Took my '94 XJ to echeck today. It failed on the NOx levels. 1014.0 was the limit and it came in at 1055.8. Anyone know what I need to fix to get it to pass?

 

Found this, might help:

http://answers.yahoo.com/question/in...1125843AAH2rgS

 

Thanks Jason! I'll probably give that try. The thing is running good, it's stupid that I have to do anything to it.

 

You're welcome. Let us know how it works out.

 

Elevated NOx is a product of elevated combustion temperatures. It could also be a sign of a failed/failing catalytic converter.

First thing - get some Seafoam and a catalytic converter flange gasket. Disconnect the converter at the flange, and tie out of the way (the downpipe should be open.) Run the Seafoam through the engine - that should clear out any accumulated carbon deposits, which can (and usually do) elevate compression (and therefore combustion temperatures.) Carbon deposts can also start 'glowing' after a while, which can lead to preignition (and other nasty problems.)

If there's a lot of carbon, that's why you disconnected the catalytic conveter. Shine a light into the converter (to look for a carbon-coated matrix) before you connect it back up - the matrix may have become coated with carbon and that will degrade the efficiency of the thing - and it is sixteen years old (if it's still original.) Retain the flange when you change the converter - cut it off of the old one and have a stub of pipe welded to it. You'd rather have that flange in place, believe me - especially if you have a five-speed!

You've dealt with the carbon now, and you've inspected the cat (and replaced it if necessary.) The only other high-NOx issues I can think of at the moment would be the spark plugs (mentioned - if they're "too hot," they'll retain too much heat and cause trouble,) and the base timing (which should not be adjustable, but can be checked with a timing light. Make sure it isn't wandering, and I am inclined to think it should be 4-6*BTDC (but don't hold my feet to the fire - I'm not sure.))

But, the carbon deposits and the plugs will give you a good place to start. Good luck, and report back if you didn't pass again (with any new symptoms and a history of repairs you've done and noted oddities.)

Make sure your spark plugs are the correct ones for your engine, and aren't "too hot." The retained heat from a hotter plug can elevate chamber temperatures - while this will reduce HC and CO, it will increase NOx.

 

...just saw this 5-90! Thanks, I'll give it a try. I found that as well-that the timing is not adjustable.

 

I replaced the cat, and voila! It passed!

 

I just found this thread today, that's what I was going to tell you. Glad you got it solved

 

Yeah, you told me over here! That's how I knew what to fix! LOL!

 

LOL Thats so funny.....I guess I should pay more attention.

 

Good info there 5-90, thanks for posting it for everyone that doesn't know.
Dan