Emissions...

Hey,


88 XJ Cherokee Laredo 4.0 Auto. When I had the closed radiator system which was just barely adequate to keep the 4.0 cool I always had a hell of a time trying to pass NYS Emissions Inspection.

Especially, since they place the rear wheels on a DYNO and rev the vehicle on a test track on a computer screen. Since the vehicle wasn't actually moving there wasn't any air flowing thru the radiator and engine therefore the 4.0 would redline or overheat!

All readings were usually pretty low except NOX. The latest Emissions Test I barely squeeked by with NOX at 2.49; Failing is 2.50! During this particular test the engine was borderline overheating and almost into the red 240+ degrees. I know it takes high temperatures to produce NOX so that is one of the reasons I chose to do the Closed to Open Radiator mod (not to mention that nasty pressure bottle and pain in the butt burping the air out).

Here's my question: Do you think that [now] with this mod and running much cooler (normal 210+) I will have much lower NOX readings during the Emissions Test on the DYNO? btw - When it Passed (see above paragraph) I changed these sensors (O2, MAP, TPS, CRANK), Spark Plugs, Wires, Rotor, Cap, Distributor, EGR (When Reman 4.0 put in), Used OLD Manifold (no cracks!) on new Long Block, oil change and tune-up, added a Cold Air Intake or CAI, High Flow Cat and Flowmaster Exhaust. I put Fuel Injector Cleaner in the Gas Tank and something called: "Guaranteed to Pass" - a Fuel System Cleaner.

Time permitting I will have the CAT replaced this week (with another High Flow Cat of course).

btw - The funny thing is when the Engine was changed around 5 yrs ago (133k mi now at 138k mi, didn't drive it much since was still rebuilding her), she passed Emissions with flying colors and no overheating (with the closed radiator system). Go figure.


Thx, Phil:confused:

NOX is produced by the high combustion inside the cylinders. Overheating will definetly increase nOx, as will too little egr flow and any lean cylinders. Most 4.0's I have tested had very good emissions. I'd be very suprised if you need a new cat at 5k miles.

I have seen some cheap cats that do not have nox beds.

If your other readings were fine, I'd think you likely dropped the nox emissions by fixing the cooling system.

Are your fan clucth and electric kicker fans both working properly?

NOX is produced by the high combustion inside the cylinders. Overheating will definetly increase nOx, as will too little egr flow and any lean cylinders. Most 4.0's I have tested had very good emissions. I'd be very suprised if you need a new cat at 5k miles.

I have seen some cheap cats that do not have nox beds.

If your other readings were fine, I'd think you likely dropped the nox emissions by fixing the cooling system.

Are your fan clucth and electric kicker fans both working properly?


Hey OlJeepTek,


Before the most recent Emissions Test, I turned the screw (on the ThrottleBody, the one with the knockout cover. I had to use a punch to remove the cover that was covering the screw. Did that cover mean leave it alone?) counter-clockwise all the way out. Someone said it will give me more air and better Emissions?

Question, could this also be making it lean causing high NOX?

Yes, both my electric fans are working fine. I did away with the clutch fan when I upgraded to a DUAL FLEX-A-LITE Electric Fan set-up. Much better since the engine is not spinning a five pound clutch around all the time. A little bit more torque and HP.


Thx, Phil

The torx screw under the cover is for the "Minimum air" adjustment. Its the idle speed adjustment provision. Yes, it is an air leak, but unless you have gone beyond the normal idle, the ecu will adjust for it.

I need to double check the spec, but here's the nutshell on how it goes..

Start with a warm engine, tps sensor set to .7v with the throttle closed.

1. Repair any vac leaks you can find. Especially look to see that the pcv grommet at the rear of the valve cover is in good shape, and the plastic tube is not pushed past the orifice at the bottom. This is where most of the carbon gets into the throttle bore. Also replace it if the hole is swollen shut. (This is where a lot of oil leaks get started. It also gets oil blown onto the air filter if its plugged.)

2. Clean out the throttle body bore with carb spray and a rag. Scrub the back side of the throttle plate as well.

3. You need to plug the IAC passage in the throttle bore. If you don't have the "Popsicle" tool, you can do this; remove the two screws holding in the iac valve, manually extend the center pintle so it bottoms in the bore when you reinstall it. Clean out the passages while you're in there. Note: if its very hard to move, you need a new one. Push it in gently after it bottoms and install the screws, but NOT the wires.

