Failed smog, what to fix?
April 26, 2010 3:16 PM   Subscribe

Hello AskMe Mechanics! My 1999 Chevy S-10 just failed its emissions test and I need some reliable information as to why and towards what I can do to make sure it passes next time.

The truck is relatively low-mileage: ~70,000mi.
It just had a fluids-change last week, but I probably need to change the air filter soon.

The test was conducted in Los Angeles, CA at a Test Only station (they didn't have the capacity to work on the truck) and measures a bunch of stuff at two different speeds: 15mph and 25mph. My truck passed every metric except the NO (PPM) at 15mph. At 25mph the NO measured very low at 35ppm, but at 15mph it was waaay over the max (533) at 843ppm.

Ray, the man with the machine, wasn't able to fathom why it would pass the 25mph test with flying colors but fail at 15mph.

Do you know why? And what should I ask the mechanic to do when I go to have it fixed?
posted by carsonb to Travel & Transportation (8 answers total) 1 user marked this as a favorite
Response by poster: The consensus on the msg boards I'm finding seems to be "Use an additive!" but I'd prefer to have the issue taken care of more permanently than that if possible.
posted by carsonb at 3:22 PM on April 26, 2010

This is all from half remembered stuff I learned from dad, I haven't lived anywhere requiring a smog check in many years, so my first advice is go find a good mechanic nearby.

Was the car thouroughly warmed up when you did the test (like been running on the freeway for at least 15 minutes?)? NO is nitrous oxides (precursor to smog) which is taken care of by the catylitic converter, which must be warm (actually really hot) to work right. So may guess would be not warm enough to work when the engine is running in a rpm range that isn't very effecient (lugging at 15 mph) or it is clogged/dirty as this would also produce the same range. It could also be a number of other problems but most of what i can thing of would make your check engine light come on. I would get some shop to pull the codes of the engine to see if that tells you anything. Also do all the normal tune up kind of things like new spark plugs, new air filter, new oil (and filter) etc. This could be a tough problem to track down and if you need a new cat it could be expensive (some are, some aren't).
posted by bartonlong at 3:28 PM on April 26, 2010

seconding warm it up before next pass. I used to live in a state where *every car* had to pass inspection - even my early 60s bug! (go figure). I went to a shop that advertised "inspection repairs" (you should see these around town - such shops have state-type inspection gear and settings) and had them tune it to pass. it ran turrible. after I got my sticker, I went back and had them tune it to run right.
posted by toodleydoodley at 3:31 PM on April 26, 2010

EGR valve working properly (keeps combustion temperature low, to reduce NOx)?
AIR pump OK (auxiliary air in the exhaust to help catalytic converter at low RPM or cold starts)?

As mentioned above, get any failure codes from the fuel injection computer, and see what's on its little mind. Check your hoses for the PCV, and make sure everything is tight.
posted by the Real Dan at 3:51 PM on April 26, 2010 [1 favorite]

Last time I had this problem (high NO, 1983 Toyota Landcruiser), a valve in one of the hoses from the smog pump was bad--it wasn't passing nearly enough air (which could certainly cause this problem if mentioned above).

Unfortunately it failed again this time due to high CO, so I have to figure out why it's running rich. Passed last time OK. Who knows? Good luck.
posted by RikiTikiTavi at 3:58 PM on April 26, 2010

A check engine light is usually an automatic failure, so I'm assuming the OPs CEL is not on. My wife's Z28 failed back in the day, and the tech told her to take for a 20 run in the Interstate and rev it up. When she came back it passed. So I'm N'thing the recommendation to make sure it is good and warmed up before the retest.
posted by COD at 4:20 PM on April 26, 2010

Response by poster: A check engine light is usually an automatic failure, so I'm assuming the OPs CEL is not on.

Correct on both counts. My truck only failed the NO test at 15mph.
posted by carsonb at 4:37 PM on April 26, 2010

It may be your mass air sensor, or your idle air bypass valve. Running some Sea Foam through one of the vacuum hoses may clean out the system. Be forewarned that sea foam will blow ALL of the carbon built up in your engine out the exhaust pipe-think Spy Hunter. It works extremely well, and makes for a hell of an attention getter in your drive way.
If you need the truck to pass before you can get it to your mechanic you can run a few gallons of Sunoco Racing fuel on an empty tank. The increased octane (104 if I recall correctly) will make the entire combustion process run hotter, almost guaranteeing a pass. Remember that this is just a band aid to get you through inspection; If you use racing level octane fuel for extended periods in a system not designed to handle the increased temps you'll most likely burn out your cats. But you'll be fine a few gallons at a time. Years ago while in college in Chicago I had a very souped up Ford that wouldn't pass emissions if my life depended on it (I'm sure it didn't help that it had no factory emissions equipment on it to speak of). After failing 2 times in a row a mechanic friend suggested that I toss 4 or 5 gallons of race fuel in it the morning of my inspection. It passed with flying colors every year until I sold it. Not the best option, but an option none the less.
posted by chosemerveilleux at 5:40 PM on April 26, 2010 [1 favorite]

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