failed smog high NOx

Just failed a smog test, Passed the last one about 3000 miles ago. The Jeep doesn't see much street duty. The motor is stock.

Since the last smog check:

Recent oil change
New 02 sensor
New TPS sensor (set closed position voltage at 0.85V)
New Cat
New Dynomax exhaust
4.89 gears

Seems like a lot of changes but not really.

I've checked functionality of the EGR and it is operating when I bump the throttle. My concern is the fact that since it's a positive backpressure EGR and I'm wondering if the combination of the free-er flowing exhaust and the low gears are affecting the backpressure. Gears? When he was on the rollers the car did get into overdrive so the RPMs were pretty low on the tests. This is moving less air through the motor so the backpressure is less. The EGR will not function correctly w/out backpressure in the exhaust. Low enough backpressure for the EGR to not function? I don't know. I wouldn't think so. My first thought is to retest and have them leave it in 1-2 so it doesn't go through the gears to keep the RPMs higher to help the EGR function. I've searched high and low for a vacuum leak and can't find anything.

I did notice that the measured oxygen levels were higher than they were the last time I smogged it and that is consistent with higher combustion chamber temps which lead to higher NOx. But I'm not getting any "lean exhaust" trouble code so it's probably within an acceptable range. I pulled off the EGR and cleaned it and checked it for functionality and it seems fine. Like I said earlier I did notice it moving when racing the engine. I wish there was an EGR position sensor on it so I could verify that it was open during teseting.

High NOx is from high combustion chamber temps. This is caused by vacuum leaks (lean mixtures) and/or an EGR that isn't functioning. I can't find a problem with either so I'm at a loss. Fuel pressure checked out fine by the way. Maybe it is just the combination of the gears/exhaust and I need to have them test in 1-2 on the transmission? Any other CA jeepers run into this? I'm sure aftermarket exhaust and low gears isn't unique on this forum. Help!!!! chasing my tail right now.:repair:

didn't mention, it's a 1987 and it doesn't have the vacuum transducer like 88 and up RENIX

Warm the engine to normal operating temperature.


Operate engine at idle speed.


Check vacuum at solenoid vacuum source.


Disconnect the hose and attach a vacuum gauge to it.


Vacuum should be at 15 inches.


If vacuum is low, check the line for kinks, twists, or a loose connection at vacuum connector on intake manifold.


If vacuum is ok, remove gauge, reconnect the line and proceed to next step.


Check vacuum at solenoid output port.


Disconnect the line and attach a vacuum gauge to output port. Vacuum reading should be 0 at this side of solenoid.


If vacuum reading is 0, leave the gauge connected and proceed to the next step. However, if vacuum is present check solenoid/ECU operation with the DRB-ii tester.


Disconnect electrical connector at solenoid and note vacuum at output port. Vacuum should now be present at output port.


If vacuum is present, proceed to EGR valve test. However, if vacuum is not present, replace the solenoid. Test the EGR valve as follows:


Leave solenoid electrical connector disconnected. Note the engines idle speed.


The engine should idle roughly or stall. If this occurs the valve is ok. If the idle does not change, proceed to next step.


Disconnect hose from the EGR valve and connect a hand vacuum pump to EGR nipple.


Apply a minimum of 12" of vacuum to the valve and note engine idle. If engine now idles rough inspect the vacuum line between the EGR valve and the solenoid.


If the idle did not change, remove the EGR valve and inspect the valve and the exhaust passage in the manifold for blockage, repair as necessary. If no blockage is present replace the EGR valve.


Vacuum Transducer Testing as follows:


Disconnect the transducer vacuum lines and back pressure line. Remove transducer.


Plug transducer output port. Apply 1-2 PSI air pressure to transducer back pressure port. Air pressure can be supplied with a hand operated air pump or compressed air (regulated to correct pressure). Apply a minimum of 12” of vacuum to the input port.


Replace transducer if it will not hold vacuum.

Just a quick note on positive backpressure EGR valves. They are designed to only open with backpressure in the exhaust (the engine under load). They will not open at idle. The system is designed that way. There is a port in the diaphragm that is closed with exhaust backpressure. When the port is opened all the vacuum will bleed off. The backpressure feeds up through the rod that connects from the valve to the diaghragm (the rod is hollow).

That being said the first thing I did was trouble shoot the EGR solenoid and verified that it worked. I applied direct vacuum off of the tree to teh EGR valve and it didn't move. It DID move when I raced the engine. It moved even more when I raced the engine with a shop rag stuffed in the exhuast pipe. But no matter what it was fully closed with 17" of vacuum pulling on it at idle.

