Just got thebad news on the e-test. HC and CO well within limits but NOX approx 30% over limit. I was 1213ppm limit is 0805 ppm at2000 rpm. I'm thinking vac leak and/or temp was too low and possibly stuck thermostat. I'm up in Toronto, Ontario and it was approx 45F andI don't think engine was up to temp during the test. All in all she runs great and I have no other problems. Anybody have suggestions. Only about 190,000miles on it, full tune-up done and I use Castrol high mileage oil.

Oxcides of Nitrogen are produced by high cyl temps, not to be mistaken with high engine temp.

High Cyle temp is created by several factors.
Ping or knock from overadvanced timing, or a knock sensor signal or Bad EGR valve.

Lean A/F mixture, like if you leaned the mixture down to pass the CO portion of the test.
Or a clogged fuel filter forcing a lean mix.

High carbon deposits on valves and pistons.
This will increase compression ratio/cyl temp and even if everything else is perfect IE: timing, mixture, egr, ect...will cause high NOx.
One cure is having a motorvac service at a quality shop.
($150-$300) depending on the shop.
Also...the way I personally recomend...
Only one fuel additive is proven to reduce carbon depostis
"BG44K" at about $20 per can, the price alone indicates you don't want to get this stuff on your clothes or hands.
It does not harm your engine and works in one or two tank fulls. It is a good product to use once or twice a year.
It is found at NAPA or Car Quest Stores.

SD

Never thought fuel filter would/can be the problem.I would have changed it but my parts store was out of stock atthe time. Always figured plugged filter would starve fuel and cause a serious miss/cut out situation.AsI said in my original post she runs great otherwise. Will change filter right away. What about using colder plugs.Iwas told different plugs will increase/decrease temp as well. I understand the difference between coolant temp and internal combustion chamber temp.

A new O2 sensor and cat got my 94 through smog check after high NOX readings.....

Couldn't get the business truck to pass with high NOX readings...even with a brand new cat and O2 sensor....put 4 bottles of some chemical in there...will have to get back when I remember what it was called...but filled the tank, put the 4 bottles in...ran the tank down to about a 1/4 tank...and it got the trcuk to pass...took 4 tries to pass, and that stuff worked...it was getting tiring going to the station over and over. <img src="/forums/images/graemlins/pfft.gif" alt="" />

Changing plug heat range has no effect on changing engine or cyl temp. Plug heat ranges are designed for matching engine and induction types to a given plug.
A hotter plug will glaze in a leaner engine a cooler will carbon foul.

BG44k is the best bet if everything else tests OK.

O2 sensor can have an effect on NOx, but most times it would have to be so bad it would effect other areas, like very poor MPG.

NOx is the most missdiagnosed etest failures.

The only way it is measured is under load, making it tough to duplicate in a shop.

SD

What about running premium gas for the test.Would this help bring down the NOX ?????

If you failed by VERY tiny amount it might make enough change to get a point or two.

Have you found BG 44K yet? That stuff really works.

SD