Sierra :: Smog Test Failed - O2 Don't Cycle Rapidly
Nov 4, 2011
My truck is a 91 sierra 2500 over 8600 gvw (8 lug) 5.7 4l80E. After my truck failed a Calif smog test I set about testing the different sensors that are attributed to the reason it failed (a few points high on HC's). All the smog equipment and sensors tested ok except for the 02.
I tested the one in the truck when it failed smog, it came out at 400 millivolts key on engine off as my Haynes manual and the Autozone website said it should, after running the truck for 10 minutes and running it at around 1200 RPM's for 2 min to get the O2 cycling it doesn't, best I have gotten was a slow rise of about 0.01 millivolt rise per second to about 0.590 MV where it seemed to level off, the problem is 2 seperate new sensors do the exact same thing, none cycle rapidly between 100-900 MV as the Haynes manual and the Autozone website say they should.
The 02 is pre-cat stock location, no changes to the exhaust since my last smog check. I don't even know if this is the reason I failed smog, it's just the only thing that didn't pass my testing. the truck does seem to be running a bit rich (per my ultra sensitive tailpipe sniff test).
I did do all the normal repairs after I failed the smog test, tune up, air filter, clean the TB run a can of cleaner through the gas tank, ETC, then I failed the re-test by 3 points worse. The truck does not run bad or blow a bunch of smoke, the truck is not throwing any codes.
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I drive a 1990 240 with close to 300,000 miles on it. Haven’t had any troubles with it in the 1-2 years I’ve had it.
Recently went in to get my smog checked and failed two of the three tests: the functional and emissions. I've attached the VIR.
Mechanic said there might be a leak, so a buddy of mine checked the engine for a vacuum leak but couldn’t find anything.
What are the common sources for a failed smog check? I plan on getting a diagnostic later in the week/
smogcheckVIR.png ( 1.93MB )
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