My 1994 4.0 failed California emissions today.

Failing numbers: NOX 15mph max=517 measured=881
NOX 25mph max=539 measured=727
Hydrocarbons were real close as well 25mph max=38 measured=36

Where should I start?

Recent tune up included:
Spark plugs (gap not checked)
Oil and filter change
91 octane last couple tanks
Warmed up everything prior to emissions test

Thanks in advance

 

Gap not checked? What brand? Cap, rotor and wires? They're due every 30,000 miles.

 

Check the gap.
lower the gas to 89-maybe 87.

 

Bosch platinum 6717 fr8

 

I haven't replaced the distr. cap or wires yet.
Any brand recommendations?
I'll go ahead and put in new plugs as well.

 

what were all the numbers? If I had to guess I would say it needs a cat and or egr.

Miles

 

Return them and get some NGK standard coppers or Champion Coppers.

 

I run the ACCEL TST16 Ignition Tune Up Kit but i did not use the plugs from that kit i ran Champion Coppers

 

Napa's premium line cap, rotor, and wires are fine.

 

Here's a pic of the numbers

 

make sure the egr and intake manifold is really clean.
if there is any gunk stuck in that area, u will get high NOX numbers.

 

Update:
Replaced
spark plugs (champion coppers)
Distributor cap, rotor, wires
O2 sensor
Coolant temp sensor

Went back for a pre-test and the NOX failed on both levels in the 1100's. it seemed to get worse.

What should I look at next? I'm thinking about getting some diagnostics done or replacing the cat.

 

Is there a provision of the law that lets you register anyway if you've spent money on trying to pass?
Could you register it at family member's house in an emissions-free county?

 

we have smog test yearly here in Vancouver as well. There are many gas additives that you can put in your tank to "fool" the smog sensor into thinking everything is fine. I've never had to use it myself but it apparently works (some even give you money back if it fails). Your cat maybe also be just borderline serviceable for your PCM not to pick it up as a fault.

Also, everything mentioned should helped but also your upstream O2 sensor has a huge role in how efficiently and cleanly your engine runs. Sometimes those sensors get "lazy" and are to slow to respond.

as a history note: Before OBDII (Renix jeeps), Cherokees had a clock that set off the engine light after 3-5 years of driving and the only way to reset it was to replace it. When this happened, several components where to be replaced on spec and the O2 sensor was one of them. Now, PCM waits for total failure before giving any indication. Not ideal

 

There is no EGR so don't waste your time looking for it. That function is handled by cam overlap.
Is the motor running hot? What do the plugs look like, overheated by any chance?
NOx is often caused by combustion temps that are too high. Running a bit lean will do it, could be O2 sensor, as mentioned. Carbon deposits, plug heat range too high, an injector or two not operating correctly..
There is a good possibility your cat is diminished. High oxygen numbers kill the cat's ability to clean up NOx, but yours are very low. It should be able to clean that up, yet isn't.