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What causes a measurement of 133 HC (PPM) and 1033 NO (PPM) for emissions which I just failed

Tiny
SHAWNALU
  • MEMBER
  • 1991 CHEVROLET SILVERADO
  • 200,000 MILES
What would cause my truck to measure 133 HC (PPM) and 1033 NO (PPM) and fail emissions
Thursday, September 27th, 2012 AT 6:50 PM

10 Replies

Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
A slow response o2 sensor could be the hc along with plugs and dirty oil.

The nox is mostly either a cat converter or egr valve

Roy
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Thursday, September 27th, 2012 AT 6:53 PM
Tiny
SHAWNALU
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When I took it in 2 weeks ago it didnt fail emissions it only failed timing, and EGR valve. Now my timing is only off by 5 and my EGR valve is fine but I failed emissions this time.
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Thursday, September 27th, 2012 AT 6:54 PM
Tiny
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I also just changed my oil this morning.
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Thursday, September 27th, 2012 AT 6:55 PM
Tiny
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I changed my EGR valve a few days ago.
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Thursday, September 27th, 2012 AT 6:55 PM
Tiny
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I also have an after market cat converter. But it is legal in CA
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Thursday, September 27th, 2012 AT 6:59 PM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
Drive the truck for 10-15 miles on eway if you can then get it tested. It may have not been fully warmed up when you got it tested.
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Thursday, September 27th, 2012 AT 7:07 PM
Tiny
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Reading my report, my co2 and o2 passed they read 14.2 @ 15 MPH and at 25 MPH its 14.7, and o2 is @15 MPH.5 and @ 25 MPH.1. How would it fail the first time because the EGR Valve and timing off by 11 degree I fix the timing, replace the valve. The first report said the CO2 and o2 were ok, now the second report says HC and NO is off but EGR valve is fine but my tming is off by 5 but he said he would have passed it if my timing was the only issue. I dont understand how they pass the first time when I fix the original issues they fail the second time
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Thursday, September 27th, 2012 AT 7:08 PM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
It could be wear in distributor and timing chain. Check to see if there is vacuum leaking under the tbi unit, those like to blow the gaskets and make it lean. It still could be a convrter problem or an air leak prior to converter.
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Thursday, September 27th, 2012 AT 7:12 PM
Tiny
SHAWNALU
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I'll check the Vacuum again but 2 weeks ago it was ok, but I know how things break fast. I'll try that hmac300, Thanks I hope this fixes it.
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Thursday, September 27th, 2012 AT 7:17 PM
Tiny
WRENCHTECH
  • MECHANIC
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Hi NOx is caused by high combustion temperatures. Advanced timing will increase the combustion temp and the EGR is designed to reduce it so the 2 things that you worked on are the 2 most common causes of high NOx. HC is unburned fuel and usually caused by either a misfire or gas diluted oil. I would verify that the EGR is working properly now as the valve may not have been the issue in the first place.
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Thursday, September 27th, 2012 AT 7:40 PM

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