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emissions fail; causes for high NOX?

macarchie

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91 Ranger; 3.0 L; 5-spd. Failed emissions due to high NOX readings. CO% and HC counts were good. Any ideas where to start. New EGR, new cat or just an oil change? Anybody else have this problem. Any assistance would be much appreciated.

- Jimmy
 


mechano

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check for blockage or plugged EGR passage(mostlikely) and also check operation of the EGR if you have the know how...Usually when you have 1 particular gas failure reading eg. Nitrogen Oxides, but the other readings are good it usually is a good indicator of a good converter.. I would try as folllows 1/loosening the pipe at the EGR valve completely
2/starting truck while sitting in it as the RPMs will start to climb rapidly if there is no blockage and it will sound like a terrible exhaust leak...but...if the it idles without jumping in rpm then you probably got a blockage issue..(that is my opinion, I have seen this in the past, BUT, there is still always a possiblility of catalysis failure or bad vacuum leaks, bad intake...My advice , start off with the easy stuff.
 

locknar

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speaking of the easy stuff first, over-advanced timing will cause high NOX
 

triumphrider-1

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Have you checked for any codes thrown?
 

mechano

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+1 locknar. I was thinking in terms of an OBDII. check timing for sure
 

graff104

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Have you checked for any codes thrown?
+1 when you have a problem always pull the codes first. When you go into the doctor he asks you what wrong. When your car is sick why not ask it what is wrong?
 

macarchie

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UPDATE. Thanks for the replys. This is a 1991 - so no OBDII. It has EEC-IV. This engine does not have an EGR valve. I hooked up a scanner and got a "41" code which is " Exhaust Gas Oxygen (EGO) sensor always lean (low value) - does not switch " I have replaced the O2 sensor, the PCV, used BG-44K and changed the oil. The emissions tech seems to think the coolant temperature sensor is bad and is not telling the computer that the engine has warmed up. The computer is thinking that the engine is still warming up and is keeping the mixture rich, which is killing the emissions test. It has been tested 3 times. My next step is to replace the thermostat with a higher temp (195) and replace the ECT sensor (easy and cheap). I didn't get any codes stating that the ECT or MAF were acting up. If this doesn't work, then I guess the cat is next. There is only one O2 sensor (pre-cat) and the scanner can't tell me if the cat is bad.
Of course, it's running great - I hate to mess with the engine. Maybe I should move away from town where emissions are not checked.
Comments are welcome.
 

busopyukon

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No Advance Timing

Would the problem that you have also throw off the advance timing?
Joe
 

joe.dollar

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I live in an emissions province in Canada. My truck failed 3 times on NOX. I did all kinds of stuff, but the only thing that got me through was a new cat. I got one cheap, installed it myself and didn't fail emissions again for years.
 

BlackBII

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The code says your motor is running lean, however the tech thinks it's running rich?

Interesting..I would suspect a vaccum leak and high combustion temps.

Good info here. http://www.nichols.nu/tip766.htm
 

macarchie

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Thanks for the replies... Another Update: I put in a 195 deg thermostat and a new ECT sensor. Ran it up to operating temp and then hooked up the scanner - still got the 41 code. I don't understand what the tech was talking about when he said it was running rich... it didn't compute.
Timing - I have a timing light (from my younger days) but the timing is set by the computer. I looked all over the front of the engine and cannot find the timing marks or scale. Any ideas how to check the timing?
MAF - I unhooked the air flow sensor and the engine was hard to start, ran rough and then the check engine light came on after a few seconds. I hooked it back up and it started right up and was back to normal. I don't think the MAF is bad.
Now I'm looking at the throttle position sensor and the intake air temp sensor. According to the book, both of these can affect the mixture. I can get a replacement cat for $299 (I'll have to install).
What am I forgetting?

thanks.
 

351rangerxxx

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a vacuum leak will cuz a lean code. found a bad boot on the pvc valve sucking air on a ford taruas before
 

SMTGREG

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NOx reading high ....engine slightly lean but not lean enough to cause misfire. NOx goes down when misfiring due to colder cylinder temp. Vacuum leaks (unmetered air entering intake system) or low fuel pressure will make it lean. You can test the Catalytic Converter.

Performing the Catalyst Delta-Temp Test.
To perform this test you will need a infrared temperature gun.
http://www.toolprofessional.net/Mastercool-Infrared-Thermometer-with-Laser-in/M/B000TM7HXC.htm

Disable the AIR System, if equipped.
Run engine for two minutes at 2500 RPM to fully heat the exhaust system.
Engine off.
Disconnect and ground one spark plug wire from each cylinder bank.
Start engine and run at 1000 RPM.
Disconnect the IAC solenoid.
Measure the surface temperature of both the inlet and outlet of each catalytic converter using an infrared temperature probe.
Turn Engine off.
Compare the difference in temperature between the inlet and outlet readings.
If the difference is 28°C (50°F) or greater: The catalyst is operating correctly. Reconnect spark plug wire(s), IAC solenoid and AIR system (if required). Perform PCM Reset to clear any DTCs set during testing.

If the difference in temperature between the catalyst inlet and outlet is less than 28°C (50°F): Testing indicates the catalyst is not working. Repeat test to verify results. If the temperature difference is still less than 28°C (50°F), replace the catalyst. Reconnect spark plug wire(s), IAC solenoid and AIR system (if required). Perform PCM Reset to clear any DTCs set during testing.
 
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countryboy

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i don't know anything about your emissions problems but i see from your avatar you are in va what part do you livein i am in va and have never had to do a emissions test just wandering
 

macarchie

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Thanks for the replies...

1. countryboy: I'm in Stafford County (we are considered to be part of northern virginia). The EPA says we have a population density that is high enough to require emissions testing.

2. Timing questions: First of all, the underhood label says to disconnect the "small" inline spout connector. I have found the main connector on the ICM (located beside the radiator on the driver side. Cannot find an "inline" connector. Any ideas where it might be or what it looks like? Secondly, unlike the chevy V8, the #1 cylinder on the 3L is on the passenger side. There is (what looks like) a reference mark attached to the block beneath the #1 cylinder. With the air intake in place - you cannot see it.

3. Thermostat: I put in a new 195 deg thermostat. I warmed up the engine and the temp gauge never climbed above about 1/4 of the dial. It only got up to the N in Normal. When the stat opened, the temp gauge dropped to the bottom edge of the Normal operating range. When the truck was new, the gauge usually stayed right around the center. Any ideas why?

4. mtnrgr: I have looked over the entire exhaust system and I can only find one O2 sensor (pre-cat).

Again, thanks for the input and discussion.
 

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