1993 Jeep Emissions


Old 08-23-03, 09:10 AM
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Question 1993 Jeep Emissions

My son's 1993 Jeep Cherokee failed the NOX portion of the emissions test. All other values are well within spec. Jeep has the in-line 6 cyl. eng., manual trans, A/C and 168,000 miles. After reviewing this forum I thought it'd be an easy diagnosis of the EGR valve. BUT, I have a different emissions system. I did the "self diagnosis" described in my Haynes Manual and only got a 12 code for the battery being disconnected (it was replaced about a month ago). Where is the best place to start checking now? Oh, emission test values were:
HC 44 (max 159) 62 (max 163)
CO 0.14 (max 1.78) 0.17 (max 1.38)
CO2 14.6 (no spec) 14.7
O2 0.1 (no spec) 0.1
NOX 1711 (max 1014) 1678 (max 1110)
Dilution 14.8% (min 4%) 14.8% (min 6%)
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Old 08-23-03, 03:34 PM
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If the vehicle has an EGR valve, start there.

If not, how is the cooling system condition? High NOx can be traced to a cooling system not up to par (fan not coming on, etc).
Old 08-23-03, 04:08 PM
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NOx is from high combustion temps and pressures. EGR and cooling system are usually normally whats wrong like Joe said, but I had a few that were failing becouse excessive cylinder carbon. A top engine cleaner seems to work good for cleaning these deposites if done correctly. Also could be the cat.
Old 08-23-03, 09:49 PM
mike from nj
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those are some pretty clean readings, except NOx of course, i would say the other readings are almost perfect for something this old.

like previously stated, (if it has one) the egr valve must be working both ways---the valve must be moving with vacuum applied(by revving the engine) and the passage has to be completely clear to let the egr gasses flow. when pulling up the diaphram it idle(with your hand), the engine should stumble or stall---that means the passage is clear.

if no egr valve, a good combustion chamber cleaning should help a little, follow the directions on the can closely, i usually let it soak longer than the instructions say though---you should see the amount of junk coming out the tailpipe.

one good trick is to set the timing way back, if the spec is 12 btdc, i'll set it back to 4 or even 2 to get it through, power goes way down, but all you're trying to do is pass the test.

make sure the thermostat is opening properly, you might want to try a cooler one (160 or 180), you won't have heat in the winter with a 160 though, do not run without a thermostat. make sure the fan is working, whether it's electric or a viscous clutch. also make sure the air coming off the rad fan is very hot, if it feels cool on one side and slightly warm on the other(with thermostat fully open and coolant flowing) the radiator might have lost it's ability to transfer heat out, look for missing fins or green or brown fins at the bottom of the radiator, this is a common problem on 'all' cars and trucks with high mileage on the clock.

i think if the cat was bad, all the reading would have failed, especially HC and CO, both of which were very low. i could be wrong though

try to get the vehicle inspected on the coolest day practical, and make sure the a/c is off, a cool engine is your goal.

let us know how it turns out
Old 09-08-03, 10:12 PM
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I found the O2 sensor heat element was bad so replaced it. I also tuned it up and replaced the drive belt and ran some fuel additive through it. AFter driving the dog out of it to help blow out the carbon I had it retested with the following results:

HC 42 (max 159) 40 (max 163)
CO 0.17 (max 1.78) 0.2 (max 1.38)
CO2 14.7 (no spec) 14.7
O2 0 (no spec) 0
NOX 1074 (max 1014) 1235 (max 1110)
Dilution 14.9% (min 4%) 14.9% (min 6%)

STILL TOO HIGH ON THE NOX!!!!! arrrggghhh!!!!!

The guy at the shop said he was sure that if I changed the catalytic converter it would pass. I was skeptical but with the bad O2 senser and high mileage I decided to give it a try. I should have been more skeptical! I had another retest today with the following results:

HC 15 (max 159) 13 (max 163)
CO 0.01 (max 1.78) 0.01 (max 1.38)
CO2 14.8 (no spec) 14.8
O2 0.1 (no spec) 0.1
NOX 1137 (max 1014) 1339 (max 1110)
Dilution 14.8% (min 4%) 14.8% (min 6%)

Tonight I retested the coolant sensor, MAP sensor, and manifold temp sensor. They all seem fine. I've also looked at the cooling system: coolant is not yukky, viscous fan seems toreally kick in about 210 deg., auxillary electric fan kicks in with A/C on and at about 215 deg without A/C on. I the parts house tonite to see what the standard thermostat was. IT's 195 deg. I plan to change the thermostat tomorrow. Any other ideas????????
Old 09-09-03, 06:27 AM
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Because the O2 sensor typically has NOTHING to do with NOx.

