High NOx at 25mph

Lurch

Lurch

NAXJA Forum User
Location
Long Beach, CA
Took my '92 MJ 4.0L to get smogged, passed everything except NOx at 25mph. Below is a comparison of the previous smog (top) and the current failing smog (bottom).



Truck has almost 350,000 on the clock, not sure the history of the motor but theres no evidence of a swap or rebuild. The previous owner put a new cat on ~2 years ago. Since the last smog, the only engine related work I've done is swap out the existing 180* thermostat for a 195* unit.

I'm going to go ahead a put a new fuel filter in, the current one is of unknown age. What else can I do to get this to pass?

The plugs:



Thanks.
 
From the look of those plugs, new cap/rotor, plugs and wires at the very least should be on the list. Maybe run some seafoam or acetone in your fuel tank. IIRC, high Nox typically means that it is running lean. Maybe test/replace the O2 sensor as well.
 
Johnnie Walker said:
From the look of those plugs, new cap/rotor, plugs and wires at the very least should be on the list. Maybe run some seafoam or acetone in your fuel tank. IIRC, high Nox typically means that it is running lean. Maybe test/replace the O2 sensor as well.
Click to expand...

I thought a bad O2 sensor made it run rich?

This engine is getting swapped for my '98, I just wanted to smog it before the transplant since it's due in April and I'm not sure when I'll have it running again.

I pulled an airhead move and drove it straight to the smog shop after the truck had been sitting in my garage for a few weeks without warming everything up good. Probably didn't help anything.
 
Running lean would mean your CO and HC emissions would be lower, if anything, they went up.

Does your engine have an EGR? Some engine have them, some do NOT, but the primary function of the EGR is too reduce NOx emissions, its clogged up or non-functioning would result in increased NOx.

A tune up can't hurt, and poor ignition can create additional emissions on all fronts. Remember the suggestion of new ignition cables, the ignition/spark plug cables are long flexible resisters (so they don't create radio noise), the conductor is graphite impregnated latex, it wears out with age and use, even if the insulation looks good, so fresh ignition wires can be like fresh plugs as well. Just like the cap and rotor.
 
try colder plugs, take it in cold and find all of your exhaust leaks from the manifold back and weld them up. Replace o2 sensor as well.
 
Mine was caused by a crack in the manifold, i couldn't hear it but the guy next to the smog place told me he could hear it. Yep there was one, getting old makes things harder to hear i guess. I got a new one from Rock auto i think.
HTH

This was a 1994
4.0
 
Another note, from experience.. EVAP test mostly fails if fuel tank is full, or at/below 1/4 tank. But, definitely do tune-up and some kind of fuel cleaner. And then drive the crap out of it before you take it in.
 
Johnnie Walker said:
Another note, from experience.. EVAP test mostly fails if fuel tank is full, or at/below 1/4 tank. But, definitely do tune-up and some kind of fuel cleaner. And then drive the crap out of it before you take it in.
Click to expand...


I've failed the EVAP test, drove to the gas station, filled it up all the way, and took it back and it passed.

My theory- when its low on gas it takes forever to fill up with that pressurized gas, but when the tank is full, it fills faster, and it only has to hold for so long.

I always take my rig in full now and it always passes evap.

failed evap twice with very low tank.
 
A cracked exhaust manifold would suck air into the manifold, cause the O2 sensor to conclude the A/F mixture is lean and the engine would run rich. Rich doesn't cause extra NOx, it causes extra HC and CO. The exhaust in the manifold pulses, it is NOT at a constant overpressure, so exhaust leaks out of the crack and sucks air in through the crack as well.

eHall had a good suggestion, and the age of XJ's that are equipped with a lot of vacuum lines, a cracked or dry rotted vacuum line could create a vacuum leak and could cause this.

But keep in mind, the EFI works off a switching O2 sensor, meaning it constantly going a little lean/rich trying to keep an average. So its NOT impossible for any malfunction to cause the opposite effect of what you think it should. i.e. even though I just argued a cracked exhaust manifold should NOT cause extra NOX, I would NOT be surprised in the least if someone proved me wrong.

And that is sorta of the point, the approach should be to totally inspect the motor and fix anything wrong with it so that it operates in tip top shape, slight malfunctions, leaks in intake/exhaust, etc will effect A/F ratio and resulting pollution. The fact the motor needed a tune-up prior to the inspection could purely be the cause of failing the test.

The EVAP, the charcoal canister stores the gasoline fumes from the tank. The fumes cling to the charcoal and condense and don't build up pressure, without the charcoal canister the fumes would build up pressure in the tank and have to be vented, which that is the pollution the system is trying to prevent. Every once in a while a solenoid valve opens to let the motor suck the fumes out of the charcoal canister. Why would it fail between the tank being full or empty? I guess it depends on what they are checking, and I don't know for sure, but a full tank has less volume for the fumes to collect and maybe that makes it easier for the charcoal canister to collect all the fumes and hold them. While an empty tank has lots of volume for fumes to linger and NOT get cleared out by an old charcoal canister past its prime. Maybe they check for pressure and its just harder to hold down the pressure on an empty tank by a charcoal canister than it is for a full tank.
 
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Swapped plugs, changed the fuel filter and added some fuel system cleaner. As soon as I did this, the check engine light came on. Had a buddy pull the code, 51, which means its running lean. I'm assuming that was the cause of the high NOx to begin with, but the CEL never came on before. Not sure how doing any of those three things would have triggered that.

Have a new O2 sensor on order and going to test the fuel pressure this week too. Hopefully I can get to the bottom of the lean condition.
 
exhaust leaks can make it seem like its running lean. Replace the manifold gasket perhaps and look for vacuum leaks? and hook up a shop vac on reverse to the tail pipe and you will find all of the leaks very easily...
 
Exhaust leaks upstream of the O2 sensor or near the O2 sensor will get oxygen into the exhaust and run over the sensor and make it sense lean. The only way that changes what blondejoncherokee said, is you find exhaust leaks after the CAT, I doubt that is what is causing it to set a fault code for running lean.

Retorquing the intake/exhaust manifold bolts can't hurt. I was shocked how loose the bolts were on my 2.5L I4, I discovered it when I found the intake manifold could wiggle up and down a very noticeable amount, I was surprised the motor even ran with that much open space between the intake manifold and head.

How are the Distributor Cap and Rotor? The ignition cables? Did you check the resistance of each Ignition Cable? If its been more than 60k miles for any of those items, I would replace them. Ignition cables, at least check how many ohms each cable has and look up what is the permissible ohm figure.

Check for vacuum leaks, especially the line from the manifold to the MAP sensor.

And finally, the EFI system is adaptive, it stores corrective values that it learns from the feedback loop in the closed loop mode. It takes some time of operating the motor for it to learn the new corrective values and overwrite the old ones. So, if your problem was just the plugs, its possible the corrective values learned from operating with bad plugs, now make the motor run lean with the new plugs, and it will take a little while for the EFI to relearn the new corrective values and get the motor back to running normal. If all you've done it start the motor and run it a few minutes, that is NOT enough. You have to get the motor warmed up and drive in various conditions, i.e. stop and go, highway, accelerating at various power levels (light, moderate, hard), passing maneuvers and idling for 5 minutes or more when the motor is at full operating temp. Google "Drive cycle".
 
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I passed! New O2 sensor, fuel filter, plugs and injector cleaner. NOx was at zero at 15mph and 106 at 25mph. Average is 246.

Thanks for everyone's advice!
 
Use NTK, do not waste time with new Bosch crap.
 
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