I would have thought that putting a shop rag into the exhaust would create quite a bit of backpressure. It could be that it's working but not that well? I pulled it off yesterday and blew 5psi of shop air into the inlet and put a vacuum pump on the EGR valve. it held vacuum just fine and opened right up with no problems. I really don't think it's bad, but I have no way of quantifying how much it's opening. It could be that a new one opens up twice as much and it would fix my problem, but right now that just seems like throwing money at it without any sort of smoking gun from the diagnostics. I do know that cars with positive backpressure EGR valves throw EGR codes like crazy when you install headers remove the cats put high flow exhaust etc. but I don't think a Dynomax cat back is lowering the backpressure to the point where it won't work. I wish it were just computer controlled so there wouldn't be a question.

Use your vacuum pump, open the EGR, measure the distance.

Go to the friendliest parts store in town, ask them to let you test and measure a replacement EGR valve. If the travel is different, buy the new valve.

They won't hold vacuum. Must have a air source going into the seat to simulate backpressure. Still doable, but nobody in town stocks them. Found it online though. hard to imagine that this part is that rare (it's just a standard looking GM EGR valve). Just in case anybody is curious and they have a non transducer 87 and everyone is trying to sell them an EGR with the transducer, the Mopar PN is 53003446 and the Napa PN is 26417. it says Comanche but verified the Mopar PN with my VIN at the dealer today. I'm going to order it eventually but since I haven't verified that mine is bad I don't expect it to fix anything. Maybe I'll get lucky. I hate throwing parts at problems without a positive diagnosis.

Does anybody know how much vacuum this valve should hold with the bleed off valve fully closed by backpressure? Last night when I was blowing shop air in it it wouldn't hold over 5", but it did hold it. This could be totally normal for this valve though.

Also check the following.....

Timing advance-Too much advance creates high cyl temps and causes nox to be formed. Check for proper timing specs on your white fed sticker under the hood.

Catalytic converter-I had my 93 fail on NOX and it had an aftermarket cat. Got a new one and it ran the I.M. 240 test for 30 seconds (meaning it was running very clean) and it literly blew zeros on all readings. I think the aftermarket one was a reducing cat (two way) and you need an oxydizing/reducing cat(three way) with the paladium and rodium to help break up NOX.

EGR port not clogged.- I have seen this on older cars where the port going to the intake or from the exhaust crossover is just crammed full of crud. If possible start the engine and reach into the EGR and manualy open the diaphragn with your fingers. It should kill the engine at idle or at the least make it run rough. If not you have a port clogging issue.

If you can please post your readings from your test or maybe if you got the graph (you have to ask for that at the smog cafe when you fail)that will help as well.
I used to be a fleet inspector for the state of AZ for five years so I still have a ton of info and notes. PM me if you have any questions.

tjhawk said:

Also check the following.....

Timing advance-Too much advance creates high cyl temps and causes nox to be formed. Check for proper timing specs on your white fed sticker under the hood.

Catalytic converter-I had my 93 fail on NOX and it had an aftermarket cat. Got a new one and it ran the I.M. 240 test for 30 seconds (meaning it was running very clean) and it literly blew zeros on all readings. I think the aftermarket one was a reducing cat (two way) and you need an oxydizing/reducing cat(three way) with the paladium and rodium to help break up NOX.

EGR port not clogged.- I have seen this on older cars where the port going to the intake or from the exhaust crossover is just crammed full of crud. If possible start the engine and reach into the EGR and manualy open the diaphragn with your fingers. It should kill the engine at idle or at the least make it run rough. If not you have a port clogging issue.

If you can please post your readings from your test or maybe if you got the graph (you have to ask for that at the smog cafe when you fail)that will help as well.
I used to be a fleet inspector for the state of AZ for five years so I still have a ton of info and notes. PM me if you have any questions.

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Not really adjustable, controlled by the ECU--however (don't you just love it) Hesco has a modified CPS/CKP crank sensor so you can adjust the air gap, why not elongate the holes side to side for some adjustment, sort of a reverse "high altitude" crank sensor?