4. Start the engine. You may need to give it throttle initially. I believe the rpm spec should be 550-650 in drive, or (neutral for a manual). You set this with the torx screw mentioned above. It should end up about 1 turn below flush at most if everything else is correct. If you need to open it more, you need to open the throttle some with the throttle stop screw.
If you have to screw the torx screw way in, either the throttle stop is too far out, or you have a vacuum leak still. (The throttle body shaft can wear. Quick check is open the throttle slightly and see if it wiggles up and down. there shouldn't be any looseness here.)

5. Reconnect the aic 4 wire connector, shut off the engine. When you restart it after 30 seconds, the iac should have reset it self. The engine RPM should flare up and come down to base on each start.

6. Reset the tps to .7v key on, engine off by using the 2 mount screws if necessary.

The torx screw under the cover is for the "Minimum air" adjustment. Its the idle speed adjustment provision. Yes, it is an air leak, but unless you have gone beyond the normal idle, the ecu will adjust for it.

I need to double check the spec, but here's the nutshell on how it goes..

Start with a warm engine, tps sensor set to .7v with the throttle closed.

1. Repair any vac leaks you can find. Especially look to see that the pcv grommet at the rear of the valve cover is in good shape, and the plastic tube is not pushed past the orifice at the bottom. This is where most of the carbon gets into the throttle bore. Also replace it if the hole is swollen shut. (This is where a lot of oil leaks get started. It also gets oil blown onto the air filter if its plugged.)

2. Clean out the throttle body bore with carb spray and a rag. Scrub the back side of the throttle plate as well.

3. You need to plug the IAC passage in the throttle bore. If you don't have the "Popsicle" tool, you can do this; remove the two screws holding in the iac valve, manually extend the center pintle so it bottoms in the bore when you reinstall it. Clean out the passages while you're in there. Note: if its very hard to move, you need a new one. Push it in gently after it bottoms and install the screws, but NOT the wires.

4. Start the engine. You may need to give it throttle initially. I believe the rpm spec should be 550-650 in drive, or (neutral for a manual). You set this with the torx screw mentioned above. It should end up about 1 turn below flush at most if everything else is correct. If you need to open it more, you need to open the throttle some with the throttle stop screw.
If you have to screw the torx screw way in, either the throttle stop is too far out, or you have a vacuum leak still. (The throttle body shaft can wear. Quick check is open the throttle slightly and see if it wiggles up and down. there shouldn't be any looseness here.)

5. Reconnect the aic 4 wire connector, shut off the engine. When you restart it after 30 seconds, the iac should have reset it self. The engine RPM should flare up and come down to base on each start.

6. Reset the tps to .7v key on, engine off by using the 2 mount screws if necessary.


Hey OlJeepTek,


Sorry, I was not clear.

I didn't need to adjust my idle. I wanted to know if adjusting that torx screw would allow too much air in thereby making a lean condition and producing too much NOX.

So, I screwed it in all the way (the way it was). Now to concentrate on having the CAT changed. My Emissions Test is not until Sept or Oct (I forget... old age, lol).

Question, I can see the spindle on the EGR moving in/out (about 1/8-3/32") when reving the engine. Is this a good amount? I also changed the EGR Solenoid on the Driver's side Fender Wall (thinking maybe it wasn't opening the EGR enough). I had done all this in my QUEST for lower PASSING NOX LEVELS!

btw - I also changed the grommet and things on the valve cover you had mentioned (forgotten I had done this). I had gotten tired of diagnosing and troubleshooting so I replacing anything I could find. The vacuum lines seem ok.

Thx, Phil

Sorry, I didn't mean to wander off the question on you..

The airscrew is in a little far but it wont affect nox. Nox is produced under load.

The egr should open further than that under load, but it is backpressure sensitive (See if there is a P or an N stamped on it) and won't open very far in the shop. You should be able to manually squeeze it open and get a major idle change if the passages aren't plugged. Do you get any pinging on acceleration?

Have you had the emissions checked at all since you fixed the temp problem? You may have already fixed it.

