SMOG LEVELS NOx

My wifes 94 Grand Cherokee just failed her MA smog check. It passed all areas great except NOx. Its a new car to her, and I know was well taken care of by the prior owner. I did a complete tune-up (plugs, wires, cap/rotor),K&N air filter, fuel filter, and the O2 sensor. Ran fuel injector cleaner thru prior to the check and got it good and hot. No check engine light or poor performance. What avenue should I pursue to get the NOx level down??


EDIT: I also cleaned out the TB with some Seafoam when I did the tune-up

Did you do the tune up before or after the test? If after then just go through again. If not, which plugs did you install? If you installed one of the after market performance variety (Splitfires, Platinum, etc) then you need to change back to Champ truck plugs or NGK to stock specs.

My 90 failed too and by changing the plugs it got much closer to passing. My next step was to install an adjustable MAP modification that allowed me to adjust the signal back to the ECM. I dialed it a little rich and now am getting some of the best MPG I have seen since I bought it new. The engines are known to run lean which is why you failed if your NOx was high..

I did the full tune-up before the test. It failed with Autolites and the stock O2 sensor. I threw in 6 bosch platinums and a new O2 sensor, and retested, failed again. Both failures are high NOx levels

High levels of NOx are formed because of higher than normal exhaust temperatures, which imply an overly lean condition. I'd recheck your vacuum hoses and look for exhaust leaks.

How many miles on engine? MAP sensor within tolorance? My 87 passed last year (in MA) with:
HC=.25
CO=3.9
NOX=1.47
CO2=339.37

No exhaust leaks that I can find or hear. I am going to shoot around some carb cleaner and check for vacuum leaks. I did notice something while I was just looking at the TB. The vacuum line that goes to the fuel pressure reg, was pinched over by the breather hose. At the elbow, it was pushed right down. It looks like it could have blocked it off, at least to some degree.

Now, if NOx levels are higher in a lean running engine... The engine could be running lean if the pressure reg is not getting full vacuum, CORRECT? Also, is the TPS adjustable on this year 4.0?? I believe it is not

levels were as follows:HC 00.37 limit: 2.40
CO 7.60 limit: 60.00
NOx 5.30 limit: 3.00
and 130K on engine

SPSERG said:

No exhaust leaks that I can find or hear. I am going to shoot around some carb cleaner and check for vacuum leaks. I did notice something while I was just looking at the TB. The vacuum line that goes to the fuel pressure reg, was pinched over by the breather hose. At the elbow, it was pushed right down. It looks like it could have blocked it off, at least to some degree.

Now, if NOx levels are higher in a lean running engine... The engine could be running lean if the pressure reg is not getting full vacuum, CORRECT? Also, is the TPS adjustable on this year 4.0??

levels were as follows:HC 00.37 limit: 2.40
CO 7.60 limit: 60.00
NOx 5.30 limit: 3.00
and 130K on engine

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The adjustable fuel pressure regulator works by releasing pressure in the rail and returning it back to the tank. When vacuum is applied, a diaphragm pulls a needle-like valve inside and relieves pressure proportionately. This works this way so when you step on it (low vacuum) you have more fuel pressure, and when you take your foot off of it (high vacuum) you have less pressure at the injectors. So, it would seem that you would have been running richer when cruising and decelerating.

If I am correct, only the Renix systems of 87-90 had adjustable TPSs. Your MAP sensor helps determine the amount of fuel delivered as interpreted by the ECM. I would do a thorough check of everything as you might find other issues as you did with the crimped vacuum hose.

Just for your info, I was allowed:
3.2 for HC
80.0 for CO
7.0 for NOX

Quite a difference between the years.

Also, check the values of your tempurature sensors as they relate air intake and coolant temps to the ECM for purposes of determining mixture.
I hope this helps out some. I hate seeing good Jeeps fail!

By the way, does the check engine light come on for a second when you turn the key on, so you know the lamp isn't burned out?

The check engine light does come on upon start up. I was checking hoses while it was running and got a hit when I pulled the MAP hose. I like to troubleshoot from a rational standpoint. I looked over all the paperwork from the original owner, all NOx levels were withing passing limits less than 1 year ago. The engine compartment is spotless, no tampering or half baked repairs. I really think this has to be something simple thats right in front of me.

