CA smog question

[Rob D]

I failed my CA smog check. All the emissions passed except the NO emission. The NO passed for the 25 mph portion but failed the 15 mph portion. My question is: Is there a common cause foe this high reading of NO gases where I can just replace something like an O2 sensor, or will I require the full diagnostic test (at $100+) just to figure uot which part is giving the problem. Thanks for any help. Rob D

 

[Dave in Eugene]

NO is a source of brown smog. It's produced by high combustion temperatures. EGR reduces NO by lowering the combustion temperature. There is a line from the EGR valve that carries an exhaust backpressure signal to the EGR transducer. Looks to me like there's a bottleneck in that passage in the EGR valve that could get plugged over time. Cheapest test might be to just pull off the egr valve and clean out all those passages, and also make sure the diaphragm isn't blown and vacuum will pull it open. If all is clear there go back one step and make sure the transducer is working. You can do a road test with a gauge hooked up to the vacuum line that directly operates the EGR valve and make sure it's getting vacuum - the signal should be there just about any time you're in motion. If it isn't, go back to the line from the throttle body through the EGR solenoid on to the transducer and see if there is vacuum there - it should have some pretty much all the time, and if it doesn't it is probably plugged with some gunk in the TB. If all is clear on either side, then the transducer is probably bad. Don't know if it's possible to clean or repair it, but if you're cheap like me you could probably pick one up at a junkyard for five bucks. A high flow CAT-back setup might delay the valve's activation enough to cause a failure at lower speeds even though the transducer is good, I dunno.

 

[4wheelin']

I just passed mine with flying colors after putting a new O2 sensor and a bigger cat converter on it. They Dropped from almost 2x for the 25mph NO levels to 1 and from just barely passing at the 15mph for NO to just 3. I had also done a flush of the cooling system, fuel system cleaner (gauranteed to pass stuff from kragen), spark plugs (std. champions), air filter, switched to synthetic at 128,500 i'm at 130,500 now, with improved mpg and power! This was passed today 7/16/04 after i think only three tries... tried the first two times going the cheap way out and ended up doing everything i could think of but replace the other three fuel injectors (three went out earlier this year) and to check for a vacuum leak.. my $.02 is that you can't go wrong with a big cat converter and if you can get ahold of a bosch O2 sensor the people that do a good job will put it on for like $10-20 if you get a big cat from them... I paid $140 for a large cat installed then put the O2 sensor in myself while they were doing the cat converter. THis is also an '89 4x4 Cherokee...