Sure, the converter could be bad, but you are likely attacking the symptom and not the actual problem.

If the converter is truly bad, it can be verified with a good temperature probe at a shop.

I believe this vehicle ALSO has an electric fan. Is it working correctly?
Old 09-09-03, 07:41 PM
mike from nj
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if the oxygen sensor was bad, the car would have failed miserably in HC and CO, it didn't, ever. it would also affect nox to a certain degree

if the cat was completely bad, it would have failed all three readings, it didn't. cats fail gradually, their efficiency gradually tapers off from 100% down to 0%---a dead cat.

the sensor 'heater' was bad, meaning it could take a while for it to reach operating temperature, but once it reaches operating temperature---it works.

you have a NOX problem, which is directly related to combustion chamber temperature.

you have to lower the combustion chamber temperature for the nox to come down.

a cooler thermostat is a good start, i can't remember if timing is adjustable, if it is, that will fix your problem(backing it off) if not, ignore that sentance.

combustion chamber cleaner worked for me a few times, sometimes it didn't(not fuel injection cleaner, not stuff you pour in to the tank) stuff you spray down the throttle body while it's running, any chrysler/dodge/jeep dealer in america stocks this stuff and it's usually under $10. follow the directions closely on the can and watch the smoke come out of the tailpipe(carbon)

let us know if this helps
Old 09-10-03, 06:41 AM
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Better known as "Top Engine Cleaner" by the folks at GM, but it's probably all the same stuff marketed by a couple of companies and sold to the OEM .

I would agree with Mike, but I wouldn't mess with the lower thermostat temperature. I think that will foul the works more and leave you with lackluster heat in the winter time. Find the root cause the problem and then go from there.

Of course, change your oil and filter after all of this "fogging" is done. Whew, the smoke will be unreal!!
Old 10-04-03, 08:50 AM
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Thumbs up Success

Here's what I did to get it to pass:
1. Replaced the 195 deg thermostat with a 185 deg. Not a heater problem for us as we live in Texas.
2. Flushed the radiator.
3. Realized there was no fan shroud on the viscous fan so installed one.
4. Heard a rumor that changing the oil can sometimes reduce NOX. I only had 2,500 miles on the oil but it'd been really hot so changed it.
5. The weather finally cooled down and we were out of the 100+ deg days when I had the last test. It was 90 deg when I had the last emission test that past.

NOX went down to around 300. It's my opinion that the oil change made the biggest difference. I guess, with the heat, the oil had degraded as after the oil change the engine ran significantly cooler. I couldn't get it over 200 deg. where before it'd get up to 210. Obviously the lowere ambient temperature help also.

Thanks for your help and I hope this helps someone else.
Old 10-04-03, 10:27 AM
mike from nj
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good news, glad to hear it's finally fixed!

usually when oil wears out, it causes the HC to go up, usually not the nox. i think the new fan shroud and the cooler thermostat were your biggest contributors to the lower readings, just MY opinion though. unless the oil was very old or very low, then it might bring the temps down a little.
Old 10-04-03, 10:49 AM
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I just got my Cavalier inspected last weekend and it "failed" the first time.

Second try rerun, flew right through. I had to wait an hour or so as my friend had someone in front of me. Noticed the NOx on mine was a little high, but noticed that he doesn't have a fan in the shop and the test rig the state wants has a fan as part of the setup. I'll have to talk to him about that.

Yes, a missing fan shroud will cause a problem. Glad that you got it going.
Old 10-04-03, 10:54 AM
mike from nj
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nj requires us to use a fan on anything over 70 degrees, i always use it because it does make a difference at any ambient temp.
Old 10-05-03, 08:35 AM
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I think NYS does too, I'll ask my friend about it .

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