Smog Q 95 4.0 - help please

Please excuse this question if you don't need to deal with the California smog nazis.

My '95 Cherokee just failed smog. I was wondering if anyone could give me their opinion:

115K miles. Had new cat at 75K. New O2 sensor at 90K. Platinum plugs at 65K.

NOX is high at 15MPH, barely pass at 25MPH. All other standards pass.

Thank you for your input.

Try standard plugs. I had to switch to pass here in Oregon from Splitfires to Champs. It made a difference in NOx.

Spoke to the SF applications guy who said the aftermarket plugs cause the combustion temperature to rise. Thats how they get the better milage numbers - lean out the mixture.

Check EGR next. Don't know the Kalifornia test, but if it is failiing only at one data point it may only be a marginal failure in which case the plug change might do it.

Thanks for the info. No EGR on a 95, though. I will try the plugs. I'm also putting stock size tires back on, I read somewhere that can make a difference, I don't know why.

I can not imagine why oversize tires would make a diffference!
How about codes out of the ECU? you might have a bad sensor.

No codes, runs great, too!

There is no reason for tyre size to matter on a smog check (but the 8a$t4rd$ will find a way somehow...) but here are my thoughts...

1) Since there is no EGR, and you are failing on NOx, that indicates a slightly lean condition. Check MAP voltages against FSM spec to be sure - the MAP is a major player in setting fuel/air mix, and that is why an adjustable MAP can be a rewarding mod...

2) If MAP voltages are nominal (and we can assume that injector pulse width is nominal, unless you have a ScopeMeter in your pocket...) let us then go with the spark plugs. I know from experience that most platinum plugs run "hot" - I will typically get mine one range colder than catalogue specification to offset this. Therefore, AP3924 becomes AP3923, while a copper-core plug will remain 3924. The "heat range" of a plug is determined by the relative length of the ceramic nose that protrudes from the steel collar - you can tell the difference with a piece of wire if you are so inclined. The "colder" plugs will make contact between ceramic and steel closer to the electrode than the "hotter" plug.

Heat ranges in spark plugs are mainly set for purposes of self-cleaning - the plug retains enough heat to literally burn the crud off, if everything else is up to scratch. If the plug is retaining too much heat, it will have a similar effect to a lean mixture - specifically, slightly increased operating temperature and elevated NOx levels.

If you already have the adjustable MAP, this can also be mitigated rather quickly by turning it slightly to enrich your mixture. The ideal mix for the spark-combustion engine is actually slightly richer than purely stoichiometric, which will give a slight cooling effect to the combustion chamber overall (heat transfer/absorbtion in vapourising the slight extra fuel) and a reduction in NOx emissions - careful tuning will achieve this with minimal impact upon CO and HC emissions, but a five-gas exhaust analyser is still a device whose price must yet decrease to within reach of the hobbyist mechanic...

5-90

I had a coworker who had problems passing an emissions test, I think he had high NOx also.

He ended up replacing the cap/rotor/wires and that solved the problem. IN my 88 XJ owner's manual these components are to be replaced every 50,000 miles.

I also notice you have 50,000 on your spark plugs. My owners manual says change those every 30,000.

I recommend you go to your dealer pick up plugs plus a cap, rotor and wires and install them. You will be pleasantly suprised how competitive they are priced. I walked out paying less at the dealer than I'd pay at autozone for their "stuff".

Also remember DEALER parts HAVE TO work, aftermarket parts SHOULD work.

If you are running large tires and have not changed the speedo gear, it can affect emissions because the engine doesn't know how fast the wheels are going. Put on tires that are matched to your speedo gear, or match the speedo gear to the tires you run.

And by all means change the spark plugs, cap and rotor.

Re: Smog Q 95 4.0 - solved

Thanks everyone.

Inspected the car last night. The CCV hose from the intake to the top of the valve cover was split (or cut by the smog guy). Vacuum leak=lean condition=higher combustion chamber temps=more NOX.

Changed the hose (plus new plugs/cap/rotor/air filter) and it passed.

arty: