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Failed for NOx

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New User

Feb 19, 2007, 12:41 PM

Post #1 of 3 (3027 views)
Failed for NOx Sign In

My NJ based 92 Plym Acclaim w/2.5L 4 cyl failed insp for NOx emissions: 1st time 1529 on 970 max std. Then replaced cat converter as muffler shop said it was bad. NOx went down to 1469 still failing. I do not know where to look next. Any ideas?

Tom Greenleaf
Ultimate Carjunky / Moderator

Mar 19, 2007, 11:49 AM

Post #2 of 3 (2982 views)
Re: Failed for NOx [In reply to] Sign In

Well the converter helped but not much.

How old are common things like air filter, plugs and wires? That and even small vacuum leaks can throw things off but usually hit HCs and CO also when bad enough.

Are you a diyer for some of this stuff? T
Long retired now

Veteran / Moderator

Mar 20, 2007, 12:04 AM

Post #3 of 3 (2977 views)
Re: Failed for NOx [In reply to] Sign In

Does this engine have an EGR (exhaust gas recirculation) valve? Back in 92 some had some didn’t but this being a 2.5 I think it might.
If it does that’s the first thing I would make sure was functioning properly. The EGR’s purpose is to reduce NOX by introducing exhaust gases back into the engine at the proper time and amount; which will cool the combustion chamber.
Heat and load are the main contributors in the production of NOX; even though exhaust gases are hot, they are nothing compared to combustion temperatures.
Plus in theory all of the combustibles have already been burnt in this exhaust gas; it only takes up space in the combustion chamber so it doesn’t add to the combustion process or hinder combustion when added in the proper amount.
Again if this engine has one you’ll find it bolted to the intake manifold, more so to the passenger side of the throttle body injector unit.
It’ll look like the one pictured below; yours may not have an electrical connector on the backpressure transducer. The transducer is the round black plastic object with the vacuum lines.

To test it, allow the engine to reach operating temperature. With the engine idling, crack the throttle quickly from idle to approximately 2000 RPM. While you’re doing this watch the small shaft in the center of the valve; you should see it flutter up and down during the rev. This shaft is not pictured but it's between the round metal top and the mounting base.
If you cannot see it fluttering, check the vacuum supply from the engine to the transducer. There are more tests to go along with this but check this and we’ll go from there. Most times it's either the valve or transducer that is at fault as long as there is vacuum supply.
If you do end up changing the EGR assembly, do yourself a favour get the transducer as well. I believe if you get the replacement valve from the dealer, you’ll get the transducer with it.

Canadian "EH"

Failed for NOx

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