QuestionI hope you can shed some light on me. I have a 1991 GMC 1500 with a 4.3 eng. Everything else is good but I failed the Nox. portion of the emissions test. I replaced EGR, EGR sil. ,MAP, Rebuilt trottle body, Changed plugs, wires, cap and rotor. Removed intake and checked and cleaned EGR ports for carbon buildup. Basically the only things I haven't changed is the cat. and the carbon canister. Wnet back to retest it and failed again by the same levels. The insector told me that if it was the cat, then all the gasses would be bad and the levels of Nox. would be equal at low and high RPM. Right now the Nox. is 1600+ @ high, and 900+ low RPM Here is what really puzzled me. He said he had a truck that failed with a new aftermarket EGR but passed with a factory AC Delco from the dealer. I have already spent $50 on the aftermarket one and don't want to spend another $80 on the AC Delco one, then have it still fail. Any help you can offer would be great. Thx.
AnswerThis is one of the reasons why I drive old diesels.
In California, I had two 1985 Toyota MR2s. One failed smog and required a new catalytic converter. After changing the converter, I retarded the timing slightly and increased the idle speed (fuel injection). The vehicle passed the second test. I took the converter back off and put it on the shelf.
A converter with more than 50,000 miles on it may prevent passing a smog test. By 100,000 miles, the converter is dead.
The second MR2 was also going to fail its smog test. That time, I refused to even ask why. When the tech saw my "I don't give a d--m" look, he ran the car at fast idle for about three minutes. The converter, when hot, passed the test.
I would either replace the converter or look for one of those chemicals that guarantee passing emission tests (sold at most auto parts stores).
