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Catalytic Converter Not Hot Enough

cij805

NAXJA Forum User
Location
San Diego
1993 XJ 4.0 Auto 4x4

Hello all,

Been failing smog tests, and have tried pretty much everything, even got a brand new catalytic converter and though my numbers have improved, it still isn’t passing...come to find that the catalytic converter isn’t getting up to temperature (colder in the rear). And ideas of what could be causing this?



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Post the results of your smog test.
 
15 MPH: RPM-1987, CO2-15.0%, O2-0.0%, HC-15 PPM, CO-0.01%, NO-641 PPM

25MPH: RPM-1857, CO2- 15.1%, O2-0.0%, HC-13 PPM, CO-0.00%, NO-622 PPM
 
Yes, NOX is the only spot I’m failing on, and it would seem that I’m running lean but the O2 sensor is brand new and has also just been tested to ensure that it’s not faulty...


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if your XJ has an egr valve as my 1990 does, check it for operation, make sure the vacuum is opening it. the egr valve is what is used to reduce NOx. failure of the egr valve causes NOx to increase.

of course a california style three way catalyst also can reduce NOx verses the 49 state catalyst.

do you have an egr valve?

good luck with the smog nazis.
 
No, no EGR on the ‘93...and the cat is a brand new 3-way. Checked fuel pressure today, 15 PSI with ignition in on position, 35 PSI at idle and 41 PSI with vacuum removed.
 
Have a shop run a BG induction service to clean the carbon off the top of the pistons.
We’ve used it with fantastic results in my shop.
Cheap way to try is to run water through the intake via mist or drips for 30 minutes. Make sure the engine is warmed up. It may help some.
 
High NOX reading are usually caused by either a lean mixture or defect catalytic converter.
Your new 'converter may be defective or something, such as plugged injector or weak fuel pump is cause it to run lean.
Since it's OBDI, the converter does not have a downstream O2 sensor, its performance is not monitored by the ECM.
I would do a temperature test on the converter by getting it good and hot and then reading its inlet and outlet temps, using an infrared thermometer.
The inlet should be considerably cooler than the outlet if the converter is functioning .
 
I have done the water mist as well as the Mopar CCC, and my lambda numbers show optimal A/F ratios, it runs great, and have checked for carbon in combustion chamber which is minimal after treatments. I have thought of the CAT possibly being defective but want to see if there is anything else before spending more money on that, seems to me that it isn’t getting enough unburned fuel to kick off the reaction to heat up. Have checked CAT temps and I know they’re wrong, they are cooler in the rear by 50-75 degrees.

Replaced parts:
-Catalytic Converter
-O2 Sensor
-TPS
-Distributor Cap/Rotor
-Spark Plugs and Wires
-Timing Chain and Gears
-Thermostat (195 Degree)
-Injectors
-Radiator
-Fuel Filter
-Oil Pressure Sensor
-Manifold Gasket and Manifold Crack Welded
-All obviously cracked vacuum lines

Have also checked voltage to O2 sensor, Fuel rail pressure, voltage to MAP sensor with vacuum pump, and checked via vacuum gauge for exhaust restriction...all check out....

Possibly hidden cracked vacuum lines? (Idles about 600 RPM)
Exhaust leak before cat?
Worn distributor causing advanced timing?
Worn lifters not opening exhaust valve enough (no egr)?
Clogged cooling passages?
 
Normally, when a cat is working, the inlet temp is lower than the output. A bad cat will have the output temp the same or lower than the inlet. temp.

How do you know that the converter is OK, just because it's new doesn't mean it cannot be defective. Without a rear O2 sensor, the computer has no idea if the cat is performing within the expected parameters . Your catalytic converter may not be getting hot because isn't working.
Unless a major leak or blockage exist between the headers and the converter, it's getting more than enough heat to light off the cat, assuming it is functioning. If either was occurring, you would already know it.
I assume you have checked the PN of the cat and verified it's the correct CARB-approve three-way cat. Your smog guy should have checked it too.

A major vacuum leak could lean the mixture but you already have checked the A/F mixture and found to be fine.
 
Obviously I don’t know for sure that it’s not a bad cat, but seeing as if you search these forums for this same issue, it is common, and I highly doubt that a professional company like Magnaflow or Walker would stay in business very long if they were putting out that many defective products. It is possible, but considering the condition of the Jeep when I got it, it seems more likely that the issue stems from somewhere upstream.


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you may have a hot cylinder, say from an intake leak. you wont see this on the Ow sendor, cause that sendor takes the avergae of all six cylinders and the computer compensates my making the other five all richer, your lean cylinder will still be lean if it is from an intake leak.

test for intake leaks using a flammable aerosol like carb cleaner, spray along intake with motor warmed up at steady idle. you will get an rpm change when the aerosol is sprayed on a leak.

Good luck
 
I will try that, I just changed the injectors so I just assumed all the o-rings were good, but at this point I will try anything. Thanks.

Also had a Bosch O2 sensor and saw a lot of people saying to use NTK so I got one of those, going in tomorrow.


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x2 on the NTK

Unless Bosch was the factory supplier, I wont use them.
 
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