Hmmm... Well, since yours is still OBDI, it is possible for a sensor to be out of limit and still not throw a code. I can't help you any further than that, sorry. Good luck. Hopefully, someone else can offer some suggestions. Seems unusual as you describe it in good, clean and maintained condition.

I think you are on the right thought with one sensor being slightly out of tolerance. All of my research so far, tells me these 4.0 run lean to begin with. I am leaning towards the MAP sensor. My Renix Xj runs so darn clean, funny my wife wont drive it beacuse of the noise it makes-who's laughing now

Increased NOx levels are caused by increased combustion chamber temperatures.

Noticed any detonation or preignition lately?

Failing that, I'd suspect the EGR valve - but you don't have one. (it was deleted in 1991 on the XJ, and I don't htink it was ever put in place on the ZJ/WJ.)

I'd next suspect a lean mixture, since that will cause elevated combustion temperatures. Since you said you've run injector cleaner through it, that's not the place I'd start looking - but maybe a restriction in the fuel flow might come to mind? Run the underbody to make sure that a fuel line hasn't been pinched somehow (or a dent in the hardline,) or maybe change the fuel filter if that hasn't been done recently.

Apart from that, I can't think of much - since you don't have an EGR valve, that eliminates a failure point. I think it was done with "intake valve reversion" rather than a dedicated system, there's less to fail.

Maybe I'll come up with something else - but a very slightly lean AFR comes to mind...

5-90

Thanks 5-90. Yes, I changed out the fuel filter about 1 month ago. Its in correct and in the right direction. Cleaned out the TB, and ran a full can of Seafoam down the throat. After the first fail, I ran a bottle of the pass emissions decarbonizer. Like I said, the car runs great. No hesitation, ping etc. My wife uses 89 octane fuel and loves the way the car drives.

Didnt I buy my battery cables from you??

Just out of curiosity, I was reading over at Alldata. According to the guide, this year Jeep has a 3 way Cat that reduces NOx emissions.

I know everyhting I have read indicates something else, but what are the chances its a cat thats on its way out??

It's possible. A two-way cat converts HC and CO, the three-way cat adds in the conversion of NOx.

HC + 2O2 -> H2O + CO2
2CO + 02 -> 2CO2

I don't recall the conversions for NOx, and there are several anyhow. (NO, NO2, N2O, &c.) Essentially, the catalytic converter reaches "light-off" temperature (~1000*F) and the catalyst allows for the oxidation of CO and HC, and the reduction of NOx - which cuts everything down to H2O, CO2, N2, and O2.

Hadn't thought of that, thanks for the reminder!

5-90

Well, now that you guys mention it, my cat was replaced three years ago, and I think I mentioned my EGR isn't even recieving any vacumn to engage it (I have the line capped), and my NOX are low.

But, that's all I could think of as far as running lean were the sensors.

OK, so its running lean or the cat has come to the end of its life. If the engine is indeed running lean, wouldnt the 02 sensor pick that up and compensate? Does anyone know for sure that the factory cats are 3 way?

You would think it would...but not always. Read this.

http://www.autoshop101.com/forms/h64.pdf

SPSERG said:

OK, so its running lean or the cat has come to the end of its life. If the engine is indeed running lean, wouldnt the 02 sensor pick that up and compensate? Does anyone know for sure that the factory cats are 3 way?

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On OBDII models, this should set off a check-engine light. On the Renix systems, if the O2 sensor is reading outside it's parameters, it goes into open-loop mode, regardless of whether it is a bad sensor, or the system has reached it's designed capabilities to alter the fuel mixture. I couldn't find anything pertaining to which Chrysler's Single Board Engine Controller II, which I believe you have (used in 91), would do.

Just for curiosity's sake, have you cycled you key to see if there are any stored codes (it will flash a sequence), even though the light doesn't stay on?

What the procedure for breaking out the codes? On-off of the ignition how many times? Then there is a preliminary code right? To verify the system is operating. Thanks for the help

Cycle On-Off-On-Off-On within five seconds. Should blink five times, pause, then five times (55) if nothing stored.