One more thought that may or may not help. Higher octane fuel burns slower(cooler) it might drop nox a little.

Phil - Where exactly is the cover and screw? The only one I seem to have ('88XJ with MPI) is on the TB just above the EGR. Looks like a small metal cover pressed into the boss. My guess is that if it's covered, we're not meant to srew (pun intended) around with it.

OLJeeptek - Nice writeup. However, I could find no mention of this torx screw adjustment in the 88 FSM. Where did this come from?

Phil - Where exactly is the cover and screw? The only one I seem to have ('88XJ with MPI) is on the TB just above the EGR. Looks like a small metal cover pressed into the boss. My guess is that if it's covered, we're not meant to srew (pun intended) around with it.

OLJeeptek - Nice writeup. However, I could find no mention of this torx screw adjustment in the 88 FSM. Where did this come from?

It's not in the service manual. It's from a technical service bulletin for warm stalling at stops from way on back. I must have been through hundreds of these. The tps voltage is an observation after performing the proper setting to 13% with the DRBII scanner.

Sorry, I didn't mean to wander off the question on you..

The airscrew is in a little far but it wont affect nox. Nox is produced under load.

The egr should open further than that under load, but it is backpressure sensitive (See if there is a P or an N stamped on it) and won't open very far in the shop. You should be able to manually squeeze it open and get a major idle change if the passages aren't plugged. Do you get any pinging on acceleration?

Have you had the emissions checked at all since you fixed the temp problem? You may have already fixed it.

One more thought that may or may not help. Higher octane fuel burns slower(cooler) it might drop nox a little.


Hey OlJeepTek,

NP, Will check to see if there's a letter P or N stamped on it. The part that I was refering to that moves is the SPINDLE that's on the EGR and moves 1/8-3/32" under load. Do you think that's enough movement?

Haven't tried manually squeezing it yet to see if changes in idle speed. When doing this does the idle go up or down?

She runs Great no PINGING under heavy acceleration. I used to get it until I changed the O2 sensor but no more :)

Won't have the Emissions checked again until Sept or Oct. Is there a way to test the NOX levels w/o going through the entire test on the DYNO (A short version so to speak)? Hopefully, I fixed the NOX.

Good Tip about the Higher Octane Fuel to drop NOX Levels. That's a new one on me.


Saudade - The Torx Screw is on the Throttle Body on the Driver's Side and is hidden by a pressed in cover. Unsure if I have MPI so your TB may be different that mine. How can I find out if mine has MPI?


Thx, Phil

Been down this road before. My 95 failed for high NOX twice in four years here in Ohio. The first time I did all the usual tune up items and the re-test was worse. I went across the river to Kentucky where they sell the state mandated "re-formulated, low emission gas" and tried again with no change in NOX reading or any of the other items they measure. I took it to a state recommended emission repair shop. They put on a new convertor and it sailed right through the test. Two years and 25,000 miles later, same problem. High NOX reading with all other readings OK. I could find nothing wrong so I put on a new convertor. It passed.

I know high combustion temps cause high NOX readings. The EGR system is supposed to lower combustion temp. Everything appears to be working properly. I don't think the convertor should last such a short time. I am very interested in hearing what you find the cause of your high NOX readings to be. The emissions repair shop guy told me that a lot of cars they see with this problem have no clear cut cause but will pass with a new convertor.

Phil, MPI=Multiport Fule Injection. If you have individual injectors on a rail along the mainfold, you have an MPI. If your fule lines connect to a module under the throttle body, you have TBI (Throttle body injection). Check with a smog station and ask if you can smog your car without a cert since your registration is not up for renewal.

I thought all 88, AW4s had MPI but my FSM and FPM are vague on this.

Been down this road before. My 95 failed for high NOX twice in four years here in Ohio. The first time I did all the usual tune up items and the re-test was worse. I went across the river to Kentucky where they sell the state mandated "re-formulated, low emission gas" and tried again with no change in NOX reading or any of the other items they measure. I took it to a state recommended emission repair shop. They put on a new convertor and it sailed right through the test. Two years and 25,000 miles later, same problem. High NOX reading with all other readings OK. I could find nothing wrong so I put on a new convertor. It passed.

