Arizona
Car: 89 Chevrolet Camaro RS
Engine: V8-305 5.0L
Transmission: 700r4
Failed emissions, high HC and NOX
I've replaced the catalytic converter, egr valve, and the electronic control module. I even ran some seafoam through it.
HC: 2.37 (passing standard: 1.60)
CO: 7.52 (standard: 15.00)
NOX: 2.61 (standard: 2.50)
After I got the catalytic converter my HC passed and my NOX was over by .04, and it went up after I got the new egr valve and electronic control module.
I bought new spark plugs but have yet to put them in. Does anyone have any tips or thoughts? Also why would my levels go up so much after getting work done?
HC: 2.37 (passing standard: 1.60)
CO: 7.52 (standard: 15.00)
NOX: 2.61 (standard: 2.50)
After I got the catalytic converter my HC passed and my NOX was over by .04, and it went up after I got the new egr valve and electronic control module.
I bought new spark plugs but have yet to put them in. Does anyone have any tips or thoughts? Also why would my levels go up so much after getting work done?
Supreme Member
Joined: Apr 2010
Posts: 4,337
Likes: 29
From: Aurora, OR
Car: 87 IROC Z28
Engine: 355 cid TPI
Transmission: Custom Built 700R4 w/3,500 stall
Axle/Gears: QP fab 9" 3.70 Truetrac
Re: Failed emissions, high HC and NOX
High NOx is caused by high combustion temps. Anything that affects combustion temperature, lean afr, too much timing, poor cooling system function, wrong spark plugs, EGR not working, will cause NOx. High HC with high NOx sounds like a lean condition or too much timing advance. Look for any air leaks in the intake. Retarding the timing 2 degrees will reduce both. You can put it back after you pass the test. The main function of the EGR valve is to reduce NOx by reducing combustion temps. Make sure it's working. The valve is only one part of the system. Make sure the passages are clear and that the control solenoid is working. If in doubt you can bypass the solenoid for the test. At idle, the engine should stall if the EGR valve is opened by hand. If the engine doesn't stall, or at least severely fall on its face, the passages need to be cleaned.
The catalytic converter when working right reduces HC by about 200 to 300 ppm. It has less effect on NOx. By the sound of this, I guess you are having to test on the wheel dyn and not just at idle. If the test is just at idle, remember that a colder engine produces little or no NOx.
The catalytic converter when working right reduces HC by about 200 to 300 ppm. It has less effect on NOx. By the sound of this, I guess you are having to test on the wheel dyn and not just at idle. If the test is just at idle, remember that a colder engine produces little or no NOx.
Last edited by ASE doc; Jan 19, 2014 at 11:38 AM.
Joined: Aug 2011
Posts: 1,262
Likes: 196
From: Jackson NJ
Car: 1984 T/A
Engine: LQ4
Transmission: T56
Axle/Gears: Ford 8.8 4.10 gears
Re: Failed emissions, high HC and NOX
i failed for the same thing. I found out it was my o2 sensor. I had a nonheated o2 and when the car sits idling (waiting in line for inspection) the sensor cools off and would run rich, and i failed. I replaced it and passed with flying colors
Supreme Member
iTrader: (1)
Joined: Jul 2000
Posts: 1,290
Likes: 0
From: Carson, CA
Car: '88 GTA, 90 Formula
Engine: 5.7 TPI, fed growth hormones
Transmission: 700r4 4u2?
Axle/Gears: 9bolt
Re: Failed emissions, high HC and NOX
I and several others have also failed because an old cap and rotor were arcing causing a misfire. Everything else can be right, but as tight as the specs for our cars are now, one thing will put you over the limit, so just throwing parts at it can be pretty expensive.
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