failed emissions

[ol_ben]

My 93 xlt failed emissions for the first time since new.

Emissions testing is performed on a dyno at 15 and 25 mph.

All tests at 15 mph passed, at 25 mph NOx was out - almost double.

At 15 mph the NO was about 20% less than the max target.

As I understand excessive combustion tempertaure is the major contributor to NO production.

The Expl has a little over 200k miles. The spark plugs have not been changed in quite some time. Could worn plugs cause this problem.

would retarding the timing help?

any suggestions appreciated.

thanks,
ben

 

[Maniak]

You can't mess with timing...

Plugs/Tune up would help.. But look at the EGR circuit.. The egr is what is supposed to lower the combustion temp.. (I think 93's have EGR.. my '92 doesn't).

~Mark

 

[TheJMan]

New plugs and wires are a start, but try running a heavy dose of fuel injector cleaner on the next tankful. If there is a lot of carbon buildup (possible cause of high combustion temperature) the fuel injector cleaner may help. If funds aren't too big of an issue, some shops can professionally clean the injectors and the induction system, this worked much better on my old '93. (Seafoam is also an option, but the professional cleaning made a much bigger difference.) Also make sure the air filter is new, and that the Mass Airflow Sensor is clean. A fresh oil change can't hurt, either.

Also, not all 1993s had EGR, it was added midway through the model year.

Hope this helps.

 

[RangerX]

Here's a link from the ranger station: http://www.therangerstation.com/tech_library/CaliforniaEmissions.htm