Thread: not passing smog; high NOx

i live in CA and have a 93 sentra se-r that has always had trouble passing smog. right now:

15mph is 1800
25mph is 2979

passing is 537!

any ideas where to start troubleshooting? I got a used MAF from a parts place, but no change.

Check your cat or replace it.. Or dump a bottle of alcohol on a full tank that sometimes works.

Start with a normal tune up, and then go after the bigger guns.

-Oil+filter change
-Sparkplugs
-Airfilter
-Fuelfilter

Next stage:

-02-sensor (expensive!)
-Catalys. (even more expensive)

Still not working?

-Distributor cap + rotor
-Plug wires

Still not working?

Is the engine running right? Compression test? Leaking oil? Is headgasket OK? Is your exhaust system stock?

Retard your timing. High nox is caused by cylinder temps that are to high.

Retarding timing will increase your hydrocarbons and CO however

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A restrictor plate in the exit of the cat will help if you're running a bigger exhaust. The idea is to restrict flow behind the cat to get it hotter. Free flow exhausts will lower temps in the cat and make them not work as well.

High NOX is often associated with EGR valve malfunction. You can do a quick check of your EGR by locating the EGR valve and holding your finger on the bottom of it as you rev the engine by moving the throttle under the hood. (Take off your tie before you try this!)

If the plate on the underside of the EGR moves up as you rev, the valve is probably working OK. If not, then you may have an issue here. The most common cause is a blockage in the metal tube that operates the EGR through the BPT valve. This tube is connected to the BPT via a rubber hose and delivers a vacuum signal from the larger pipe that goes from the exhaust manifold to the intake manifold.

If none of this is making sense to you, do some searching on this and the other forum. There are many pictures and detailed explanations. Your car won't pass NOX if the EGR is not working.

Our cars are getting so old that many shops don't even know how to work on the emissions systems any more. We have to fend for ourselves.

where you located at?

To whom are you addressing your question?