Oh yeah....I was thinking of the 4.2 liter but then again they were also carburated and only in CJs and WJ's.

this is the dumbest question ever, but how do you set the base timing on the 4.0 renix system? I'm familiar with the GM and Ford systems where you unplug the spout or another lead to disable the computer control for base timing settings but I can't find anything about this in my lame Chilton. I know the computer makes adjustments assuming a particular base setting, and if your base if off it will screw things up. I poked around with a timing light the other day and couldn't even find a tab on teh crankshaft pulley/balancer. That is the one thing that concerns me, I've never verified the base timing setting. What is the base setting for the Renix system, where is the tiiming tab, and where is the 'spout' or whatever that I unplug for base settings? I'll post the smog results later when I get a chance. I have my old one from about 3k miles ago also. One interesting thing is that my percent O2 increased from 0.5 to 1.0/1.2. higher O2 reading is definitely consistent with a high NOx reading. I can't find a vacuum leak, fuel pressure is perfect so I'm not sure where that is coming from or if I should be concerned or not. If it was significant I would think that I would get a lean exhaust trouble code.

BTW the cat is a three way. I verified that with Magnaflow the other day. I purchased it before CA changed so it's now officially a 49 state cat, but that shouldn't keep it from passing the sniffer. It meets or exceeds the original equipment.

THE TIMING IS COMPLETELY SET BY THE ECU BASED ON INPUT FROM THE CRANK SENSOR.

Even if you were to pull the distributor and move it a tooth or two forward or backward THAT WOULD HAVE NO EFFECT ON IGNITION TIMING.

As posted earlier, there was an "off-set" crank sensor available for the very early 4.0s for high-altitude applications. I imagine you could modify the crank sensor in that fashion--off setting it left or right to get the same results.

2008
mileage 220043

15 mph 1656RPM 14.38%CO2 0.57%O2 78ppmHC 0.04%CO 831PPM_NO
25 mph 1582RPM 14.60%CO2 0.23%O2 38ppmHC 0.01%CO 551PPM_NO

2010
mileage 223142
15 mph 1759RPM 13.80%CO2 1.20%O2 17ppmHC 0.00%CO 1500PPM_NO
25 mph 1385RPM 13.90%CO2 1.00%O2 18ppmHC 0.00%CO 1684PPM_NO

but it should be noted that this is at sea level as opposed to ~4000 feet where the last smog test occured. Also the shop equipment is probably calibrated slightly different. The lower CO2 tells me that I'm burning less fuel and the higher O2 says that it's leaner... not sure why.

okay thanks for the note about the timing. Is there any way to check it to verify the timing? Maybe the crank sensor is loose. If it's working fine then I'm not inclined to modify anything. It should pass with the stock equipment working since it's a bone stock engine.

btw manually opening the EGR will make the motor run very rough, I don't think there's a significant blockage in the passage.

Sure, just throw a timing light on it, you can observe the marks, you just can't change anything.

where is the timing tab?

jeddins said:

where is the timing tab?

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I believe its on your timing cover right below your water pump on the drivers side. Its actially just a few maybe five bumps. You may have to degrease it andlook at the numbers with a light first. Mark it with white out on the cover and the crank pulley. There is a small groove on the crank pulley mark that too.

I also think you are running lean by the way the numbers look. Are you sure your Oxy sensor is good? Is it switching from rich to lean(checked with a dvom) after it is warmed up and you rev the engine? Pull it off and make sure its not partly obstructed by a piece of carbon or something. Also I don't know too much about the early 4.0 but my 1993 needs a special 50 dollar O2 that is made from cubit titania and it you just got a bosch generic you may have the wrong sensor. Also a scanner is helpful to read info about the engine but I think you need a DRB scanner and those are pretty ancient. I guess I am out of ideas for the time being.....I will look for some more info tomorrow.

I did replace the O2 about a year ago. Broke the old one off by accident pulling off the exhaust. I'll have to check to make sure it's the right one. I usually try to get AC delco or molar replacements but since this wasn't planned I may have run to auto zone for it. I'm pretty sure it's a three wire heated sensor though. I'll scope it to make sure it's working right.

http://books.google.com/books?id=QuQDAAAAMBAJ&lpg=PA149&ots=6nDgaJpyPO&dq=troubleshooting%20positive%20back%20pressure%20EGR%20valve&pg=PA149#v=onepage&q=&f=false

Just found this article. Pretty good.

Original EGR had an intermittent leak. I tested it by blocking off the outlet (to the intake side) and blowing into the inlet. This closes the valve inside that allows the vacuum to move the diaphragm. I could pull 15" but it would slowly bleed off. I sprayed some carb cleaner up the hole and sometimes it would hold vacuum. The internal valve must just be dirty. Now I see why they went away from the all in one backpressure EGR and accomplished the same system intent with the transducer. Probably less failure prone because it doesn't get as much carbon and other exhaust particles in the valve.

Nox went down to 36 at 15 and 158 at 25mph!!! Ave values were 406/349.

Great. Yeah there is nothing worse than chasing a partly broken part that is only working partime.