cant pass emission

ok my 90' 4.0 failed NOx on the cruise portion of the drive cycle. so i put a new dyno max cat, muffler, and tail pipe on, switched from bosh plugs to chanpions. so i went to get it inspected the other day and i was border line the first time and the second time they said i was gonna fail and the machine broke down. so when i go back monday is there anything that i can do to do guarntee a pass. my buddy told me to put some rubbing achol in the tank is this a good idea?

High NOx indicates a lean condition so I can think of two possible reasons for this; EGR valve stuck open, or clogged injectors. If you still have the old factory injectors, it's probably time to swap in a set of orange Ford 19lb/hr replacements (there's plenty for sale on e-bay).

The 90 will have an EGR valve - check that. With the engine at idle, use a hand vacuum pump to pull a vacuum on the EGR diaphragm. The engine should at least stumble - if not die outright.

If that happens, the EGR vavle is fine and the solenoid circuit and lines should be checked. If not, you probably need a new EGR valve (and it is a MAJOR pain to replace, and OEMR only!)

Most of the rest of the components are not going to greatly affect NOx - reducing NOx is (in production vehicles) a function of having some reversion or recirculation of exhaust gasses to lower combustion chamber temperatures. The later 4.0 uses reversion, eliminating the EGR valve and keeping the sensors in the intake stream from getting crapped up with exhaust crud...

The EGR vacuum circuit can be checked by starting at the valve and working back toward the source - the line goes to the solenoid on the firewall, thence to the vacuum "tree" on the top front of the intake manifold plenum.

Good luck!

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What temp is your engine running at? I'm assuming, since you talk about the drive cycle, that it's being run on a dyno-type thing. Are they putting a fan in front of your jeep to keep it cooling properly? If the engine is running too warm, then your engine may be producing more NOx than can be "neutralized" by the exhaust gases being introduced into the combustion chambers via the EGR valve.

It's just a thought, make sure that they are putting a fan in front of your XJ during the test....

I had a the same experince and learned that the plugs were the biggest contributor. Never did solve the problem with the stock configuration. When I installed an adjustable MAP, I was able to tweak the A/F to get it to pass. Funny thing is my cooling problem also subsided (duh)! With 176K on my the clock, my original injectors should be replaced.

why is the erg such a pain to replace. it looks like you need to remove the air box and washer fluid bottle at theres only a couple of bolts holding it on. i havent been out to look at it but i think that how i would do it. the bolts must be pretty baked on though. which is a good excuse to buy some more pb blaster.

I just remember spending a lot of time wrestling with the throttle linkage (until I unbolted it) and working behind the M/Cyl - and getting a lot of scrapes on the back of my hand...

One thing I did do that made it easier later - I replaced the OEM bolts (whatever they were) with Socket Head Capscrews - that way, I needed a little less clearance for wrenching (short arm Allen keys are great!) and there was less fighting with the bolts. The bolts are a standard thread - I just don't remember what...

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