I know high combustion temps cause high NOX readings. The EGR system is supposed to lower combustion temp. Everything appears to be working properly. I don't think the convertor should last such a short time. I am very interested in hearing what you find the cause of your high NOX readings to be. The emissions repair shop guy told me that a lot of cars they see with this problem have no clear cut cause but will pass with a new convertor.


Hey Jeepme,


Does your Jeep have the Open Radiator System (with a Rad Cap and Overflow Bottle) or the Closed Radiator System? Maybe it was hot like mine and failed?

I just hope my new cat will last a long time. I can't afford to keep changing it every two years!

btw - I heard that during Emissions Testing they are supposed to put a big fan in front of the vehicle to help keep it cool since the vehicle is stationary and not moving. But I have yet to find such a caring shop. LOL Maybe I'll go get a high powered fan and bring it with me next time.


Saudade - Yes, my 88 XJ has individual fuel injectors on a rail so I guess I have MPI. WOO HOO! YAY!


Thx, Phil

95's have an open cooling system.

I've had to take mine to a "Test Only" station my last 2 times and they put a HUGE fan in front of the radiator.

One thing is that the CAT needs to get up to temp to work properly. I've had mine fail after a very short drive to the test station (just across the street), then did a few laps around the block and came back in and passed.

My 95 does have a open cooling system. My Jeep can idle all day without over heating. I am running a 195 deg. thermostat. The drive from my house to the check station is long enough to bring the convertor up to the proper temprature.

I have to wonder if a few degrees coolant temp is enough to effect the NOX output as it is caused by high temprature during the burn in the cylinder and that is measured at a much higher temp than 195 degrees. The usual recommendation is that the timing is off causing a hotter burn or the EGR is not adding the proper amount of exhaust gas to cool the burn. If exhaust gas which is hot can cool the burn then I doubt that running the coolant at say 185 or 205 degrees would affect it very much, but who knows?

The emissions check station does run a large fan blowing into the radiator while they run it on the rollers. Not only are these tests notoriously unreliable but the testers have damaged a lot of cars while doing them. Here in Ohio the tests will be halted in January and Kentucky is halting them too. I'm all for clean air but there has to be a better way than this goofy system.

My 95 does have a open cooling system. My Jeep can idle all day without over heating. I am running a 195 deg. thermostat. The drive from my house to the check station is long enough to bring the convertor up to the proper temprature.

I have to wonder if a few degrees coolant temp is enough to effect the NOX output as it is caused by high temprature during the burn in the cylinder and that is measured at a much higher temp than 195 degrees. The usual recommendation is that the timing is off causing a hotter burn or the EGR is not adding the proper amount of exhaust gas to cool the burn. If exhaust gas which is hot can cool the burn then I doubt that running the coolant at say 185 or 205 degrees would affect it very much, but who knows?

The emissions check station does run a large fan blowing into the radiator while they run it on the rollers. Not only are these tests notoriously unreliable but the testers have damaged a lot of cars while doing them. Here in Ohio the tests will be halted in January and Kentucky is halting them too. I'm all for clean air but there has to be a better way than this goofy system.

Hey Jeepme,


With your Factory Open Rad System when you're idling do the electric fans kick in at some point or no?


Yeah, here in NY they've wrecked many cars too with this retarded DYNO Emissions Test. I've seen on occasion in the local paper pix of cars that broke free from the DYNO and smashed everything in their paths! I hope that never happens to mine!


Thx, Phil

The electric fan cycles on and off frequently.

The testing here was stopped for a while when it came out that the testers routinely continued to use the equipment when it was broken resulting in failure for each car that went through that lane. In one notorious incident here the tester put a front wheel drive car on the rollers with them under the rear wheels. When he hit the gas the car took off right into the car ahead of it. Another lady's car was left sitting in gear with the engine running while the tester got out to test the gas cap. The car rolled out the door, across the parking lot and into a pond. They told the owner that she would have to call a wrecker to get her car out of the pond and have the repairs done at her own expense. Then she could file a claim for the damages. She had to sue them to recover what it cost her. One of the highly skilled testers jumped my 5.0 Mustang off the rollers power shifting it from first to second. Lucky for me there was no car ahead. The manager threatened to call the cops on me for getting upset! At least they passed it.